Heat Recovery Calculator — MVHR Energy Savings

A high-efficiency MVHR unit recovers 85-95% of the heat from extract air. For a typical 3-bed UK house with 60 L/s ventilation, this equates to approximately 3,000-5,000 kWh/year — roughly equivalent to a third of the total heating demand. The actual saving depends on outdoor temperature, ventilation rate, and unit efficiency.

With 90% heat recovery efficiency, outdoor air at 2°C (typical UK winter) and indoor extract at 20°C, the supply air is delivered at approximately 18.2°C — only 1.8°C below room temperature. This dramatically reduces the heating load compared to unheated outdoor air ventilation.

Yes, in well-sealed dwellings. A typical MVHR uses 30-60W of fan power (roughly 260-530 kWh/year of electricity at year-round running) but recovers 3,000-5,000 kWh/year of heat. Even at higher electricity rates, the net saving is substantial. However, in leaky older buildings, MVHR may not be cost-effective as infiltration already provides ventilation.

The UK heating season is typically 200-220 days (October to April) with heating degree days varying by region. Southern England averages around 2,000 degree-days, while Scotland may exceed 2,500. The calculator uses heating season length to estimate annual energy recovery.

Approved Document L Volume 1 (Dwellings) 2021, incorporating the 2023 amendments (para 6.56), requires every unit that provides both supply and extract ventilation to have a heat-recovery system with a minimum efficiency of 73%, a summer-bypass facility and a variable-speed controller. The specific fan power for a balanced supply-and-extract system must not exceed 1.5 W/(l/s) (para 6.55(d)). Most modern MVHR units exceed the 73% minimum, achieving 85-95% on the Passivhaus Institute test protocol.