How MVHR works
An MVHR system continuously extracts stale, moist air from kitchens and bathrooms and supplies fresh air to habitable rooms, passing both streams through a heat exchanger. Up to ~90% of the heat in outgoing air transfers to the incoming supply — ventilation without the heat penalty of trickle vents and extract fans.
The payoff: constant filtered fresh air, no condensation problems, and lower heating demand.
The airtightness rule of thumb
MVHR only makes sense in airtight dwellings. If the building leaks, uncontrolled infiltration delivers (cold) fresh air regardless and the heat exchanger achieves little — wisdom is that MVHR starts earning below roughly q50 = 3, and the tighter the better. That makes design-stage commitment essential: the air test must deliver what the SAP calculation assumes.
In the SAP calculation
SAP models MVHR through the system’s heat-exchange efficiency and specific fan power, balanced against the dwelling’s airtightness. In a tight dwelling it cuts ventilation heat loss substantially and helps the DER and DFEE; in a leaky one it can model worse than natural ventilation once fan energy is counted.
Duct design matters too — short, rigid, insulated runs. Get the SAP assumptions and ventilation design agreed together; we can advise on both. Ask us.