MVHR - How it works, and the benefits, sustainability and alternatives

What is an MVHR system?

An MVHR (Mechanical Ventilation Heat Recovery) system is one of the most common and important methods of ventilation in today’s highly sealed properties. Fundamentally, an MVHR system brings in fresh air from outside and removes stale air from inside whilst improving indoor comfort levels by tempering the incoming air. The system uses the temperature difference between the incoming and outgoing air to recycle energy. It can also provide ‘free cooling’ in that if the room temperature is higher than ideal, and the outside air is cooler than the inside air, then the incoming air bypasses the heat exchanger to cool the space down.

How does it work?

1. Fresh air is drawn into the building by the supply fan.
2. At the same time, stale air from inside is blown out of the building by the exhaust fan.
3. The two air streams pass through a heat exchanger in the MVHR unit itself, where heat is transferred from one to another depending on the external temperature.
4. The tempered supply air is distributed throughout the building, and the stale air is extracted through dedicated ductwork systems.

What are the benefits?

MVHR as a form of ventilation allows incoming air to be conditioned to a more comfortable temperature, as well as improving indoor air quality (read more in our other editorial here). Health benefits include the removal of pollutants such as carbon dioxide and volatile organic compounds from the inside of the house, which can reduce human allergy reactions, as well as preservation of the building by reducing the possibility of condensation and mould.

Is MVHR sustainable?

There have been sustainability concerns raised over MVHR systems and energy efficiency, as the fans used to distribute the air require energy to operate. However, the tempering of the fresh air saves more energy than the fans consume, as the energy used to heat the property is reduced. An MVHR system meets this need whilst also providing ‘free cooling’ in spring and autumn.

What are the alternatives and how do they compare?

Many designers prefer to use passive ventilation instead, which is perceived as being much greener and more sustainable as it requires no energy at all to operate. However, the concept of passive ventilation conflicts with the drive to save energy as the air drawn in by passive ventilation is not temperature controlled and adds to the heating or cooling load of the building. It is not therefore as sustainable as it first looks, and users must choose between untampered fresh air or no fresh air. The ductwork required for it is also very bulky, making it less than ideal for many of today’s properties where spatial constraints are paramount.