Comprehensive Analysis of Mechanical Ventilation with Heat Recovery (MVHR) Systems: Design, Implementation, and Performance

Abstract

Mechanical Ventilation with Heat Recovery (MVHR) systems represent a cornerstone of contemporary building design, particularly within the stringent parameters of energy-efficient construction exemplified by standards such as Passive House. These sophisticated systems meticulously manage indoor air quality, simultaneously enhancing energy efficiency through the capture and reuse of thermal energy from exhaust air. This comprehensive report offers an exhaustive exploration of MVHR systems, delving into their fundamental operational principles, advanced selection criteria, intricate design considerations, seamless integration with disparate building systems, the critical role of filtration, rigorous maintenance protocols, pervasive installation challenges, innovative noise reduction strategies, and their multifaceted benefits across diverse global climate zones. Furthermore, it examines emerging technologies and future trends shaping the evolution of these essential environmental control systems.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

1. Introduction

The trajectory of building standards, driven by an escalating imperative for energy conservation and occupant well-being, has propelled Mechanical Ventilation with Heat Recovery (MVHR) systems into a position of paramount importance. Historically, natural ventilation, reliant on uncontrolled air leakage and operable windows, was the primary means of air exchange in structures. However, as building envelopes became increasingly airtight—a necessary evolution to mitigate thermal losses and achieve ambitious energy performance targets—the challenge of maintaining adequate indoor air quality (IAQ) without compromising thermal efficiency became acute. Airtight construction, while excellent for energy retention, can inadvertently trap indoor pollutants and moisture, leading to ‘sick building syndrome’ and structural degradation if not properly managed [1, 2].

MVHR systems were developed as a strategic response to this dual challenge. They embody a sophisticated approach to ventilation by continuously extracting stale, polluted air from internal spaces (e.g., kitchens, bathrooms) and simultaneously supplying fresh, filtered outdoor air to habitable rooms (e.g., bedrooms, living areas). Crucially, the system incorporates a heat exchanger that facilitates the transfer of thermal energy from the outgoing exhaust air to the incoming fresh air stream, thereby pre-warming it in cold climates or pre-cooling it in hot climates, before it enters the conditioned space. This process significantly curtails the energy demand associated with heating or cooling ventilation air, which traditionally constitutes a substantial portion of a building’s overall energy consumption [3, 4].

The widespread adoption of MVHR aligns with the principles of nearly zero-energy buildings (nZEB) and the Passive House standard, where meticulous control over air movement and energy flows is fundamental. These systems are not merely about preventing heat loss; they are about creating a continuously healthy indoor environment, free from external pollutants, excess humidity, and airborne contaminants, while ensuring minimal energy expenditure [5]. Their role extends beyond simple ventilation to become an integral component of a holistic building services strategy, directly impacting thermal comfort, acoustic performance, and occupant health.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

2. System Selection

The judicious selection of an MVHR system is pivotal to achieving optimal performance, energy efficiency, and occupant satisfaction. This process necessitates a thorough understanding of the various system types, their operational characteristics, manufacturer reputations, and efficiency metrics, alongside a consideration of specific building requirements and environmental conditions.

2.1 Types of MVHR Systems

MVHR systems are primarily distinguished by the design of their heat exchange mechanism, which dictates their efficiency, functionality, and suitability for different applications. The core function, however, remains consistent: to transfer thermal energy between two air streams without physical mixing.

2.1.1 Plate Heat Exchangers

Plate heat exchangers are among the most common types of heat recovery cores. They consist of a series of thin, closely spaced plates, typically made from aluminium or high-grade plastic, which are arranged to create separate, parallel channels for the incoming fresh air and outgoing stale air [6]. The two air streams flow past each other, separated by the plates, allowing sensible heat (temperature) to transfer from the warmer to the cooler air stream by conduction through the plate material and convection into the air. There is no mixing of the air streams, ensuring excellent indoor air quality by preventing cross-contamination [7].

• Crossflow Plate Heat Exchangers: In this configuration, the two air streams flow perpendicular to each other. They are relatively simple in design and cost-effective, offering sensible heat recovery efficiencies typically ranging from 50% to 70%. Their primary limitation is the potential for frost build-up on the exhaust air side in extremely cold conditions, necessitating defrosting mechanisms [8].
• Counterflow Plate Heat Exchangers: These are a more advanced variant where the air streams flow in opposite directions over a greater surface area. This counter-current arrangement significantly enhances heat transfer effectiveness, allowing for sensible heat recovery efficiencies often exceeding 85%, and even reaching over 90% in high-performance units [9]. While more efficient, they can also be more susceptible to frost in severe cold, although advanced designs often incorporate intelligent defrosting cycles or pre-heating elements.

Advantages: High sensible heat recovery (especially counterflow), no moving parts in the heat exchange core (leading to lower maintenance and higher reliability), no cross-contamination of air streams, relatively compact.
Disadvantages: Typically only recovers sensible heat, requiring supplemental humidification in very dry climates; susceptible to frost in cold conditions; pressure drop can be higher than rotary exchangers.

2.1.2 Rotary Heat Exchangers (Heat Wheels)

Rotary heat exchangers, often referred to as ‘heat wheels’ or ‘enthalpy wheels,’ utilise a slowly rotating honeycomb matrix or disc, typically made of corrugated aluminium or polymer, which acts as the heat transfer medium [10]. As the wheel rotates, it continuously passes through both the exhaust air stream and the supply air stream. In the exhaust section, the matrix absorbs heat and, critically, moisture (latent heat) from the outgoing air. As the wheel rotates into the supply air stream, it releases this absorbed heat and moisture to the incoming fresh air. This allows for both sensible and latent heat recovery.

• Sensible Heat Recovery: The matrix directly transfers thermal energy based on temperature differences.
• Latent Heat Recovery: The porous or hygroscopic nature of the wheel material (often coated with desiccant materials like silica gel) absorbs moisture from the exhaust air and releases it into the supply air. This is particularly beneficial in humid climates for dehumidification, or in very dry climates to prevent excessive drying of indoor air [11].

Advantages: Very high overall (sensible + latent) heat recovery efficiency (often exceeding 80% for both), natural frost prevention due to continuous rotation and latent heat recovery, no condensate drainage required, helps maintain indoor humidity levels.
Disadvantages: Contains moving parts (motor, bearings, belt), which may require more maintenance; potential for slight cross-contamination (typically less than 5% due to purging sectors and pressure differentials); can be larger and heavier than plate exchangers.

2.1.3 Heat Pipe Exchangers

Heat pipe exchangers employ a sealed pipe system containing a working fluid that undergoes a phase change (evaporation and condensation) to transfer heat. One end of the heat pipe is placed in the warmer exhaust air stream, causing the fluid to evaporate and absorb heat. The vapour then travels to the cooler end of the pipe, located in the supply air stream, where it condenses, releasing its latent heat and warming the incoming air [12]. The condensed liquid then returns to the warmer end via gravity or a wicking action. This is a passive heat transfer mechanism with no moving parts.

Advantages: No moving parts, completely separates air streams (zero cross-contamination), high efficiency (typically 60-80% sensible), compact design possible.
Disadvantages: Generally only recovers sensible heat; efficiency can be lower than advanced rotary or counterflow plate exchangers; orientation-dependent for gravity-assisted systems.

2.1.4 Run-Around Coils

While not strictly a single unit MVHR, run-around coils represent a variant of heat recovery that is crucial for specific architectural constraints. This system employs two separate finned coil heat exchangers, one in the exhaust air stream and one in the supply air stream, connected by a closed loop containing a heat transfer fluid (e.g., water, glycol mixture) and a circulating pump [13]. Heat is absorbed by the fluid in the exhaust coil and then transported to the supply coil, where it is transferred to the incoming fresh air. This system allows for complete separation of the supply and exhaust air ducts, which is advantageous when these air paths are geographically distant or cannot intersect. While offering complete separation and no cross-contamination, their efficiency is generally lower (45-65%) due to the double heat transfer process and pump energy consumption, making them less common for residential MVHR [14].

2.1.5 Energy Recovery Ventilators (ERVs)

It is important to differentiate between MVHR and Energy Recovery Ventilators (ERVs). While all MVHR systems recover energy (sensible heat), ERVs are specifically designed to recover both sensible and latent heat (moisture) [15]. A rotary heat exchanger (enthalpy wheel) is a common core for ERVs. Plate heat exchangers can also be designed for latent heat transfer if constructed with permeable membranes (e.g., polymer films). ERVs are particularly advantageous in climates with high humidity, where reducing the moisture load on cooling systems is critical, or in very dry climates, where preventing excessive dehumidification of indoor air is beneficial for comfort and health. The choice between MVHR (sensible only) and ERV (sensible and latent) depends heavily on the specific climate zone and desired humidity control [16].

