Over the last decades homes have become much more comfortable and energy efficient, due to increased standards for insulation and air tightness. On average, modern houses are more than three times as air tight, compared to the pre-millenium housing stock, which additionally benefitted from ventilation through the chimney stack effect of fire places. On top of this, the indoor humidity was regulated by hygroscopic materials, such as lime render.
With such great changes in the fabric and function of buildings, concerns have arisen over the indoor air quality (IAQ) of modern and renovated dwellings. A recent study has shown that the average IAQ in modern homes with natural ventilation via trickle vents is alarmingly poor – unless users open windows regularly. Long-term exposure to poor Indoor Air Quality is likely to have serious negative health implications.
dMEV systems have become very popular with the mass house builders, as they are cheap and easy to install. But do they really provide good indoor conditions?
Like MEV, dMEV systems have been designed to deal with excess moisture and in that way they work well.
However, the indoor air quality in habitable rooms is hit and miss, as the ventilation path through these rooms depends on changing variables:
MVHR is the only ventilation strategy where the ventilation rate in habitable rooms can be controlled. Thus the indoor air quality can be predicted and set to a good standard and not just left to chance. How does it work:
Compared to conventional Air Conditioning Systems, MVHR uses very little energy. It also does not re-circulate air and thus avoids the associated health risks. It is a completely different technology than A/C.
At the heart of an MVHR system is a counterflow heat exchanger, which passively transfers heat from the stale waste air to the incoming air. The only active element are normally only two fans that move the air.
It goes hand in hand with the fabric first approach, based on passive design criteria, which is becoming increasingly more common as people’s awareness and understanding of its advantages rise. Compared with any other renewable technology, it has potentially the greatest impact on the primary energy use, CO2 footprint and heating costs.
that heat recovery ventilation systems have been installed as standard in Sweden since the 1980s? This followed rising cost of fuel and subsequent legislation aimed at minimising energy consumption. Although there are various ways to recover heat, MVHR is the most simple and cost-effective solution. The technology and control systems have progressed from simple systems with plate heat exchangers to highly efficient counterflow heat exchangers.
With the rise of Passive House in the 1990s, Heat Recovery Ventilation became an integral part of air tight buildings and was also successfully used in retrofit projects, such as EnerPHit.
An efficient MVHR system will reduce the heating demand from 35kWh/(m2a) to 15kWh/(m2a) at equal ventilation rates – compared with natural ventilation.
Heat Recovery Ventilation is not just for the super efficient Passive Houses. All new-build dwellings, even those without special attention to energy efficiency, can potentially benefit from MVHR. The air tightness standards have come down a lot, compared to historical buildings.
MVHR systems, especially those to Passive House* standard have a number of key advantages:
MVHR systems tackle excess humidity in a powerful way, actually in three ways:
As such they counteract mould growth and the proliferation of house dust mites, which thrive on higher humidity.
In some cases MVHR systems can actually dehumidify too much in winter. This danger arises within larger dwellings, occupied by few people.
In such cases, better MVHR systems offer an alternative heat exchange core, called ‘ERV’ – Energy Recovery Ventilaiton or Enthalpy Core. These recover heat and about 70% of the extract humidity back into the supply air. Excess humidity from using a shower or cooking is still exhausted enough. We usually recommend ERV systems for dwellings over 230 m2 floor area.
In summer, ERV can also lead to lesser indoor humidity, especially when it is really hot outside and solar gains can be avoided. The ‘ERV’ core fits into the same MVHR unit as the standard ‘HRV’ core. MVHR systems with ERV core normally don’t need a condensate connection.
With all ambition to reduce energy and carbon, we should never loose sight of the fact that it’s real people living in the dwellings that we handle.
Besides the energy savings, many occupants feed back to us, that MVHR systems with pollen filters can make a difference to their health & wellbeing. Occupants with certain health challenges have experienced tremendous benefits.
Also, if there is a lot of noisy night traffic in the vinicity, MVHR systems with good noise attenuation will be a welcome solution, as the bedroom windows do’t need to be kept open.
Fan noise is the primary obstacle for users to accept ventilation systems. Noise attenuators or commonly called silencers can powerfully reduce fan noise and make systems potentially inaudible.
However, these are often omitted from budget specification, in order to be competitive. The question arises, how useful the investment in such a cheap system is, if the users don’t use it or don’t use it correctly, e.g. switch it off at night, as they feel disturbed by noise.
Where should attenuators be installed? Typically in the supply and extract ducting between the MVHR unit and the first t-piece or manifold. We recommend at least 1000mm supply and 500mm extract attenuators. Attenuators below 500mm are not too much helpful.
