Airtightness and MVHR

Once a house becomes as airtight as we hope to make ours, a mechanical ventilation system with fans is needed to make sure there is enough airflow to keep the house fresh. This opens up a further opportunity for energy saving, using Mechanical Ventilation with Heat Recovery (MVHR).

In this system, “stale” air is extracted from all the kitchens and bathrooms and ducted to a central unit in the basement. We are using a Genvex Combi unit. It will extract nearly all of the heat from the stale air and pass it to the incoming fresh air via a high-efficiency “counterflow heat exchanger”. More ducts will carry this fresh, warm air to the bedrooms and living room, where specially designed grills will mix it thoroughly with the rest of the air. The placement of these ducts is a difficult issue, and we have drawn up schematics of the house to plan their exact positions. These can be seen in the picture above. Where possible, the ducts will run (invisibly) inside the floors, but sometimes exposed boxing is unavoidable.

On very cold days the house will require a little heating, and the air in the ventilation system will be warmed by a small heat pump before flowing into the ducts. This means that no radiators, stoves or electric heaters are required. Certain family members (i.e. mothers) have raised their eyebrows about the idea of not having central heating…we’re hoping they’re wrong and we’re right!

Impressively, the same Genvex system also heats the water when there is not enough sunshine for the solar panels. Thus the “Combi” unit provides fresh air, extractor fans, heating and hot water; all in one box!