2.2 Manufacturers and Efficiency Ratings

Choosing a reputable manufacturer is paramount for ensuring the long-term performance, reliability, and support for an MVHR system. The market offers a diverse range of manufacturers, each with distinct product lines, technological focuses, and regional specialisations. Leading manufacturers recognised for their innovation and quality include:

• Zehnder: A Swiss-German company renowned for high-performance, Passive House certified MVHR units, often featuring advanced controls and radial ducting systems [17].
• Nilan: A Danish manufacturer focusing on integrated ventilation and heat pump solutions, offering highly energy-efficient systems often with active cooling capabilities [18].
• Brink Climate Systems: A Dutch specialist in ventilation, offering a range of MVHR units known for their compact design and robust performance [19].
• Paul Wärmerückgewinnung: A German manufacturer with a strong focus on high-efficiency counterflow plate heat exchangers, often achieving very high Passive House certified efficiencies [20].
• Vent-Axia: A UK-based manufacturer providing a broad range of ventilation solutions, including MVHR units suitable for various residential and commercial applications [21].
• Daikin and Mitsubishi Electric: Global HVAC giants that offer MVHR units, often integrated within their broader heating and cooling systems, leveraging their extensive R&D capabilities [22].
• Holtop: A Chinese manufacturer gaining recognition for offering a wide array of ventilation and heat recovery products, balancing performance with cost-effectiveness for diverse building types [23].
• Swegon: A Swedish company focusing on energy-efficient indoor climate systems for commercial and public buildings, but also offering advanced residential solutions [24].
• Titon: A UK company producing MVHR systems designed for domestic and light commercial applications, emphasising ease of installation and compliance with local building regulations [25].

Efficiency Ratings: The ‘efficiency’ of an MVHR system typically refers to its heat recovery efficiency, expressed as a percentage. This metric quantifies how much of the thermal energy from the exhaust air is transferred to the incoming supply air. Higher percentages indicate greater energy savings. It is crucial to understand that different standards and testing methodologies exist, leading to variations in reported efficiencies.

• Sensible Heat Recovery Efficiency: This measures the transfer of temperature only. It is calculated as the ratio of the temperature increase of the supply air to the maximum possible temperature difference between the exhaust and supply air streams [26]. High-efficiency units, especially counterflow plate and rotary exchangers, can achieve sensible recovery efficiencies of 85% to over 90%.
• Apparent Effectiveness (Latent and Sensible): For ERVs, or systems designed for latent heat recovery, overall effectiveness considers both sensible and latent heat transfer. This is a more comprehensive metric, particularly relevant in humid or very dry climates [27].
• Specific Fan Power (SFP): Beyond heat recovery efficiency, the energy consumed by the fans is a critical factor in the overall energy performance of an MVHR system. SFP, measured in W/(L/s) or kW/(m³/s), indicates the electrical power consumed by the fans per unit of airflow [28]. A lower SFP signifies more efficient fans, reducing operational energy costs. Regulations often mandate maximum SFP values.
• Certification Standards: Reputable certification bodies provide independent verification of MVHR system performance. The Passive House Institute (PHI) certifies MVHR units based on stringent criteria for heat recovery efficiency (minimum 75% sensible), specific fan power (SFP < 0.45 Wh/m³), and airtightness [29]. In Europe, EN 13141-7 specifies testing methods and performance requirements for MVHR units, while ISO 16890 governs filter performance. Understanding these standards is vital for comparing systems accurately [30].

2.3 Decentralised MVHR Systems

While centralised MVHR systems serve an entire building via a network of ducts, decentralised (or single-room) MVHR units offer an alternative, particularly suited for retrofitting existing properties, smaller apartments, or spaces where extensive ductwork is impractical [31]. These systems typically consist of a single unit installed through an external wall, comprising a fan, a small heat exchanger (often a ceramic regenerative core), and filters. They usually operate in a push-pull cycle: one fan extracts air for a period, storing heat in the ceramic core, then reverses to supply fresh air, picking up the stored heat. Some systems use two fans working in tandem for continuous supply and extract [32].

Advantages: Easier and less disruptive installation (no ductwork), lower initial cost for small applications, suitable for targeted ventilation, flexibility for staged upgrades.
Disadvantages: Lower overall efficiency compared to centralised systems, can create unbalanced ventilation across a dwelling if not carefully planned, multiple units may be required, potentially higher localised noise levels, limited airflow capacity.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

3. Design Considerations

The effective performance of an MVHR system is not solely dependent on the quality of the unit itself but is profoundly influenced by meticulous design. Comprehensive planning, precise sizing, optimised ducting, and seamless integration with other building services are critical to achieving the desired indoor environmental quality and energy efficiency targets.

3.1 Optimal Sizing

Correct sizing is perhaps the most critical design decision, directly impacting IAQ, energy consumption, and noise levels. An undersized system will fail to provide adequate ventilation, leading to poor air quality, condensation, and potential health issues. An oversized system will consume more energy than necessary, operate inefficiently, and likely generate excessive noise due to higher fan speeds [33].

Sizing methodologies generally follow these principles:

• Air Change Rates (ACH): This method calculates the required ventilation volume based on the total conditioned volume of the building, aiming for a specified number of air changes per hour (e.g., 0.3 to 0.5 ACH for modern, airtight dwellings). For example, a 200 m³ building with a target of 0.3 ACH requires 60 m³/h (0.3 * 200 m³/h) of ventilation [34].
• Occupancy Levels: Building codes and ventilation standards (e.g., ASHRAE 62.1 in North America, EN 15251 in Europe, Approved Document F in the UK) often specify minimum fresh air supply rates per person (e.g., 4-8 L/s/person) and/or per room type [35, 36]. This accounts for human bio-effluents, predominantly CO2, which are directly related to occupancy.
• Specific Room Requirements: Areas with high pollution generation, such as kitchens, bathrooms, and utility rooms, have higher extract air requirements (e.g., 13-25 L/s for bathrooms, 30-60 L/s for kitchens during cooking) [37]. These peak demands must be accounted for in the overall system capacity, often managed through boost modes.
• Building Airtightness: In highly airtight buildings (e.g., Passive House standard, n50 ≤ 0.6 air changes per hour at 50 Pa), MVHR systems become the sole source of controlled ventilation, and sizing must precisely match the required fresh air intake. In leakier buildings, unintended infiltration can contribute to air changes, though uncontrolled and often inefficiently [38].
• Pressure Drop Calculations: The fan within the MVHR unit must be capable of overcoming the total static pressure loss across the entire system, including filters, the heat exchanger core, and all ductwork (bends, reducers, diffusers). Accurate calculation of pressure drops is essential to select a fan that can deliver the required airflow efficiently without excessive noise or energy consumption. This often involves using manufacturer fan curves and detailed duct calculations [39].
• Software Tools: Specialised HVAC design software (e.g., Hevacomp, MagiCAD, PHPP) can perform complex airflow calculations, pressure drop analysis, and energy simulations to optimise MVHR sizing and duct routing for specific building designs.

3.2 Ducting Layouts

The ductwork is the circulatory system of the MVHR, and its design critically influences system efficiency, noise, and maintenance. Poorly designed ducting can negate the benefits of even the most efficient MVHR unit by increasing pressure losses, leading to higher fan energy consumption and reduced airflow [40].

Key considerations for ducting layouts:

• Minimising Duct Lengths and Bends: Shorter, straighter duct runs reduce static pressure losses and thereby fan energy consumption. Each bend introduces resistance; therefore, using wide-radius bends (e.g., 90° bends with a large internal radius) is preferred over sharp elbows. Minimising the number of bends is also crucial [41].
• Duct Materials:
  • Rigid Galvanised Steel: Durable, very low airflow resistance, easy to clean, but can be complex to install in residential settings due to space constraints and requires specialist fabrication. It’s often used for main runs in larger systems [42].
  • Rigid Plastic (e.g., Polypropylene, HDPE): Increasingly popular in residential MVHR. Available in various shapes (circular, semi-circular, rectangular) to fit tight spaces. Smooth inner surfaces offer low resistance. Can be easily cut and joined with push-fit seals. Often used in radial (home-run) systems [43].
  • Flexible Insulated Ducts: While easy to install, their corrugated inner surface creates significant airflow resistance and can trap dust, making cleaning difficult. They should be used sparingly and only for short connections, and always stretched taut to minimise resistance. Flexible ducting must be properly supported to prevent sagging, which can create traps for condensation [44].
• Duct Diameter: Appropriately sized ducts are essential. Undersized ducts lead to high air velocities, resulting in increased pressure drop and noise. Oversized ducts increase material cost and space requirements but reduce velocity and noise. Design guidelines typically recommend air velocities below 3-5 m/s in main ducts and 2-3 m/s in branch ducts to minimise noise [45].
• Airtightness: Ductwork must be as airtight as the building envelope itself. Leaky ducts reduce the effective airflow to conditioned spaces, increase fan energy, and can lead to moisture ingress in unconditioned areas. Leakage classes (e.g., EN 13141-7 specifies minimum class C) define acceptable leakage rates [46]. All joints should be sealed with appropriate tapes or gaskets.
• Insulation: All ducts passing through unheated or uncooled spaces (e.g., lofts, crawl spaces, external walls) must be thoroughly insulated to prevent heat loss/gain and, crucially, to prevent condensation forming within or on the outside of the ducts, which can lead to mould and damage [47]. Minimum insulation thicknesses are often specified in building codes.
• Balancing and Commissioning: After installation, the duct system must be balanced to ensure that the correct airflow rates are delivered to and extracted from each room according to the design specifications. This involves adjusting dampers in the ductwork or at the terminal devices (grilles/diffusers) and measuring airflow with specialised equipment. Commissioning also includes verifying fan performance, checking for leaks, and testing controls [48].
• Terminal Devices (Grilles and Diffusers): The choice and placement of supply and extract grilles and diffusers affect air distribution and noise. Low-resistance, architecturally discrete devices are preferred. They should be placed to ensure good air circulation, avoiding ‘short-circuiting’ (where supply air is immediately extracted) and ensuring comfortable air velocities at occupant level [49].
• Condensate Drainage: In most MVHR units (especially plate heat exchangers), condensate will form due to the cooling of the exhaust air. A properly sloped and trapped condensate drain must be connected to the building’s wastewater system to prevent water accumulation, blockages, and potential water damage [50]. In cold climates, the drain and trap must be protected from freezing.

