Mechanical Ventilation with Heat Recovery (MVHR)

Mechanical Ventilation with Heat Recovery (MVHR)

As modern houses become more airtight to avoid energy leakage, they risk not having enough fresh air entering to keep the environment in the house pleasant or healthy. Historically, houses were quite “leaky” – air both entered and escaped from the building through floorboards, windows, doors, wall joint details etc and sometimes deliberately with the use of extract fans.

As we look to make buildings as energy efficient as possible the idea of heating the air in the building and then either allowing it to escape to outside through window trickle vents or even worse, to plan to blow it out of the house with an extract vent seems crazy as we then have to consume more energy to heat the new cold air that has come in from outside. That said, it is not possible to just keep the same air in the building or air quality deteriorates and humidity builds up leading to mould and smells.

The current best solution to this for new build properties is to fit an MVHR system.


MVHR units generally all work in the same way:

They have a central heat exchange unit where fresh air is taken from outside and passed adjacent to the warm stale air from inside the house on its way to being exhausted to the outside. As the fresh, cold air passes against the warm, stale air, it picks up about 90% of the heat from the warm stale air.

The end result is fresh warm air coming into the house and stale, cool air passing to outside. This allows a good rate of fresh air entering the building with only about 10-15% of the heatloss that would occur if the same ventilation rate was maintained without MVHR.

In practical terms, this can make the difference between 2 sizes of heatpump and a significant real-world saving on energy consumption for heating. There is also a noticeable less tangible benefit of the house having a nicer “feel” due to the better air quality.

H2ecO recommends MVHR where budget and design allow on all new build properties or major renovations.

There are a few issues to consider with MVHR.

  1. The main unit is quite large and MUST be located within the insulated perimeter of the building – it is not feasible to install it in an uninsulated loft for example.
  2. The main air duct pipes are quite large so consideration must be made for the routing of those duct pipes. 2 pipes going from the main unit to outdoors and 2 to the air collection/distribution system.
  3. Each room will have either a supply or an extract vent depending on its type. Typically, air is extracted from bathrooms and utility rooms and is supplied to all other habitable rooms. It is possible to have both supply and extract to the same room but the most common, cost efficient way is to have either supply or extract in a room and allow the air to move between rooms internally via undercut doors.
  4. We recommend flexible ducting for ease of installation and minimal disruption to the structure of the property and other trades.
  5. We recommend the use of a unit with built in humidity sensors so that they can automatically speed up when they detect increasing humidity (such as when a shower is being used).
  6. Other features such as summer bypass, CO2 sensors etc can be specified if required.
  7. We are often asked if MVHR can be used to move heat around a building – for example to allow an oversized solid fuel burner to heat the whole house by moving the warm air around. The answer is no, MVHR does not move large quantities of air and air is a poor carrier of heat energy so MVHR is not suitable for moving heating or cooling by itself.
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