Cross-talk attenuators should be considered for any branch ducting system.
The simple answer is: No. Although a good MVHR system looses hardly any heat, temperatures between rooms can vary between 4 and 6 degrees C, in extreme cases up to 10 deg. C., depending mainly on the heat losses of the fabric of a particular room and the internal heat transfer. This is really good news as many people don’t want their bedrooms as warm as their living spaces. It also means that MVHR systems cannot be used for distributing space heating from heated to un-heated rooms, unless a post-heater or a special set up of the system is being used.
The reason for this seeming paradox is that MVHR systems as based on slow air movements and air has a limited capacity to carry heat. E.g. if we supply 30 m3/h pre-warmed air at 18 degrees C. into a particular room, which is 14 deg. C cool, the ventilation adds 41W of heat into this room, which is not much, as the room’s heat loss can range from 100W-1000W.
Although the original concept of Passivhaus incorporated space heating via air with a small heating element in the MVHR system, this setup has proven problematic in subsequent years. Reason for this is that only about 1.5-2.5kW of heat can be introduced into a normal sized dwelling via a post heater and it normally cannot be zoned. However, heating modules can supplement the space heating. Full space heating systems with MVHR are possible, but are limited to smaller certified Passive House dwellings.
Passive cooling: Most MVHR systems have an automated summer bypass, which shuts off the heat recovery, if the internal temperatures rise above a certain point. The effect of such bypass is very limited and in no way adequate to compensate for excessive solar gains. It is more like gentle night cooling.
With increased demand for cooling, cooling modules for MVHR have been successfully developed.
MVHR can take a lot of different forms:
This system lends itself to most dellings, apart from very small apartments.
These systems are best installed close to an external wall.
The supply and extract ducting to the various rooms is ideally kept fully within the thermal envelope, e.g. fully underneath loft insulation. Insulating longer lengths of ductwork in cold areas is a compromise to be avoided.
Such systems are ideal for dwellings with multiple habitable rooms, which are not always used at the same time.
These innovative MVHR systems optimise the spread of fresh air by zoning and combining it with sensor control. This way, the bulk of the fresh air is supplied only where needed. The same priciple can be used for extract ventilation, that the bulk of the extract air comes only from wet rooms or a kitchen in use.
The advantages of such system is optimised indoor air quality, powerful extractions of wet rooms, full automation and possible integration of cooling.
Combined services cupboards are of increasing interest for flatted developments, as it keeps all the services neatly together in one space and saves on site plumbing works. What do they combine?
Cascade MVHR systems are best suited in smaller dwellings, like apartments or small houses.
The main advantages are that they don’t take up any plant space and that they typically incorporate very little ducting. i.e. only one duct to a wet room. Cascade fans connect bedrooms and other habitable rooms with the MVHR unit, without ducting.
These systems are normally fully automated, so that apart from annual filter changes and regular maintenance services, there is no user-interaction needed.
Such systems are ideal for small dwellings, extensions or garden offices.
Single Room Heat Recovery Ventilators (SRHRV) range from cheap systems, as replacement for a bathroom fan to Passive House certified units.
Oszillating push-pull MVHRs are quite popular in Europe as they tick the box for background ventilation. They are normally used in tandem and change direction of roughly air flow every 80 seconds. The warm extracted air heats up a ceramic or aluminium storage element, which then passes its energy to the incoming air at reversal of the airflow. Quite clever technology, but limited in its effect on the indoor air quality. These systems can also not be used for wet rooms.
Continuously operating MVHR units provide better results in terms of the indoor air quality.
Central MVHR systems for block of flats are of an advantage if there is a communal house administration in place. They are based on one commercial MVHR unit either in the basement or on the roof with supply and extract risers, t-ing off to the various flats. Within the flats flow dampers regulate the amount of airflow, so that users can boost their system if needed.
The advantage is that most of the maintenance can be done centrally, outwith the flats. For Housing Associations access to properties is a major issue.
The Building Regulations for Domestic Ventilation have been almost harmonized within the UK since 2023.
Additionally there is additional guidance produced by the NHBC, the Passivhaus Trust, Future Homes Standard and PAS 2035.
MVHR systems have a variety of control options from fully automated systems, often called demand controlled ventilation, to more complex control platforms:
The maintenance of MVHR systems is mostly housekeeping: filter changes and keeping things clean.
How often should filters be changed? At least every year, better every 4-6 months. Filters can be carefully hoovered out with a brush attachment bettwen the yearly change over. Remember: The dust and debris caught in the intake filter, is what you didn’t have to breathe in.