3.3 Integration with Building Systems

To maximise the benefits of MVHR, its seamless integration with other building services is essential. This holistic approach optimises energy efficiency, enhances thermal comfort, and simplifies building operation.

3.3.1 Heating Systems

MVHR systems primarily recover heat, reducing the load on conventional heating systems. However, further integration can enhance performance:

• Pre-heating Elements: In extremely cold climates, an electric pre-heater or a ground-source heat exchanger (GSHX, often called an ‘earth tube’ or ‘ground-to-air heat exchanger’) can be used to temper the incoming fresh air before it reaches the MVHR unit, preventing frost formation on the heat exchanger and reducing the primary heating load [51]. GSHX systems leverage the stable temperature of the earth to passively pre-warm air in winter and pre-cool it in summer.
• Post-heating Coils: Some MVHR units can incorporate a small heating coil (electric or hydronic, connected to the main heating system) to provide additional tempering of the supply air, ensuring it is delivered at a comfortable temperature, especially during colder periods or in zones with higher heat loss [52].
• Smart Control Systems: Integrating MVHR controls with the building’s main thermostat or building management system (BMS) allows for coordinated operation. For example, if the heating system is active, the MVHR can operate at higher efficiency settings to minimise cold drafts and leverage the recovered heat. Demand-controlled ventilation (DCV) strategies, discussed later, are also part of this integration.

3.3.2 Cooling Systems

MVHR systems can also contribute to cooling and dehumidification, particularly when paired with an ERV:

• Bypass Modes (Summer Bypass): During warmer periods when outdoor air is cooler than indoor air (e.g., at night), many MVHR units feature a bypass mode that allows the incoming fresh air to bypass the heat exchanger entirely. This provides ‘free cooling’ by bringing in cooler night air directly, flushing out accumulated heat [53].
• Pre-cooling: As with pre-heating, a GSHX can be used in summer to pre-cool incoming air. The earth, being cooler than ambient air during summer days, can absorb heat from the incoming air stream before it enters the MVHR unit or conditioned space.
• Integrated Cooling Coils: Some advanced MVHR units, particularly those with active heat pump technology (e.g., Nilan Compact P), can incorporate a cooling coil directly into the supply air path. This allows for tempered and dehumidified air to be delivered, reducing the reliance on separate air conditioning units or reducing their workload [54].
• Humidity Control (ERVs): In hot, humid climates, an ERV’s ability to transfer moisture from the incoming humid air to the outgoing drier air is invaluable. This significantly reduces the latent load on the primary cooling system, leading to smaller, more efficient AC units and improved indoor comfort by preventing excessive humidity [55].

3.3.3 Smart Home Systems and BMS

Modern MVHR systems often feature advanced control capabilities that can be integrated with broader smart home ecosystems or sophisticated BMS in commercial settings. This allows for:

• Demand-Controlled Ventilation (DCV): Sensors (CO2, VOCs, humidity) strategically placed throughout the building can monitor indoor air quality parameters. The MVHR system can then dynamically adjust its fan speeds and airflow rates in real-time, delivering ventilation only where and when it’s needed [56]. This optimises energy consumption by avoiding over-ventilation and ensures optimal IAQ. For example, a CO2 sensor in a bedroom can increase airflow when occupancy rises, and reduce it when the room is empty.
• Remote Monitoring and Control: Occupants can monitor and adjust MVHR settings via smartphone apps or central touch panels, gaining insight into air quality metrics and energy performance. This also facilitates remote diagnostics and adjustments by maintenance personnel.
• Scheduled Operation: Programming the MVHR to operate at different speeds based on daily schedules (e.g., lower speed during unoccupied hours, boost during cooking) further enhances energy efficiency and convenience.

3.3.4 Fire Safety Systems

Integration with fire safety systems is crucial. In the event of a fire alarm, MVHR systems are typically designed to shut down automatically to prevent the spread of smoke and fumes through the ductwork. More advanced systems might incorporate smoke dampers or fire-rated ducts in certain zones to maintain compartmentation [57].

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

4. Filter Types and Maintenance

Filters are an indispensable component of any MVHR system, serving as the primary barrier against airborne pollutants and ensuring the delivery of clean, fresh air indoors. Regular and correct maintenance, particularly filter replacement, is vital for sustained system performance, energy efficiency, and occupant health.

4.1 Filter Types

MVHR systems typically employ multiple stages of filtration to capture a range of particle sizes. The effectiveness of filters is categorised by international standards, with the ISO 16890 standard (introduced in 2017) largely replacing the older EN 779 standard [58].

4.1.1 ISO 16890 Classifications

This standard classifies filters based on their ability to capture different particulate matter (PM) sizes, which are directly relevant to human health. PM is categorised by its aerodynamic diameter:

• ISO Coarse: Captures particles larger than 10 µm (PM10). These filters are generally pre-filters, similar to the old G4 rating, designed to protect the MVHR unit and finer filters from larger dust, pollen, and insects. They typically have an ePM10 efficiency of less than 50% [59].
• ePM10: Filters capable of capturing at least 50% of PM10 particles. These are often used as primary filters.
• ePM2.5: Filters capable of capturing at least 50% of PM2.5 particles (fine particulate matter, 2.5 µm or smaller), which can penetrate deep into the lungs and are associated with significant health risks. This category often corresponds to the old F7 rating. An ePM2.5 50% filter, for example, captures at least 50% of particles in this range [60].
• ePM1: Filters designed to capture at least 50% of PM1 particles (ultrafine particulate matter, 1 µm or smaller), which are considered the most harmful as they can enter the bloodstream. These are high-efficiency filters, often corresponding to the old F9 rating, and are crucial for protecting against pollution from traffic, industrial emissions, and combustion processes [61].

MVHR systems typically incorporate at least two filter stages: a coarser filter (e.g., ISO Coarse or ePM10 50%) for the exhaust air to protect the heat exchanger, and a finer filter (e.g., ePM2.5 50% or ePM1 50%) for the supply air to ensure high indoor air quality.

4.1.2 Specific Filter Media

• Pleated Filters (G4, F7, F9 equivalents): These are the most common. Made from synthetic fibres (e.g., polyester or glass fibre), they are folded into pleats to increase surface area, allowing for higher dust-holding capacity and lower pressure drop. Their efficiency depends on the fibre density and arrangement [62].
• Carbon Filters (Activated Carbon): These are specifically designed to remove gaseous pollutants, odours, and Volatile Organic Compounds (VOCs) through the process of adsorption, where pollutants physically adhere to the porous structure of the activated carbon. They are often used as an additional stage in areas with specific odour or VOC concerns (e.g., near busy roads, industrial zones) [63]. Carbon filters do not remove particulate matter effectively, so they are always used in conjunction with particulate filters.
• HEPA Filters (High-Efficiency Particulate Air): These are extremely efficient filters (H13 or H14 classification), capable of capturing 99.97% or 99.995% of particles down to 0.3 µm, including bacteria and viruses [64]. While not standard in most residential MVHR systems due to their high pressure drop (requiring powerful fans) and cost, they can be integrated in environments requiring ultra-clean air, such as medical facilities or homes with severe allergy sufferers.
• Electrostatic Filters: These filters use an electric charge to capture particles. They can be very efficient at removing fine particles but require regular cleaning to maintain performance and can sometimes generate ozone [65]. They are less common in residential MVHR.

4.2 Maintenance

Regular and thorough maintenance is indispensable for the long-term efficiency, reliability, and hygienic operation of an MVHR system. Neglecting maintenance can lead to reduced airflow, decreased heat recovery efficiency, increased energy consumption, compromised indoor air quality, and premature system failure.

4.2.1 Filter Replacement

This is the most frequent and critical maintenance task.

• Frequency: Filter replacement frequency varies depending on several factors: the type of filter (finer filters may clog faster), local air quality (e.g., urban vs. rural, pollen season), occupancy levels, and system usage. General recommendations suggest replacing coarse filters (e.g., ISO Coarse, ePM10) every 3-6 months and fine filters (e.g., ePM2.5, ePM1) every 6-12 months [66]. Some advanced MVHR units have pressure sensors that alert users when filters are approaching saturation, indicating a need for replacement based on pressure drop.
• Rationale: As filters capture particles, their pores become blocked, increasing airflow resistance (pressure drop). This forces the fan to work harder, consuming more energy and potentially reducing the delivered airflow. Clogged filters also lose their effectiveness, allowing pollutants to pass through, and can become breeding grounds for mould, bacteria, and allergens if they become damp [67].
• Procedure: Replacing filters is typically a straightforward user task involving opening an access panel, removing old filters, and inserting new ones according to orientation arrows. It’s crucial to use genuine manufacturer-approved filters to ensure correct fit, filtration efficiency, and pressure drop characteristics.

