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 twice 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.

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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. As most of us spend a lot of time in-doors, a long-term exposure to polluted and oxygen-depleted air is likely to have negative health implications.

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What is MVHR and what does is stand for?

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In the drive to find a suitable ventilation strategy, mechanical ventilation systems are increasingly used to provide controlled ventilation. Mechanical Ventilation with Heat Recovery, otherwise called Heat Recovery Ventilation (MVHR) is the only form of ventilation that cuts out almost all of the ventilation heat losses, which make up to 30% of the heating demand of a dwelling. No other ventilation method is as predictable and consistent in providing the required amount of fresh air into each room, as well as extracting stale and polluted air where needed.

 

MVHR uses very little energy, compared to conventional Air Conditioning Systems. 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.

 

Whether it’s a certified Passivhaus, a low energy building, or a renovation project incorporating Passive principles, MVHR systems are an essential part of the success of these projects. An efficient MVHR system will reduce the heating demand from 35kWh/(m2a) to 15kWh/(m2a) at equal ventilation rates – compared with natural ventilation.
From this, it can be seen that the process of recovering heat from stale, used air and transferring it to fresh yet cold air, has a major influence on energy and emission savings.

 

Besides the energy savings, MVHR systems have a number of other benefits and reasons for its implementation. Occupants with good MVHR systems often value the excellent indoor air quality as the most outstanding advantage.

 

How does MVHR Work?

MVHR systems are based on air supply, air extract and transfer zones within the building. A heat exchanger is at the heart of the system, whilst fans provide the air movement. Most systems have ducting to the various rooms.

Habitable rooms are provided with fresh air and wet rooms and kitchens are extracted on a continual basis.

Before the extracted air is exhausted out of the building, the heat in the air is transferred by means of a heat exchanger into the fresh air, which is introduced into the building.

The pre-warmed fresh air is introduced into all habitable rooms on a continual basis. Thereby the need to completely heat the fresh air as it enters the building is eliminated. Efficient systems typically heat cold outside air from 0ºC to 18ºC through the heat transfer, when the extract air is 20 ºC.

No extract air is re-introduced or re-cycled, thus extracted germs and pathogens will not spread through the system.

 

A gentle cooling effect in the warm season is achieved by a potential summer bypass. The cooling effect is limited and needs to be supplemented by cross-ventilation if there are excessive solar gains. However, some systems can be combined with active cooling (Zehnder ComfoAirQ cool).

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MVHR stands for Mechanical Ventilation with Heat Recovery. In the 1980’s dwellings in Europe and later also in the UK were built to much better standards in air tightness, as part of the persuit to increase their energy efficiency and comfort. Soon it became obvious that there were some negative side effects for the health of the occupants, which in some cases lead to Sick Building Syndrome or otherwise called Toxic Home Syndrome.

Building researchers were trying to find a way of ventilating dwellings without the heat losses that were associated with uncontrolled ventilation / natural ventilation through gaps and purpose built openings in the external fabric of the building. Heat Recovery Ventilation was invented and refined as a controlled system, which would cut out almost all of the ventilation heat losses. Almost the only form of ventilation which provides suitable fresh air for the occupants in habitable rooms in a controlled fashion. As we all need a certain amount of fresh air to maintain our health, the ventilation requirements for bedrooms, living rooms, studies, etc. can be calculated and achieved with such a balanced ventilation system that operates 24hr a day.

In balance with the supply of fresh air, air is also extracted from wet rooms, kitchen, WCs and utility rooms, wherever moisture or pollutants can build up. The extract ventilation is also quantifiable and operational 24hrs a day on different levels.

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Unlike air conditioning, MVHR system do not re-cycle any air. Therefore they are safe to operate in situations where the spread of viruses or pollutants needs to be avoided.

Heat Recovery Ventilation typically does not use active heating elements for taking the chill out of the incoming air, as they achieve this in a passive fashion. That is another major distinguishing factor between MVHR and A/C, as A/C uses a lot of energy to actively heat or cool the air. MVHR does this passively, which is free of charge, apart from the two fans that move the air. Please note that it is therefore no heating system as it does not add heat, but just recovers the present warmth of the building.

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​​​​​​​​​​The heart of the MVHR system is the counter-flow heat exchanger, which – in good MVHR systems – transfers about 90% of the heat from the extract air to the supply air (without both air streams mixing or physically touching).

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When solar gains warm up the dwelling and there is no need for recovering the heat, most MVHR system swich off the heat recovery and provide fresh air from the outside. Such summer ventilation is gently counteracting overheating, but as it is passive and not active it’s effect is fairly limited. But, as we don’t use active heat or cooling, like A/C, it is no problem to open windows for cross-ventilation, if needed.

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The energy use of MVHR systems is extremely low, compared to A/C. It depends on the size and ventilation requirements of the dwelling and the efficiency of the ductwork. Typically it is between 20W and 80W.

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Good system providers offer the choice between HRV (heat recovery) and ERV (energy recovery) cores within their MVHR units. ERV, also called enthalpy MVHR recovers not only heat, but also about 75% of the moisture from the extract air into the supply air through special polymer membranes in their heat exchanger. The reason for ERV is that HRV systems extract humidity from the internal environment in the cold season. This de-humidification effect is passive, but can be quite powerful, especially in larger dwellings, where there are few occupants. In larger dwellings it is therefore advisable to use ERV systems, in order to keep the relative humidity in optimally healthy levels (40-60% rh).

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Good HRV systems offer active frost protection, which keeps the system performing at its peak heat recovery rate when it gets frosty outside. Please note that MVHR systems are primarily ventilation systems and not heating or heat distribution systems. However, in some cases with special set-up they can be used for space heating and distribution.

Good filters will look after your health and keep your ductwork and heat exchanger clean. We recommend to check and clean your filters every 4 months and change them over at least once a year.

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One of the most critical features for occupants is noise – or the absence of it. With regard to noise it becomes more obvious if it is a cheap or better performing system. We recommend to avoid all short-cuts and install machine attenuators for the supply and extract air, as well as cross-talk attenuators for branch systems.

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​MVHR systems are primarily ventilation systems, which reuse and distribute warmth from internal + solar gains and space heating systems. Although extremely efficient, they are normally not to be used for heat distribution or space heating. In some cases (mostly certified Passivhaus buildings) all the space heating can be provided by an MVHR system.

 

The Benefits of MVHR

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Best indoor air quality

• Continuous supply of fresh air to provide good indoor air quality

• No drastic CO2 peaks

• No build up of air pollution, e.g. from carpets, cleaning products or radon

• Elimination of bad odours

• A controlled air flow path throughout the building, rather than ‘by-chance’ ventilation

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Nasties filtered out

• Pollen filters are a great advantage for allergic people

• Filtered air gets rid of most of the pollutants from the outside, e.g. PM from traffic

• Keeps midges out of the house

• Possibility to filter out combustion and traffic smells with activated carbon filters

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Humidity Control

• Preservation of the building fabric through steady ventilation

• Keeps mould, fungus, dust mites in check

• Active dehumidification in the cold season

• Reduced humidity keeps dust mites in check

• Some systems provide a monitoring function to check the air quality and humidity

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Comfort

• Less noise inside (windows can remain closed); undisturbed sleep

• You can open windows for temperature control whenever you want

• No drafts (in conjunction with an airtight building fabric)

• Good indoor climate

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Energy Savings

• Approximately 30% of the heating energy can be saved in airtight buildings with highly efficient MVHR systems compared to naturally (uncontrolled) ventilated buildings

• Little energy use to operate the system​

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