Do I need a professional to carry out the maintenance? At least every 4 years a professional service is strongly recommended:
In between, the owners can do a ‘spring cleaning’ of their system and terminals and change the filters, if they want to save costs of an annual maintenance service.
With a growing number of different MVHR manufacturers and systems on the market, choosing an MVHR unit is not an easy task – unless price is the only criteria. We find that there are huge differences in the quality, the real performance and functionality, which cannot be easily detected.
For simplicity, we have divided the systems into two categories:
• The basic ones, from the main UK ventilation brands
• The high performance and high quality ones, mostly with Passivhaus certification.
Please consider the following table, showing the difference between the two categories:
| Basic MVHR | Quality (PH) MVHR | |
| Fan control | The output drops with increased resistance (fan curve tails off towards zero at max. output). With increased resistance from soiled filters the ventilation rate will drop and the system will dis-balance, thus reducing its heat recovery rate. | Volume flow constant (fan curve has a vertical drop at maximum output level).
The ventilation rate will not change over time and the system will not dis-balance. |
| Insulation | Minimal insulated housing with cold-bridging. This will lead to good heat recovery rates ‘on paper’ but to increased heat losses in reality. | Well insulated, thermal bridge-fee housing. You can see the difference to cheaper systems when you compare the Passivhaus certification data. |
| Noise | High Break out noise and in-duct noise levels due to cheap fans, lack of insulation, tight spaces and sharp corners. | Low noise levels. E.g. 1x basic MVHR produces as much noise as 8x Zehnder CAQ units at the same output level. |
| Frost protection | Frost protection disbalancing will reduce the heat recovery rate gradually down to zero in the coldest time of the year, thus leading to ice-cold infiltration, exactly when you don’t want it. | A de-frost pre-heater will keep the MVHR working on its peak efficiency in the coldest time of the year. |
| Humidity control | Larger dwellings with a disbalance between occupancy levels and ventilation requirements can easily become too dry in winter, due to the de-humidification effect of MVHR. | The choice of an enthalpy heat exchanger will prevent such over-dehumidification as they recover heat and moisture. |
| Filters | Basic G3 type filters with a low surface area and often loose fitting. These will allow smaller dust to enter the system and will lead to gradually soiled ductwork. | F7 pollen filters for the air intake will filter out finer dust, pollen and pollution, which keeps the house and ductwork clean. |
| Controls | Basic controls | A choice of various types of controls |
| Durability | Often the whole unit needs to be replaced if something goes wrong. | Made to last; all components can be exchanged individually. |
Generally, the capacity of a MVHR unit should be high enough to cover the nominal ventilation rate and at least 30% additional boost capacity. Ideally, the unit should run at 60-65% of its maximum capacity to work efficiently. If the system is running on higher output levels, this will increase the noise and energy use over-proportionally.
Among all technical specifications, please also consider the level of support and after sales that a supplier will offer. Unfortunately, the domestic ventilation industry in the UK is dominated by performance gaps and a “sell-and-run” culture. We are often contacted by customers of various brands, asking for support and maintenance, which their supplier does not offer.
Most domestic ventilation systems in the UK are based on PVC ducting. A choice that most other European countries have done away with (for good reasons).
Please see below table for a brief comparison of the most common ducting types. We have not mentioned flexible ducting, which has often been used in the past, as this is so poorly performing that it should only be used for very short lengths, if at all.
| PVC ducting | Galvanised spiral ducting | Radial semi-rigid ducting | |
| Type | Rigid ducting | Rigid ducting | Rigid ducting |
| Sizes | 100, 125, 150mm or flat channel ducting, e.g. 56x 110, 60x 204, 90x 220mm | 100, 125, 160, 200mm dia. | 75/63 and 90/76 mm outer/inner diameter |
| Efficiency | High resistance due to sharp bends (0.5 d radius).
Sample pressure drops*: 204x60mm: 265Pa |
Low resistance due to better formed bends (1d radius). Low velocities due to larger size.
Sample pressure drops*: |
Low resistance (radius can be as shallow as possible). Low velocities of 90mm ducting/ multiple duct runs.