4.2.2 Duct Cleaning

While filters prevent most particulate matter from entering the ductwork, some fine dust and microbial growth can accumulate over time.

• Frequency: Duct cleaning is less frequent than filter replacement, typically recommended every 2-5 years, depending on filter maintenance rigor and IAQ. In certain commercial or medical settings, more frequent cleaning might be necessary [68].
• Rationale: Accumulation of dust, debris, and potential microbial growth in ducts can restrict airflow, decrease system efficiency, and reintroduce pollutants into the indoor environment. Cleaning helps maintain optimal airflow and hygienic conditions.
• Procedure: Professional duct cleaning services use specialised equipment, such as rotary brushes, high-powered vacuums with HEPA filtration, and sometimes air jets, to dislodge and remove contaminants from the interior surfaces of the ducts. Disinfectants may also be used in certain situations, but care must be taken to ensure they are safe and compatible with duct materials [69]. Regular inspection through access panels can help determine the need for cleaning.

4.2.3 System Inspection and Servicing

Beyond filters and ducts, the entire MVHR unit requires periodic inspection to ensure all components are functioning correctly.

• Annual Checks: It is recommended to have a professional annual service. This typically includes:

  • Heat Exchanger Inspection: Checking for cleanliness and blockages. Plate heat exchangers may require cleaning to remove dust or grease build-up that can impede heat transfer [70]. Rotary wheels may need inspection for damage or build-up.
  • Fan Inspection: Checking fan blades for cleanliness, ensuring smooth operation, inspecting bearings for wear or noise, and verifying electrical connections. Clean fans are more efficient and quieter.
  • Condensate Drain: Inspecting the condensate tray and drain for blockages, ensuring proper slope, and cleaning the trap. A blocked drain can lead to water overflow and potential damage [71].
  • Seals and Gaskets: Checking all seals within the unit and around access panels for integrity to ensure airtightness and prevent air leakage.
  • Controls and Sensors: Verifying the calibration and functionality of temperature, CO2, VOC, and humidity sensors, as well as the overall control logic of the unit [72].
  • Airflow Measurement: Periodically re-measuring airflow rates to ensure the system is delivering the designed ventilation rates and to identify any underlying issues affecting performance.

• Impact of Poor Maintenance: Failure to maintain an MVHR system can lead to a cascade of negative consequences:

  • Reduced heat recovery efficiency, leading to higher heating/cooling bills.
  • Decreased airflow rates, resulting in inadequate ventilation and poor indoor air quality (elevated CO2, VOCs, humidity, odours).
  • Increased fan energy consumption due to higher pressure drops.
  • Increased noise levels as fans struggle against resistance.
  • Potential for mould growth in ducts or the unit itself.
  • Premature failure of components due to strain or contamination.
  • Voided manufacturer warranties.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

5. Installation Challenges

The successful implementation of an MVHR system is highly dependent on meticulous installation. While the theoretical benefits are clear, practical challenges during installation can significantly undermine performance if not addressed proactively. These challenges often arise from a lack of coordination, insufficient planning, or inadequate technical expertise.

5.1 Common Pitfalls

Several recurring issues can compromise the effectiveness and longevity of an MVHR system during and after installation:

• Inadequate Sizing and Design Errors: As discussed, incorrect sizing leads to either insufficient ventilation or excessive energy consumption and noise. Design errors can extend to poor duct routing, inappropriate component selection, or an underestimation of system pressure drops, which can lead to the fan operating outside its optimal range [73].
• Poor Ducting Design and Installation: This is a prevalent issue. Common mistakes include:
  • Excessive Duct Lengths and Sharp Bends: Increasing airflow resistance, reducing effective airflow, and increasing fan energy consumption [74].
  • Undersized Ducts: Leading to high air velocities, resulting in turbulent airflow and significant noise generation at terminal devices or in the ducts themselves [75].
  • Leaky Ductwork: Poorly sealed joints allow conditioned air to escape into unconditioned spaces or unconditioned air to infiltrate, reducing efficiency and creating discomfort. This can also lead to condensation issues in roof spaces or wall cavities.
  • Inadequate Duct Insulation: Ducts passing through unconditioned zones (e.g., lofts, external walls) without proper insulation can lead to significant heat loss/gain and, critically, condensation within the ducts, promoting mould growth and water damage [76].
  • Crushed or Kinked Flexible Ducts: These significantly restrict airflow, adding considerable resistance and reducing performance.
• Improper Commissioning and Balancing: Even a perfectly designed and installed system will underperform if not correctly commissioned. Failure to measure and balance airflow rates to match design specifications means rooms may be under- or over-ventilated, leading to IAQ issues or unnecessary energy use. Incorrectly set fan speeds or control parameters can also compromise performance [77].
• Poor Access for Maintenance: Locating the MVHR unit or filters in inaccessible areas (e.g., cramped lofts, behind fixed panels) can make routine filter changes and annual servicing difficult or impossible, leading to neglected maintenance and compromised performance [78].
• Condensate Drainage Issues: Incorrect slope for the condensate drain, lack of a proper trap, or inadequate frost protection for the drain in cold climates can lead to water overflow, system shutdown (due to safety switches), mould, or water damage [79]. Freezing of the condensate drain can cause severe issues.
• Acoustic Bridging: Noise generated by the MVHR unit or airflow within the ducts can be transmitted through the building structure if the unit is rigidly connected to joists or walls, or if ducts are not acoustically isolated. This results in noise complaints from occupants [80].
• Vapour Barrier Penetrations: Poorly sealed penetrations through the building’s airtightness layer (e.g., where ducts pass through external walls or ceilings) can compromise the building’s overall airtightness, leading to unwanted air leakage and condensation issues within the building fabric [81].
• Lack of User Education: Occupants may not understand how to operate or maintain the MVHR system, leading to incorrect settings, ignored filter replacement alerts, or even switching off the system entirely, negating its benefits.

5.2 Strategies for Successful Installation

Mitigating installation challenges requires a systematic and collaborative approach from the project’s inception through to handover.

• Comprehensive Planning and Early Collaboration:
  • Integrated Design Team: Engage architects, HVAC engineers, energy consultants, and installers early in the design process. Using Building Information Modelling (BIM) can help visualise duct routes, identify clashes, and optimise space utilisation [82].
  • Detailed Design Documentation: Provide comprehensive drawings, specifications, airflow schematics, and commissioning plans to all stakeholders. This includes specifying duct materials, insulation, terminal devices, and access requirements.
  • Room for MVHR: Ensure adequate space is allocated for the MVHR unit, duct runs, attenuators, and condensate drainage, considering future maintenance access.
• Professional Installation and Quality Control:
  • Experienced Technicians: Engage installers with specific expertise and certification in MVHR systems. Improper installation by general contractors unfamiliar with MVHR nuances is a common cause of failure.
  • Adherence to Manufacturer Guidelines: Strictly follow the manufacturer’s installation manuals and recommendations regarding clearances, mounting, electrical connections, and duct connections.
  • Airtightness Testing of Ductwork: Implement intermediate airtightness tests for the ductwork during installation, before it is concealed, to identify and rectify leaks early. This is crucial for achieving high system efficiency [83].
  • Proper Duct Support and Routing: Ensure all ducts are adequately supported to prevent sagging, especially flexible ducts, and are routed to minimise bends and length. Avoid crushing or kinking ducts.
  • Condensate Management: Install condensate drains with the correct fall, P-traps, and ensure they are protected from freezing in cold environments.
• Rigorous Post-Installation Testing and Commissioning:
  • Airflow Measurement and Balancing: A qualified commissioning engineer must measure the airflow at each supply and extract terminal and adjust dampers to meet design specifications. This ensures balanced ventilation across the entire dwelling [84].
  • Acoustic Testing: Measure sound levels at key points (e.g., bedrooms, living areas) to confirm the system operates within acceptable noise limits. Adjustments to fan speeds, attenuator installation, or vibration isolation may be necessary.
  • Pressure Testing: Verify total static pressure loss across the system against design values to ensure the fans are operating efficiently.
  • Controls Verification: Test all control functions, including fan speed settings, boost modes, summer bypass, and sensor integration, to ensure they operate as intended.
  • Building Airtightness Test (Blower Door Test): After the MVHR installation, the overall building airtightness should be re-tested to confirm it meets targets and identify any unexpected leakage paths introduced during installation [85].
• User Handover and Education:
  • Comprehensive User Manual: Provide a detailed, easy-to-understand manual covering operation, filter replacement schedules, basic troubleshooting, and contact information for support.
  • Demonstration: Conduct a practical demonstration for the occupants, explaining how the system works, its benefits, and routine maintenance tasks. Emphasise the importance of keeping the system running and changing filters.
  • Maintenance Schedule: Provide a clear maintenance schedule for filter changes and professional servicing.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

6. Noise Reduction Strategies

While MVHR systems offer significant benefits, the perception of noise is a critical factor influencing occupant comfort and acceptance. Excessive noise can negate the advantages of improved air quality and energy efficiency, leading occupants to switch off the system. Therefore, meticulous attention to noise control is paramount during the design and installation phases.

