What is MVHR?

Drew Barker
Posted February 08, 2024
MVHR for Passivhaus

Heat recovery ventilation is one of the five fundamental principles of Passivhaus.

If a house is well insulated and airtight, but not sufficiently ventilated, it can lead to poor indoor air quality. A Passivhaus, however, has higher levels of Interior Air Quality (IAQ) than a conventional home due to the ventilation strategy inherent in the design process. The Passivhaus approach to ventilation also provides an effective remedy to the issues of condensation, damp and mould.

The Importance of Ventilation

Adequate ventilation is crucial for all buildings in order to maintain a healthy indoor environment for the occupants – and to prevent the fabric of the building from getting damaged. The most common method of ventilating buildings is via trickle vents, airbricks and, of course, by opening windows. Other common methods are MEV and DMEV fans, which provide a continual extraction of air. However, all these methods are highly inefficient, due to heat loss, and thus unsuitable for Passivhaus.

In order to reduce heating costs and ensure a continuous supply of fresh, filtered air, we always fit an MVHR system in our buildings.

What is MVHR?

MVHR stands for Mechanical Ventilation with Heat Recovery.

It works by extracting warm, but potentially stale or humid air, and replacing it with fresh air. The warm air is drawn through a ducting system, back to a main unit, which contains a heat exchanger. At the same time, fresh air is bought in from outside. The heat is exchanged from the warm, outgoing air to the incoming fresh, filtered air, which is then reintroduced back into the home via a ducting system.

An efficient MVHR system can retain up to 95% of the heat from outgoing air.

The main appeal of MVHR systems is the reduction in energy bills, but there are also potential health benefits. Having an MVHR system installed provides a continuous supply of fresh, filtered air – removing pollutants, allergens, such as pollen and dust, and unpleasant odours from the home. It can be very beneficial for people with asthma or allergies.

In a Passivhaus, the combination of high quality insulation, airtight construction, and Mechanical Ventilation with Heat Recovery (MVHR) often means a traditional heating system is not necessary. This is very appealing to clients that are concerned with sustainability or looking to achieve net zero carbon.

Achieving a Balance

Mechanical Ventilation with Heat Recovery (MVHR) must be a balanced whole house system that supplies fresh air and extracts stale air from a building. In doing so, it transfers the heat from the extracted air to the supply air via a heat exchanger.

It is important that an MVHR system is designed, installed and commissioned correctly in order to ensure a balance. If a system isn’t balanced correctly, it can either pressurise or depressurise the house, resulting in inefficiencies.


Clients often ask, how noisy are MVHR units?

In our experience, a correctly designed, professionally installed and well maintained MVHR system is almost imperceptible.

Good duct design and specification play a huge part in the operating efficiency and noise levels of an MVHR system. Higher specification units have constant volume fans (as opposed to constant speed fans) which operate by constantly assessing the system pressure and adjusting the fan to maintain the commissioned airflow. A poorly designed duct network adds to the system pressure and in turn prompts the unit to run at a higher speed in order to maintain flow, thus making it noisier and increasing energy usage.

A well-designed system, on the other hand, allows the MVHR fans to run at the lowest speed necessary to maintain the desired airflow, resulting in quiet, low-energy systems.

At Beattie Passive, we work closely with CVC Systems to ensure your MVHR system is calibrated and commissioned professionally. Do you want to learn more about MVHR and how it works in a Passivhaus? Give our team a call or email