Sample pressure drops*: |
| Air tightness | Flat channel ducting is difficult to get permanently air tight. Do not use tapes! | Recommended: SAFE system has double seals at all joints. | Few joints in each branch, seals used for air tightness. |
| Clean-ability | Difficult to clean. Impossible for flat channel ducts. | Can be cleaned. | Can easily be cleaned. |
| Hygiene | Questionable, as pathogens can grow on PVC over time. | Inherently excellent. | Excellent, when anti-static and anti-bacterial lined. |
| Ease of installation | Easy to cut | More time consuming. Cutting with grinder or metal shears. | Easy to install, especially when going through floor joists (web or engineered timber) |
| Price | Cheap | Expensive | Middle range |
| Our recommen-dation | Please avoid it, unless for short extract only systems. | Best for very small systems or long-stretched ones. | Best for most MVHR installations. Please note that there are major differences in quality between different makes. |
*) Sample pressure drops based on 5m ducting with 5x 90 deg. bends at 210 m3/h and 30m ducting with 20x 90 deg. bends at 55 m3/h or equivalent for radial ducting. The higher the pressure drop, the higher the resistance. Doubling the pressure drop will result in 3-4 times more noise and energy use.
Please consider that any ducting installed within your floor zone or cavities will stay there for a long time and should be of high quality.
Whenever existing properties are being upgraded with additional insulation, new windows and draft-proofing, this will affect the natural infiltration rate, with reduced fresh air coming in and stale air being taken out of the building. Statistically, following such renovation works, the risk of black mould growth triples. Unfortunately, most UK energy efficiency advice services do not consider the impact, that their suggested improvement works will have on the internal climate of the building. To avoid any surprises with negative impacts on the occupant’s health, in all such cases the ventilation strategy needs to be (re-) considered.
We have accompanied a number of retrofit-installations of MVHR systems in renovation projects. It depends on the design of the dwelling and the nature and extent of the renovation works, if it is possible to install a whole house ventilation system or a system that serves only part of the dwelling.
Visit our FAQ’s page or reach out to us directly if you may have any further questions regarding MVHR Systems.
Please note following points:
Whether it is a Passivhaus/ Passive House, a Nearly Zero Energy Building (NZEB) or a low energy building, based on passive principles, air tightness is critical; it is as important as good insulation. As such close attention should be paid to the ventilation strategy. In almost all cases MVHR is the right choice, as in air tight dwellings, natural ventilation without frequent/ all night window opening is not delivering the necessary air quality.
It is also important to choose a high performance MVHR system that not just has a good performance on paper, but also in real life and will retain its efficiency over its life-time. It also should not drop its the heat recovery rate in the coldest time of the year.
that you shouldn’t feel restricted to open windows in a Passivhaus or any other dwelling with MVHR. When you want to open them, feel free to do so. However if you don’t want to, you will still have good air quality within.
that about half of Europe’s Passivhaus dwellings have a central heating system. It is a myth that Passivhaus means a dwelling without central heating, let alone a dwelling without any space heating.
If you intend to design a house without central heating, please contact us in the early design stages. For such projects typically a warm water based or electrical post heater is installed into the supply leg of the MVHR system. We are happy to assist you in the process, as we have accompanied a large number of similar projects right from the start of Passivhaus in the Scotland.
Certain building types lend themselves more to space heating via air, e.g. double storey dwellings with a rectangular and smallish footprint. Sometimes it is better to have a central heating system at least for part of the dwelling.
We find the biggest danger with Passive dwellings in the UK is over-confidence and assumptions on part of the design and specification team, as well a lack of understanding and training on part of the installation force. Proper, concluded thermal calculations with the PHPP design tool before any work on site, experience, accuracy and a quality minded installation team, that is ‘on board’ of the methodology, make all the difference and ensure a successful project.
The MVHR system is an essential part of a Passivhaus. The heating demand of such extremely energy efficient buildings is only 15 kWh/(m2a). As a comparison, the average 1970’s semi-detached social housing property has a heating demand of about 500 kWh/(m2a).
If this was replaced by natural ventilation at the same exchange rate, the heating demand would be 35kWh/(m2a). From this it can be seen that the process of recovering the heat from the stale, used air and transferring it to the fresh yet cold air, has a major influence on the energy and emission savings.
Particular attention has to be paid to the choice of the HRV system. Claims of manufacturers regarding efficiencies have to be taken with a pinch of salt, as various test methodologies can influence the results greatly. The safest way is to compare the efficiencies of the official Passive House certification (see PHI web page). The tests are done in a complete different way than e.g. the ones for Appendix Q here in Britain and mirror the overall efficiency within a dwelling more accurately.
Also important is the level of insulation of the ducting. The intake and exhaust ducts within the thermal envelope are typically insulated with 25-50mm of vapour-proof insulation. The insulation needs to be taped correctly to the MVHR unit and the vapour barrier of the external walls. The specification of insulation needs to be checked with the Passive House Designer. All warm ducting (supply and extract) should normally be kept fully within the thermal envelope.
Please discuss with us your detailed needs.
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