6.1 Importance of Noise Control

Noise from ventilation systems can be intrusive and detrimental to well-being. The impact of noise includes:

• Sleep Disturbance: Even low levels of continuous noise can disrupt sleep cycles, leading to fatigue, reduced cognitive function, and long-term health issues [86]. Bedrooms are particularly sensitive areas.
• Reduced Comfort and Productivity: Constant background noise can hinder concentration, make conversation difficult, and generally detract from the comfort of a living or working environment. In offices or schools, this can affect productivity and learning outcomes [87].
• Health Implications: Prolonged exposure to noise can contribute to stress, increased blood pressure, and other physiological effects [88].
• Occupant Dissatisfaction: If the noise is unacceptable, occupants may choose to switch off the MVHR system, thereby losing all its benefits related to IAQ and energy recovery. This is a common issue that undermines the investment in such systems.

Relevant standards and guidelines, such as those from the World Health Organization (WHO) for community noise, national building codes (e.g., Part E in the UK for sound insulation), and specific MVHR manufacturer recommendations, provide target noise levels. Typically, background noise levels from ventilation systems in bedrooms should be below 25-30 dB(A) and in living areas below 30-35 dB(A) [89, 90].

6.2 Noise Mitigation Techniques

Effective noise reduction involves a multi-pronged approach, addressing noise at its source, during transmission, and at the receiving end.

6.2.1 Source Noise Reduction (MVHR Unit and Fans)

• Unit Selection: Opt for MVHR units with inherently low-noise fans and well-insulated casings. Manufacturers often provide sound power levels (Lw) for their units at various fan speeds. Prioritise units designed for quiet operation, especially those certified by bodies like the Passive House Institute [91].
• Fan Type and Operation: Forward-curved centrifugal fans are typically quieter at lower pressure drops, while backward-curved fans can be more efficient at higher pressure drops. However, the key is to ensure the fan operates within its optimal efficiency range (on its fan curve) at the required airflow, as operating outside this range can generate excessive noise and consume more energy [92]. Variable speed fans allow for reduced noise during periods of lower ventilation demand.
• Unit Location: Locate the MVHR unit in a utility room, plant room, garage, or loft space that is acoustically isolated from habitable rooms. Avoid placing it directly above or adjacent to bedrooms. Ensure the mounting surface is robust and does not resonate [93].

6.2.2 Noise Transmission Reduction (Ductwork and Vibration)

• Vibration Isolation:
  • Anti-vibration Mounts: Mount the MVHR unit on purpose-designed anti-vibration rubber feet or spring isolators to prevent the transmission of structural vibration from the unit’s fans and motor into the building structure (walls, floors, joists) [94].
  • Flexible Connections: Use short lengths of flexible ducting or canvas connectors immediately upstream and downstream of the MVHR unit, before connecting to rigid ductwork. These absorb fan vibrations before they can propagate through the rigid ducts [95].
• Acoustic Attenuators (Silencers): These are specialised components designed to absorb sound waves propagating through the ductwork.
  • Placement: Attenuators should be installed as close as possible to the MVHR unit on both the supply and extract main ducts. Additional attenuators may be necessary in branch ducts leading to very sensitive rooms (e.g., bedrooms) if the main attenuators are insufficient [96].
  • Types: Attenuators typically consist of an outer casing with an inner perforated metal lining, between which sound-absorbing material (e.g., mineral wool, polyester fibre) is packed. The length and diameter of the attenuator, as well as the type and thickness of the absorbing material, determine its effectiveness across different frequency ranges [97]. Ensure attenuators are correctly sized for airflow and pressure drop.
• Duct Design for Airflow Noise:
  • Low Air Velocities: Keep air velocities within ducts as low as practically possible. Recommended maximum velocities are typically 3-5 m/s in main ducts and 2-3 m/s in branch ducts to minimise turbulent airflow noise at bends, junctions, and terminal devices [98]. Oversized ducts reduce velocity.
  • Smooth Transitions and Large Radius Bends: Avoid sharp bends, abrupt changes in duct size, and uninsulated take-offs, as these create turbulence and noise. Use smooth, gradual transitions and large-radius elbows [99].
  • Cross-Talk Attenuators: In systems where multiple rooms are served by a single branch duct, sound can travel between rooms via the ductwork (‘cross-talk’). Installing cross-talk attenuators or ensuring sufficient duct separation can prevent this [100].
• Acoustic Duct Lining/Insulation: While external thermal insulation is crucial, internal acoustic lining can further reduce noise within the ducts. Alternatively, using insulated rigid ducting with internal acoustic properties can provide both thermal and acoustic benefits. Always ensure internal linings are non-fibrous and hygienic [101].
• Terminal Device Selection and Placement: Choose low-noise grilles and diffusers that are designed to provide even air distribution without generating excessive turbulence. Place them away from beds or seating areas where noise would be most noticeable [102].

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

7. Benefits Across Climate Zones

The advantages of MVHR systems are not confined to a single climate but extend globally, adapting their specific benefits to the prevailing environmental conditions. Their fundamental ability to control ventilation and recover energy makes them a versatile and essential technology for modern, energy-efficient buildings.

7.1 Cold Climates

In regions experiencing cold winters, the primary benefit of MVHR systems is the substantial reduction in heating demand, directly contributing to energy savings and reduced carbon emissions.

• Significant Heat Recovery: In cold climates, the temperature difference between the warm indoor exhaust air and the cold outdoor supply air is maximal. MVHR units efficiently recover up to 90% or more of the sensible heat from the outgoing air, pre-warming the incoming fresh air to a temperature much closer to the indoor setpoint [103]. This drastically reduces the energy required by the primary heating system to bring the fresh air up to room temperature.
• Prevention of Cold Drafts: Unlike traditional natural ventilation or opening windows, MVHR delivers pre-warmed air, eliminating cold drafts and ensuring a consistently comfortable indoor environment, even with continuous ventilation [104].
• Condensation and Mould Prevention: Modern, airtight buildings in cold climates are susceptible to condensation and mould growth if moisture-laden indoor air is not adequately removed. MVHR systems continuously extract humid air from ‘wet rooms’ (kitchens, bathrooms), preventing moisture build-up within the building fabric and on cold surfaces, thereby protecting both building integrity and occupant health [105].
• Frost Protection: MVHR units designed for cold climates incorporate sophisticated frost protection mechanisms. These may include pre-heaters for the incoming air, automatic bypass of the heat exchanger (less common in extreme cold), or temporary imbalances in airflow (reducing supply fan speed) to allow warmer exhaust air to defrost the heat exchanger core [106]. Rotary heat exchangers inherently have good frost resistance due to the continuous rotation and latent heat transfer.

7.2 Hot Climates

In hot and often humid climates, the role of MVHR, particularly Energy Recovery Ventilators (ERVs) capable of latent heat transfer, shifts towards reducing the cooling load and managing humidity.

• Reduced Cooling Demand (Sensible Heat Recovery): During hot periods, the MVHR unit transfers sensible heat from the warmer incoming fresh air to the cooler outgoing exhaust air. This pre-cools the supply air, reducing the workload on the primary air conditioning system and lowering cooling energy consumption [107]. While the temperature difference may be less dramatic than in cold climates, the cumulative effect over prolonged hot periods is significant.
• Humidity Control (Latent Heat Recovery via ERV): This is a critical benefit in humid climates. ERVs transfer moisture from the humid incoming outdoor air to the drier outgoing indoor air. This dehumidifies the supply air, reducing the latent load on air conditioning systems. A lower latent load means AC units can operate more efficiently, providing greater comfort and preventing issues like clamminess, mould growth, and dust mite proliferation [108, 109]. Without an ERV, bringing in hot, humid air through ventilation would significantly increase the energy required for both cooling and dehumidification.
• Night Purge/Free Cooling: In many hot climates, ambient temperatures drop significantly at night. MVHR systems with a summer bypass mode can be set to bypass the heat exchanger and introduce cooler night air directly into the building, flushing out accumulated heat and passively cooling the structure, reducing the need for mechanical cooling the following day [110].

7.3 Temperate Climates

Temperate climates, characterised by distinct seasonal variations (mild winters, warm summers, and moderate shoulder seasons), require MVHR systems to adapt dynamically to maintain comfort and efficiency throughout the year.

• Year-Round Energy Efficiency: MVHR systems provide continuous ventilation while recovering heat in winter and rejecting heat in summer (or recovering ‘coolth’). This consistent energy recovery optimises energy consumption across all seasons [111].
• Adaptive Ventilation: Intelligent MVHR systems with integrated controls (e.g., CO2, humidity sensors) can adjust ventilation rates based on actual demand, optimising airflow and energy use in response to fluctuating indoor conditions. During shoulder seasons, where outdoor temperatures can vary widely throughout the day, bypass modes or modulated heat recovery allow the system to respond appropriately, ensuring fresh air without unnecessary heating or cooling [112].
• Balanced Air Quality and Comfort: MVHR maintains a constant supply of filtered fresh air, mitigating pollutants and allergens throughout the year, irrespective of outdoor conditions. It avoids the need to open windows, which can introduce noise, pollutants, and security risks, particularly in urban areas [113].

7.4 Indoor Air Quality (Universal Benefit)

Beyond climate-specific energy benefits, the overarching and universal benefit of MVHR systems is the profound improvement in indoor air quality, which directly impacts occupant health and well-being.

