Mechanical Ventilation With Heat Recovery

Mechanical ventilation with heat recovery, usually called MVHR, supplies fresh air to living rooms and bedrooms and extracts stale air from kitchens and bathrooms through a central unit, passing the two air streams through a heat exchanger so outgoing warmth pre-warms incoming air without mixing the streams. In a well-sealed home, that recovered heat makes continuous ventilation affordable in energy terms, because the home is not simply throwing warm air outside and heating cold replacement air from scratch.

MVHR suits airtight homes best, because in a leaky home much of the air bypasses the system through gaps, so the heat recovery benefit falls and the fan energy is partly wasted. It also suits homes where opening windows is impractical, such as noisy streets, polluted areas or homes with occupants who need filtered air, since the unit filters incoming air and delivers it continuously.

Ducting is the part that decides whether MVHR works well or disappoints. Supply ducts run to living rooms and bedrooms, extract ducts run from wet rooms, and the layout should be short, smooth and accessible for cleaning, with the unit sited where filters can be changed easily and condensate can drain. Rigid ducting generally performs better than flexible ducting, which can sag, kink and collect dust, and every bend and long run adds resistance that the fans must overcome, increasing noise and energy use.

Controls range from simple boost switches to humidity and carbon dioxide sensors, timers and summer bypass modes that let cool night air in without heat recovery when the home is warm. Frost protection matters in cold climates, since the heat exchanger can freeze if incoming air is very cold, and the unit should have a documented frost strategy, whether a pre-heater, a bypass or a defrost cycle.

Check the home airtightness, the space for the unit, usually in a loft, utility or cupboard, the duct routes and ceiling voids, and the noise implications for bedrooms, since a unit or duct that hums at night will be switched off. Ask for the design airflow rates per room, the heat recovery efficiency, the specific fan power and the filter types and change intervals.

Also check the electrical supply, the condensate drain route and whether the system includes a summer bypass and a boost mode for cooking or showering. A proposal that gives only a unit model, without room airflow rates and a duct layout, has not yet designed a system.

In the EU, MVHR design and installation follow national building and ventilation regulations under the EPBD framework, with member states differing on required airflow rates, heat recovery efficiency minima and filter classes, especially for homes in polluted or pollen-heavy areas. Some countries require MVHR or an equivalent in new airtight homes, while others accept other ventilation strategies.

In the UK, Approved Document F covers mechanical ventilation with heat recovery for England, with devolved equivalents elsewhere, and CIBSE guidance covers design practice in more detail. Building regulations, electrical safety and condensate drainage all apply, and commissioning, including airflow measurement per room, should be documented for the homeowner.

A common mistake is fitting MVHR to a leaky home without first improving airtightness, so the system recovers little heat and the home still feels draughty through gaps. Another is using long, flexible duct runs with many bends, which reduce airflow, increase noise and make cleaning difficult, so rooms at the end of long runs are under-ventilated.

A third mistake is neglecting filters: a clogged filter reduces airflow, strains fans and can worsen air quality, yet filter changes are often forgotten because the unit sits in an inaccessible loft. If filters cannot be reached easily, the system will not be maintained, however good its design.

Keep the duct layout drawing, the room airflow rates, the commissioning measurements, the filter types and change schedule, the unit manual and the summer bypass and boost instructions. Those papers let a later engineer service the system and a later owner understand how to use it.

Ask a qualified installer: what airflow is designed for each room, what heat recovery efficiency and fan power are expected, how are filters changed and how often, and what frost protection and summer bypass are provided. Ducting and commissioning answers show a real MVHR design.