MVHR Efficiency Explained
For contractors and specifiers, understanding MVHR efficiency ratings, the factors that affect real-world performance, and how to select the right unit is essential to delivering compliant, high-performing buildings.
Well-specified MVHR systems reduce heating demand, support Part L compliance, and improve indoor air quality across residential and commercial projects.
What Does MVHR Efficiency Actually Mean?
MVHR efficiency, often referred to as heat recovery efficiency or thermal efficiency, measures the percentage of heat recovered from the exhaust airstream and transferred to the supply airstream. A unit rated at 90% efficiency recovers 90% of the heat that would otherwise be expelled to outside, using it to pre-warm the fresh air entering the building.
The figure is calculated under standardised test conditions and is expressed as a percentage. Most quality units on the market today achieve between 75% and 93% efficiency under test conditions. The higher the efficiency rating, the less work your primary heating system has to do, which directly reduces energy consumption and running costs for the building occupant.
It is worth noting that manufacturers test units under controlled laboratory conditions. Real-world efficiency depends on installation quality, ductwork design, and how well the system is commissioned and maintained.
Our article on MVHR system design covers the specification and installation factors that have the biggest impact on delivered performance.
The Difference Between Thermal Efficiency and Specific Fan Power
Thermal efficiency and specific fan power (SFP) are two separate but equally important metrics. Thermal efficiency measures heat recovery performance. SFP measures how much electrical energy the unit's fans consume per cubic metre of air moved, expressed in W/(l/s) or W/(m³/h).
A unit with high thermal efficiency but poor SFP can still be an inefficient overall system because the fans consume disproportionate electricity relative to the heat they recover. For SAP and Part L compliance, both figures feed into the energy calculations. MVHR units used in new dwellings assessed under SAP 10.2 must meet minimum SFP thresholds, so always check both metrics when specifying.
What Factors Affect Real-World MVHR Efficiency?
Real-world MVHR efficiency consistently falls below laboratory-tested figures when systems are poorly designed or installed. The gap between rated and delivered performance is one of the most common complaints on residential and light commercial projects, and it is almost always attributable to avoidable installation errors.
The main factors that reduce delivered efficiency include:
-
Ductwork that is too long, undersized, or poorly routed, increasing resistance and reducing airflow
-
Poorly sealed duct joints that allow air leakage and reduce balanced flow rates
-
Incorrect commissioning, where supply and extract flows are not balanced to design rates
-
Filter blockage from lack of maintenance, which restricts airflow across the heat exchanger
-
Thermal bridging at the unit or duct penetrations, which increases heat loss before recovery occurs
-
Units installed in unheated roof spaces or plant rooms without adequate insulation on supply ductwork
For projects where Part F ventilation compliance is being demonstrated through an MVHR system, the commissioned flow rates and SFP must be verified and recorded. A poorly installed system that does not deliver design airflow rates will fail compliance regardless of the unit's rated efficiency.
How Airtightness Affects MVHR Performance
Airtightness and MVHR efficiency are directly linked. In a leaky building, uncontrolled air infiltration bypasses the heat exchanger entirely, reducing the proportion of ventilation air that benefits from heat recovery. The efficiency gain from MVHR diminishes as air leakage increases, which is why the system delivers its full value only in buildings with good airtightness.
For new builds targeting Passivhaus or Future Homes Standard compliance, achieving an airtightness test result below 3 m³/h/m² at 50Pa is common practice before MVHR is commissioned. In deep retrofits, draught-proofing and insulation upgrades should be completed before the MVHR system is designed and sized.
Our comparison of MVHR versus a heat pump covers how airtightness affects the case for both systems in retrofit and new build contexts.
How Is MVHR Efficiency Rated and Tested?
MVHR units sold in the UK and Europe are tested under EN 13141-7 for residential units and EN 13141-2 for commercial units. These standards define the test conditions, airflow rates, and measurement methodology used to generate the headline efficiency figure shown in product datasheets.
The ErP (Energy-related Products) Directive sets minimum efficiency thresholds for residential ventilation units. From 2016, units must meet a minimum specific energy balance (SEC) under the Ecodesign Regulation. Units classified as being in the highest energy efficiency bands will display this on their product label, similar to the energy rating system used for domestic appliances.
When comparing units across manufacturers, always reference the EN 13141-7 test result rather than marketing claims.
Understanding the Eurovent and Passivhaus Component Certification
Eurovent certification independently verifies that a manufacturer's published efficiency data is accurate and reproducible. Units carrying Eurovent certification have been third-party tested and confirmed to perform as advertised. For projects requiring Passivhaus certification, the Passive House Institute (PHI) maintains a component database of approved MVHR units that meet strict efficiency and SFP criteria. Only units on this list can be used in certified Passivhaus buildings.
If you are specifying for a Passivhaus or EnerPHit project, checking the PHI component database before selecting a unit will save significant time during the certification process.
Our guide on the best MVHR systems includes units from manufacturers with strong certification track records across both residential and commercial applications.
What MVHR Efficiency Rating Should You Specify?
For standard new build residential projects complying with Part L 2021 or the Future Homes Standard, units with heat recovery efficiency of 80% or above will comfortably support the energy calculations.
For Passivhaus projects, the PHI component database typically lists units achieving 85% or higher, with many modern units reaching 90% to 93%.