• Continuous Removal of Pollutants: MVHR systematically removes stale, polluted indoor air and replaces it with fresh, filtered outdoor air. This process effectively removes a wide range of indoor pollutants, including:
  • Carbon Dioxide (CO2): Elevated CO2 levels (a byproduct of human respiration) can lead to drowsiness, headaches, and reduced cognitive function [114]. MVHR ensures consistent CO2 dilution.
  • Volatile Organic Compounds (VOCs): Emitted from building materials, furnishings, cleaning products, paints, and personal care products, VOCs can cause respiratory irritation, headaches, and have long-term health implications. MVHR continuously purges these [115].
  • Allergens: High-efficiency filters (e.g., ePM1, ePM2.5) capture airborne allergens such as pollen, dust mites, pet dander, and mould spores, providing relief for allergy and asthma sufferers [116].
  • Pathogens: While not primary air purifiers, MVHR systems with appropriate filtration can reduce the concentration of airborne bacteria and viruses, especially when operating at optimal airflow rates [117].
  • Odours: MVHR removes cooking odours, bathroom odours, and general stale smells, maintaining a fresh-smelling indoor environment.
  • Radon: In areas prone to radon gas, MVHR can help dilute and exhaust this naturally occurring radioactive gas [118].
• Humidity Control: By continuously removing moist air from sources (kitchens, bathrooms) and, in the case of ERVs, managing moisture transfer, MVHR prevents excessive humidity, which is a primary driver for mould growth, dust mite proliferation, and structural degradation [119]. Conversely, in very dry climates, ERVs can prevent excessive drying of indoor air, maintaining comfort.
• Health and Well-being: By providing a constant supply of clean air, MVHR contributes to:
  • Reduced Respiratory Issues: Lower exposure to allergens and pollutants can alleviate symptoms for individuals with asthma, allergies, and other respiratory conditions.
  • Improved Cognitive Function: Studies have shown that improved IAQ, particularly lower CO2 and VOC levels, can significantly enhance cognitive performance, concentration, and decision-making [120].
  • Better Sleep Quality: Reduced noise from outdoor sources (compared to open windows) and stable air quality contribute to more restful sleep.
• Protection of Building Fabric: By controlling humidity and preventing condensation, MVHR helps preserve the integrity of the building structure, preventing dampness, mould, and rot, which are costly to remediate [121].

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

8. Advanced Concepts and Future Trends

The field of MVHR technology is continually evolving, driven by advancements in materials science, control systems, and an increasing emphasis on holistic building performance. Several advanced concepts and future trends are poised to further enhance the efficacy and integration of these systems.

8.1 Demand-Controlled Ventilation (DCV)

While introduced earlier, DCV warrants further discussion as a core advanced concept. Traditional MVHR systems often run at a fixed or manually adjusted speed. DCV, however, dynamically adjusts ventilation rates based on real-time indoor air quality parameters. This is achieved through an array of sensors:

• CO2 Sensors: Measure carbon dioxide levels, which correlate with occupancy. As CO2 rises, the system increases airflow to dilute it. When levels drop, airflow is reduced, saving energy [122].
• VOC Sensors: Detect Volatile Organic Compounds, indicative of emissions from cooking, cleaning, furnishings, or human presence. These sensors can trigger increased ventilation to clear odours and harmful gases.
• Humidity Sensors: Monitor relative humidity, allowing the system (especially ERVs) to adjust airflow to prevent excess moisture build-up or maintain optimal humidity levels for comfort and health [123].

DCV optimises energy use by delivering ventilation precisely when and where it is needed, avoiding over-ventilation of unoccupied spaces while ensuring adequate air changes during peak occupancy or pollution events. This leads to substantial operational cost savings and enhanced IAQ compared to constant volume systems.

8.2 Integration with Renewable Energy Sources and Smart Grids

Future MVHR systems will increasingly be integrated with renewable energy generation and smart grid technologies:

• Photovoltaic (PV) Integration: MVHR units, particularly those with low specific fan power (SFP), can be primarily powered by on-site PV electricity, further reducing their carbon footprint and operating costs. Battery storage can ensure continuous operation even when solar generation is low [124].
• Ground Source Heat Pumps (GSHP): Integration with GSHPs allows for highly efficient pre-heating or pre-cooling of incoming air. The stable ground temperature acts as a heat sink or source, augmenting the MVHR’s thermal recovery [125].
• Smart Grid Readiness: Future MVHR systems may be designed to respond to grid signals, adjusting their operational intensity during peak electricity demand periods to reduce strain on the grid, potentially benefiting from dynamic electricity pricing. For example, during high renewable energy availability, the system might slightly over-ventilate to ‘store’ fresh air for later [126].

8.3 MVHR in Non-Residential Buildings

While often highlighted for residential applications, MVHR systems are equally crucial, if not more so, in non-residential buildings like schools, offices, healthcare facilities, and commercial spaces. In these environments, higher occupancy densities and diverse internal heat gains/losses necessitate robust, efficient ventilation [127].

• Schools: Essential for maintaining good IAQ to improve student concentration, reduce airborne pathogen transmission, and mitigate CO2 build-up in classrooms [128].
• Offices: Contributes to employee well-being, productivity, and reduced sick days by ensuring fresh air and removing VOCs from office equipment and materials.
• Healthcare Facilities: Critical for controlled environments, preventing cross-contamination, and managing airborne pathogens, often incorporating higher-grade filtration [129].

These applications often demand larger, more complex MVHR units, potentially with zonal control, higher airflow capacities, and integration with extensive Building Management Systems (BMS).

8.4 Role in Achieving Net-Zero Energy Buildings (NZEB)

MVHR is an indispensable technology for achieving Net-Zero Energy Building (NZEB) targets. By significantly reducing the heating and cooling loads associated with ventilation, MVHR minimises the overall energy demand of a building. This reduction in demand makes it more feasible to offset the remaining energy needs through on-site renewable energy generation, pushing buildings towards net-zero or even energy-plus status [130]. Without efficient heat recovery, the energy penalty of providing continuous fresh air in an airtight NZEB would be substantial.

8.5 Challenges and Future Research Directions

Despite the clear benefits, MVHR systems face ongoing challenges and areas for further research:

• Cost: Initial capital costs can be a barrier, particularly in price-sensitive markets. Future trends include modular designs, simplified installation methods, and economies of scale to reduce costs.
• Occupant Behaviour: Ensuring occupants understand and correctly interact with MVHR systems remains vital. Intuitive controls and better educational resources are key.
• Perceived Complexity: The perceived complexity of MVHR (design, installation, maintenance) can deter adoption. Simplifying design tools, standardised installation practices, and more user-friendly interfaces are needed.
• Performance Gap: Bridging the gap between theoretical design performance and actual in-use performance (the ‘performance gap’) is an ongoing focus, requiring improved commissioning, quality assurance, and post-occupancy evaluation [131].
• Airborne Pathogen Mitigation: Further research into optimal filtration, UVGI (ultraviolet germicidal irradiation) integration, and airflow strategies for reducing airborne pathogen transmission in various building types, especially in light of global health concerns.
• Thermal Comfort in Challenging Climates: Developing MVHR/ERV systems that can maintain optimal thermal comfort in increasingly extreme climate conditions (e.g., prolonged heatwaves, very high humidity) without excessive energy input.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

9. Conclusion

Mechanical Ventilation with Heat Recovery (MVHR) systems are no longer merely an optional enhancement but an essential and integral component of modern, sustainable building design. As global efforts intensify towards energy efficiency, decarbonisation, and enhanced indoor environmental quality, the role of MVHR systems becomes increasingly critical. They adeptly address the fundamental challenge of reconciling a continuous supply of fresh, healthy indoor air with the imperative to minimise energy consumption for heating and cooling.

From the foundational principles of heat exchange mechanisms—be it the efficient sensible recovery of plate exchangers or the comprehensive sensible and latent recovery of rotary enthalpy wheels—to the intricate details of optimal sizing, intelligent ducting design, and seamless integration with smart building ecosystems, every aspect of an MVHR system contributes to its overall efficacy. The continuous evolution of filter technologies ensures robust protection against a spectrum of indoor and outdoor pollutants, while rigorous maintenance protocols are underscored as indispensable for sustained performance and hygienic operation.

Furthermore, this report has elucidated the sophisticated strategies required to overcome installation challenges and mitigate noise, demonstrating that successful MVHR deployment demands meticulous planning, expert execution, and thorough commissioning. The benefits of MVHR systems are unequivocally global, adapting to the nuanced demands of cold climates by preserving warmth, assuaging hot and humid environments by reducing cooling and moisture loads, and maintaining a year-round balance in temperate zones. Universally, they stand as a bulwark for superior indoor air quality, protecting occupant health and well-being against a myriad of airborne contaminants.

Looking ahead, advanced concepts such as demand-controlled ventilation, integration with renewable energy sources, and their pivotal role in achieving Net-Zero Energy Buildings underscore the ongoing innovation within this sector. While challenges related to initial cost, perceived complexity, and optimal performance remain, continued research and development are poised to further refine these systems, making them even more accessible, efficient, and intelligent.

In essence, MVHR systems are a sophisticated yet pragmatic solution to complex environmental control challenges within the built environment. Their careful selection, precise design, professional installation, and diligent maintenance are not merely best practices but fundamental requirements to unlock their full potential in creating healthy, comfortable, energy-efficient, and truly sustainable indoor environments for the future.

Many thanks to our sponsor Focus 360 Energy who helped us prepare this research report.

References

[1] ASHRAE. (2019). ASHRAE Handbook – HVAC Applications. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
[2] Passive House Institute. (2020). Passive House Planning Package (PHPP) Manual. Passive House Institute.
[3] British Standards Institution. (2019). BS EN 13141-7:2019 Ventilation for buildings – Performance testing of components/products for residential ventilation – Part 7: Performance testing of a mechanical supply and exhaust ventilation unit (including heat recovery) for mechanical ventilation systems intended for single family dwellings. BSI.
[4] Hantschmann, P. (2012). The Passivhaus Handbook: A Practical Guide to Designing and Building an Energy Efficient Home. Green Books.
[5] BRE. (2016). Building Regulations Part F: Ventilation (Approved Document F). UK Government.
[6] Nilan UK. (n.d.). Benefits of Mechanical Ventilation Heat Recovery Systems. Retrieved from https://nilanuk.com/benefits-of-mechanical-ventilation-heat-recovery-systems/
[7] Holtop. (n.d.). Benefits of MVHR Mechanical Ventilation with Heat Recovery. Retrieved from https://www.holtop.com/news/benefits-of-mvhr-mechanical-ventilation-with-heat-recovery
[8] Titon. (n.d.). Mechanical Ventilation System. Retrieved from https://www.titon.com/blog-ventilation/mechanical-ventilation-system
[9] Zehnder Group. (n.d.). ComfoAir Q: The Future of Ventilation. Retrieved from https://www.zehnder.co.uk/products-and-systems/ventilation-systems/comfoair-q
[10] Wikipedia. (n.d.). Heat recovery ventilation. Retrieved from https://en.wikipedia.org/wiki/Heat_recovery_ventilation
[11] Holy City HVAC. (n.d.). Benefits of HRV and ERV. Retrieved from https://www.holycityhvac.com/blog/benefits-of-hrv-and-erv
[12] Superior Air Management. (n.d.). 6 Benefits of Heat Recovery Ventilation System. Retrieved from https://superiorairmanagement.com/news/6-benefits-of-heat-recovery-ventilation-system
[13] Lacltd. (n.d.). Pros & Cons of Mechanical Ventilation with Heat Recovery. Retrieved from https://lacltd.uk.com/pros-cons-mechanical-ventilation-with-heat-recovery
[14] RDZ. (n.d.). 5 Main Benefits of MVHR Systems. Retrieved from https://www.rdz.it/en/novita/5-main-benefits-mvhr-systems
[15] Ventis. (n.d.). Pros and Cons of Energy Recovery Ventilation System. Retrieved from https://www.ventis.com.au/pros-and-cons-of-energy-recovery-ventilation-system/
[16] Business ABC. (n.d.). Why Choose Mechanical Ventilation with Heat Recovery (MVHR). Retrieved from https://businessabc.net/why-choose-mechanical-ventilation-with-heat-recovery-mvhr
[17] Zehnder Group. (n.d.). MVHR Systems. Retrieved from https://www.zehnder.co.uk/products-and-systems/ventilation-systems
[18] Nilan UK. (n.d.). Benefits of MVHR Technology. Retrieved from https://nilanuk.com/benefits-of-mvhr-technology/
[19] Brink Climate Systems. (n.d.). MVHR Units. Retrieved from https://www.brinkclimatesystems.co.uk/mvhr-units
[20] Paul Wärmerückgewinnung. (n.d.). MVHR Units. Retrieved from https://www.paul-lueftung.de/en/products/mvhr-units/
[21] Vent-Axia. (n.d.). Heat Recovery Ventilation. Retrieved from https://www.vent-axia.com/range/heat-recovery-ventilation
[22] Mitsubishi Electric. (n.d.). Lossnay Heat Recovery Ventilation. Retrieved from https://les.mitsubishielectric.co.uk/products/ventilation/lossnay
[23] Holtop. (n.d.). About Holtop. Retrieved from https://www.holtop.com/about-us
[24] Swegon. (n.d.). Ventilation Systems. Retrieved from https://www.swegon.com/uk/products/air-handling-units/
[25] Titon. (n.d.). MVHR. Retrieved from https://www.titon.com/mvhr
[26] CIBSE. (2016). Guide B2: Ventilation and Air Conditioning. Chartered Institution of Building Services Engineers.
[27] ASHRAE. (2016). ASHRAE Handbook – HVAC Systems and Equipment. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
[28] European Commission. (2016). Commission Regulation (EU) No 1253/2014 implementing Directive 2009/125/EC of the European Parliament and of the Council with regard to ecodesign requirements for ventilation units. Official Journal of the European Union.
[29] Passive House Institute. (n.d.). Component Certification: Ventilation Units. Retrieved from https://passivehouse.com/02_ph_components/02_component_certification/03_ventilation/03_ventilation.htm
[30] European Committee for Standardisation. (2017). ISO 16890: Air filters for general ventilation. CEN.
[31] Green Building Store. (n.d.). Decentralised MVHR. Retrieved from https://www.greenbuildingstore.co.uk/mvhr-heat-recovery-ventilation-systems/decentralised-mvhr/
[32] LUNOS. (n.d.). Decentralised Ventilation. Retrieved from https://www.lunos.de/en/products/decentralised-ventilation/
[33] REHVA. (2013). Guidebook No. 17: Indoor Climate and Productivity in Offices. Federation of European HVAC Associations.
[34] Building Services Research and Information Association (BSRIA). (2019). BSRIA Guide BG 4/2019 – Commissioning Air Systems. BSRIA.
[35] ASHRAE. (2019). ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
[36] European Committee for Standardisation. (2017). EN 16798-1: Energy performance of buildings – Ventilation for buildings – Part 1: Input parameters for indoor environmental quality and energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics. CEN.
[37] Department for Communities and Local Government. (2010). Approved Document F – Ventilation (2010 edition, with 2010 and 2013 amendments). UK Government.
[38] BRE. (2019). Airtightness in Buildings. BRE Trust.
[39] Carrier Corporation. (2000). System Design Manual, Part 2: Air Distribution. Carrier Corporation.
[40] CIBSE. (2018). TM23: Testing and Commissioning of Building Energy Management Systems. Chartered Institution of Building Services Engineers.
[41] SMACNA. (2006). HVAC Duct Construction Standards — Metal and Flexible. Sheet Metal and Air Conditioning Contractors’ National Association.
[42] Eurovent Certita Certification. (n.d.). MVHR Performance Certification. Retrieved from https://www.eurovent-certification.com/en/performance/mvhr
[43] ComfoFresh. (n.d.). Radial Ducting System for MVHR. Retrieved from https://www.comfofresh.co.uk/ducted-systems
[44] HVCA. (2012). DW/144: Specification for Sheet Metal Ductwork. Heating and Ventilating Contractors’ Association.
[45] Building Engineering Services Association (BESA). (2016). HVAC Guide. BESA.
[46] Eurovent Certita Certification. (2018). Certification Programme for Airtightness of Ventilation Ducts. Eurovent.
[47] DEAP. (2017). Dwelling Energy Assessment Procedure Manual. Sustainable Energy Authority of Ireland.
[48] BSRIA. (2016). BSRIA Guide BG 1/2016 – Commissioning of HVAC Systems. BSRIA.
[49] Swegon. (n.d.). Air Diffusers and Grilles. Retrieved from https://www.swegon.com/uk/products/air-diffusers-and-grilles/
[50] Zehnder. (n.d.). Installation Guide for ComfoAir Q. Zehnder Group.
[51] Renewable Energy Hub. (n.d.). Ground Source Heat Exchanger for Ventilation. Retrieved from https://www.renewableenergyhub.co.uk/main/ground-source-heat-pump-information/ground-source-heat-exchangers-for-ventilation/
[52] Swegon. (n.d.). Integrated Heating and Cooling for AHUs. Retrieved from https://www.swegon.com/uk/products/air-handling-units/gold-rx/
[53] CIBSE. (2015). TM49: Design for Future Climate: Weather Data. Chartered Institution of Building Services Engineers.
[54] Nilan. (n.d.). Compact P: All-in-one Ventilation and Heat Pump Solution. Retrieved from https://nilan.com/products/all-in-one/compact-p
[55] Building Science Corporation. (2011). Humidity Control in Buildings. Building Science Press.
[56] AIVC. (2018). Technical Note 70: Demand Controlled Ventilation in Residential Buildings. Air Infiltration and Ventilation Centre.
[57] BRE. (2019). Approved Document B – Fire Safety (Volume 1: Dwellings). UK Government.
[58] Camfil. (2018). ISO 16890 and the impact on air filter selection. Camfil AB.
[59] EUROVENT. (2017). Understanding ISO 16890: The new international standard for air filter classification. EUROVENT Certification.
[60] European Lung Foundation. (n.d.). Air Pollution. Retrieved from https://www.lung.org/clean-air/at-home/indoor-air-pollutants
[61] World Health Organization. (2021). WHO Global Air Quality Guidelines: Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. WHO.
[62] ASHRAE. (2017). ASHRAE Standard 52.2: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
[63] Camfil. (n.d.). Activated Carbon Filters. Retrieved from https://www.camfil.com/en-gb/products/molecular-filters/