As a general specification guide:
-
75% to 80% efficiency: Suitable for standard Part F compliance in moderate airtightness buildings
-
80% to 88% efficiency: Good performance for Part L new builds and deep energy retrofits
-
88% to 93% efficiency: Recommended for Passivhaus, EnerPHit, and net zero carbon projects
Higher efficiency units typically cost more upfront, but the reduction in heating demand they deliver over the building's lifespan justifies the additional capital cost on most projects.
Our dedicated article on MVHR system costs breaks down unit pricing by efficiency band and system type, which is useful when preparing project budgets.
Single Room Units vs Whole House Systems
Single room heat recovery units and whole house MVHR systems operate on the same heat recovery principle but serve different applications. Single room units recover heat from one space and are well suited to retrofits where full ductwork installation is not viable.
Whole house systems serve all habitable rooms from a central unit via a duct network and deliver better overall building performance when properly designed.
For noise-sensitive applications such as bedrooms or open-plan living areas, SFP and sound power level are both worth checking alongside thermal efficiency. Our article on whether MVHR systems are noisy covers sound performance expectations and how unit placement and duct attenuation affect occupant experience.
How Does MVHR Efficiency Affect SAP and Energy Compliance?
SAP 10.2 uses MVHR efficiency data directly within its calculation methodology. The heat recovery efficiency figure reduces the calculated heat loss attributed to ventilation, lowering the dwelling's space heating demand and improving the energy rating. SFP adds a small electrical consumption figure back into the calculation, so both metrics feed into the final EPC outcome.
For a typical well-insulated new build, specifying an MVHR system with 85% or higher efficiency can improve the SAP score meaningfully compared to a standard MEV system. This improvement can be the difference between achieving an EPC A and EPC B on borderline projects.
If you are comparing MVHR against MEV for a specific project, our article on MEV vs MVHR sets out the compliance and performance differences in detail.
Summer Bypass and Its Effect on Annual Efficiency
Many modern MVHR units include a summer bypass function that routes supply air around the heat exchanger during warmer months to prevent overheating. Bypass mode reduces heat recovery during summer but improves overall building comfort and is increasingly important as UK summer temperatures rise.
Annual average efficiency figures account for bypass operation and give a more accurate picture of yearly performance than peak winter efficiency figures alone. For more on how bypass functions work in practice, our guide on whether MVHR can cool a house explains the bypass mechanism and its limitations.
eFans MVHR Units for Trade Supply
eFans stocks a full range of heat recovery and MVHR units from Vent-Axia, Zehnder, Elta, and S&P, covering heat recovery efficiency up to 93% and airflow rates from 32 m³/h to 900 m³/h.
Whether you are specifying a single room heat recovery unit for a bathroom retrofit or a whole house MVHR system for a new build development, the range is built around trade supply with full product data available for specification purposes.
All units are supplied with manufacturer datasheets including EN 13141-7 test data, SFP figures, and airflow performance curves. For a broader overview of what MVHR delivers for your clients and projects, our article on MVHR pros, cons, and how it works is a solid starting point.
Frequently Asked Questions
What is a good heat recovery efficiency for an MVHR unit?
A heat recovery efficiency of 80% or above is considered good for standard new build residential applications. For Passivhaus and net zero projects, units achieving 88% to 93% are recommended. Always cross-reference the efficiency figure against the unit's specific fan power (SFP) rating, as high-efficiency units with poor SFP can still result in poor overall energy performance.
Does MVHR efficiency drop over time?
Yes, MVHR efficiency decreases over time if the system is not maintained. Blocked or dirty filters are the most common cause, as they restrict airflow through the heat exchanger and reduce heat transfer. Regular filter replacement, typically every three to six months depending on usage and air quality, is essential to maintain rated performance. Annual inspection of the heat exchanger core and duct seals is also recommended.
How is MVHR efficiency measured?
MVHR efficiency is measured under EN 13141-7 for residential units, which specifies standardised airflow rates, inlet temperatures, and measurement points. The test calculates the ratio of heat transferred to the supply airstream compared to the total heat available in the exhaust airstream. Manufacturers publish this figure as a percentage on their product datasheets.
Can an MVHR system be too efficient for a building?
An MVHR unit cannot be too thermally efficient, but it can be oversized for the building's ventilation requirements. An oversized unit running at low speed to match actual airflow demand will spend most of its operation time outside its optimal efficiency range. Correct sizing based on whole-dwelling ventilation calculations under Part F is essential to ensure the unit operates near its rated efficiency in normal use.
Does MVHR efficiency vary between summer and winter?
Yes. Thermal efficiency is highest in winter when the temperature difference between exhaust and supply air is greatest. In summer, units with a bypass function route air around the heat exchanger to prevent overheating, which reduces heat recovery but improves comfort. Annual average efficiency figures, rather than peak winter figures, give a more representative view of year-round system performance.
What is the difference between sensible heat recovery and total heat recovery in MVHR?
Sensible heat recovery transfers only the thermal energy (temperature) from the exhaust airstream. Total heat recovery, also called enthalpy recovery, transfers both sensible heat and latent heat (moisture). Most MVHR units in the UK use sensible heat recovery with a counter-flow or cross-flow heat exchanger. Enthalpy recovery units are more common in continental European climates where humidity management across seasons is a greater concern.