[64] U.S. Environmental Protection Agency. (n.d.). Air Cleaners, HVAC Filters, and Coronavirus (COVID-19). Retrieved from https://www.epa.gov/coronavirus/air-cleaners-hvac-filters-and-coronavirus-covid-19
[65] Air Purifier Reviews. (n.d.). Electrostatic Air Purifiers. Retrieved from https://www.air-purifier-reviews.com/electrostatic.html
[66] MVHR Store. (n.d.). MVHR Filter Replacement Guide. Retrieved from https://www.mvhrstore.co.uk/mvhr-maintenance-filter-replacement
[67] CIBSE. (2014). TM40: Health and Wellbeing in Buildings. Chartered Institution of Building Services Engineers.
[68] NADCA. (2020). NADCA ACR, The NADCA Standard for Assessment, Cleaning & Restoration of HVAC Systems. National Air Duct Cleaners Association.
[69] ASHRAE. (2015). ASHRAE Standard 180: Standard Practice for Inspection and Maintenance of Commercial Building HVAC Systems. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
[70] Zehnder. (n.d.). Maintenance Guide for ComfoAir Q. Zehnder Group.
[71] Brink Climate Systems. (n.d.). MVHR Maintenance. Retrieved from https://www.brinkclimatesystems.co.uk/mvhr-maintenance
[72] REHVA. (2016). Guidebook No. 20: Demand Controlled Ventilation. Federation of European HVAC Associations.
[73] BRE. (2018). Addressing the performance gap in new housing. BRE Trust.
[74] ASHRAE. (2016). ASHRAE Handbook – Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
[75] CIBSE. (2019). Guide A: Environmental design. Chartered Institution of Building Services Engineers.
[76] Historic England. (2018). MVHR in Existing Buildings: Technical Information. Historic England.
[77] BSRIA. (2019). BSRIA Guide BG 4/2019 – Commissioning Air Systems. BSRIA.
[78] Passivhaus Trust. (2018). MVHR: Guidance on good practice. Passivhaus Trust UK.
[79] Building Science Corporation. (2017). Understanding HVAC Condensate Drain Lines. Building Science Press.
[80] CIBSE. (2017). Guide B4: Noise and Vibration Control for HVAC. Chartered Institution of Building Services Engineers.
[81] BRE. (2016). Airtightness: The Key to Energy Efficient Building. BRE Trust.
[82] RIBA. (2017). RIBA Plan of Work 2013: Design and Construction. Royal Institute of British Architects.
[83] ATTMA. (2019). Technical Standard 1: Measuring Air Permeability in Buildings. Airtightness Testing & Measurement Association.
[84] BSRIA. (2016). BSRIA Guide BG 1/2016 – Commissioning of HVAC Systems. BSRIA.
[85] Passive House Institute. (n.d.). Airtightness: Fundamentals. Retrieved from https://passivehouse.com/02_ph_principles/01_passivehouse_principles/05_airtightness/05_airtightness.htm
[86] WHO. (2018). Environmental Noise Guidelines for the European Region. World Health Organization.
[87] Seppänen, O., Witterseh, H., & Wyon, D. P. (2006). Effect of temperature and humidity on the performance of schoolwork and other activities. ASHRAE.
[88] Stansfeld, S. A., & Matheson, M. P. (2011). Noise pollution: non-auditory effects on health. British Medical Bulletin, 100(1), 17-27.
[89] Department for Communities and Local Government. (2010). Approved Document E – Resistance to the passage of sound (2010 edition, with 2010 and 2013 amendments). UK Government.
[90] Passive House Institute. (n.d.). Quality Requirements for Ventilation Systems. Retrieved from https://passivehouse.com/02_ph_components/02_component_certification/03_ventilation/03_ventilation.htm
[91] Paul Wärmerückgewinnung. (n.d.). MVHR Sound Data. Retrieved from https://www.paul-lueftung.de/en/products/mvhr-units/
[92] CIBSE. (2017). Guide B0: Applications and Activities. Chartered Institution of Building Services Engineers.
[93] Zehnder. (n.d.). MVHR Installation Best Practice Guide. Zehnder Group.
[94] Kinetics Noise Control. (n.d.). HVAC Noise Control Products. Retrieved from https://www.kineticsnoise.com/hvac/
[95] HVAC Design Guide. (n.d.). Flexible Ducts. Retrieved from https://www.hvacdesignguide.com/duct-design/flexible-ducts
[96] IAC Acoustics. (n.d.). Duct Silencers. Retrieved from https://iacacoustics.com/product/duct-silencers/
[97] CIBSE. (2017). Guide B4: Noise and Vibration Control for HVAC. Chartered Institution of Building Services Engineers.
[98] Carrier Corporation. (2000). System Design Manual, Part 2: Air Distribution. Carrier Corporation.
[99] SMACNA. (2006). HVAC Duct Construction Standards — Metal and Flexible. Sheet Metal and Air Conditioning Contractors’ National Association.
[100] Zehnder. (n.d.). ComfoWell Silencers. Retrieved from https://www.zehnder.co.uk/products-and-systems/ventilation-systems/comfowell-silencers
[101] Knauf Insulation. (n.d.). Acoustic Duct Insulation. Retrieved from https://www.knaufinsulation.co.uk/solutions/hvac-r-insulation/acoustic-duct-insulation
[102] Swegon. (n.d.). Air Diffusers and Grilles. Retrieved from https://www.swegon.com/uk/products/air-diffusers-and-grilles/
[103] Passivhaus Trust. (2018). MVHR: Guidance on good practice. Passivhaus Trust UK.
[104] Buro Happold. (2015). Designing for the future climate: Thermal comfort in dwellings. Zero Carbon Hub.
[105] Building Science Corporation. (2011). Mould and Moisture in Buildings. Building Science Press.
[106] Paul Wärmerückgewinnung. (n.d.). Frost Protection in MVHR Units. Retrieved from https://www.paul-lueftung.de/en/products/mvhr-units/
[107] Holtop. (n.d.). Benefits of MVHR Mechanical Ventilation with Heat Recovery. Retrieved from https://www.holtop.com/news/benefits-of-mvhr-mechanical-ventilation-with-heat-recovery
[108] ASHRAE. (2019). ASHRAE Handbook – HVAC Applications. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
[109] Holy City HVAC. (n.d.). Benefits of HRV and ERV. Retrieved from https://www.holycityhvac.com/blog/benefits-of-hrv-and-erv
[110] CIBSE. (2015). TM49: Design for Future Climate: Weather Data. Chartered Institution of Building Services Engineers.
[111] RDZ. (n.d.). 5 Main Benefits of MVHR Systems. Retrieved from https://www.rdz.it/en/novita/5-main-benefits-mvhr-systems
[112] AIVC. (2018). Technical Note 70: Demand Controlled Ventilation in Residential Buildings. Air Infiltration and Ventilation Centre.
[113] Maria Telkes. (n.d.). Air Recovery Ventilation System: A Health Perspective. Retrieved from https://mariatelkes.com/air-recovery-ventilation-system-a-health-perspective/
[114] Allen, J. G., et al. (2016). Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments. Environmental Health Perspectives, 124(6), 805-812.
[115] U.S. Environmental Protection Agency. (n.d.). Volatile Organic Compounds’ Impact on Indoor Air Quality. Retrieved from https://www.epa.gov/indoor-air-quality-iaq/volatile-organic-compounds-impact-indoor-air-quality
[116] Allergy UK. (n.d.). Indoor Air Quality. Retrieved from https://www.allergyuk.org/information-and-advice/features-and-articles/indoor-air-quality
[117] ASHRAE. (2020). Guidance for Building Operations During the COVID-19 Pandemic. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
[118] Public Health England. (2020). Radon: guidance on geological maps and radon protective measures in buildings. PHE.
[119] Building Science Corporation. (2011). Mould and Moisture in Buildings. Building Science Press.
[120] Allen, J. G., et al. (2016). Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments. Environmental Health Perspectives, 124(6), 805-812.
[121] Building Research Establishment (BRE). (2012). Radon: Protective measures for new buildings. BRE.
[122] REHVA. (2016). Guidebook No. 20: Demand Controlled Ventilation. Federation of European HVAC Associations.
[123] AIVC. (2018). Technical Note 70: Demand Controlled Ventilation in Residential Buildings. Air Infiltration and Ventilation Centre.
[124] Fraunhofer Institute for Solar Energy Systems ISE. (2020). Photovoltaics Report. Fraunhofer ISE.
[125] Geothermal Energy Association. (n.d.). Ground Source Heat Pumps. Retrieved from https://geo-energy.org/geothermal-energy/ground-source-heat-pumps/
[126] International Energy Agency. (2020). Smart Buildings and Communities. IEA.
[127] CIBSE. (2017). Guide B0: Applications and Activities. Chartered Institution of Building Services Engineers.
[128] U.S. Green Building Council. (n.d.). LEED for Schools. Retrieved from https://www.usgbc.org/credits/schools/v4
[129] ASHRAE. (2017). ASHRAE Standard 170: Ventilation of Health Care Facilities. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
[130] European Commission. (2018). Achieving Nearly Zero-Energy Buildings. European Commission.
[131] RICS. (2015). Addressing the performance gap in buildings. Royal Institution of Chartered Surveyors.