How much heat does an MVHR recover?

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.

What is the supply air temperature from an MVHR?

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.

Does MVHR save money after accounting for fan electricity?

Yes, in well-sealed dwellings. A typical MVHR uses 30-60W of fan power (500-530 kWh/year electricity) 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.

How long is the UK heating season?

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.

What is the minimum efficiency for an MVHR under Building Regulations?

Approved Document F recommends MVHR units with at least 70% heat recovery efficiency, though SAP calculations reward higher efficiencies. Most modern MVHR units achieve 85-95% according to the Passivhaus Institute test protocol. The unit should also meet SFP limits of 1.5 W/(L/s) for balanced systems.

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Heat Recovery Calculator — MVHR Energy Savings

Calculate supply air temperature rise, annual energy recovered, and running cost savings from MVHR heat recovery ventilation systems.

Supply Air Flow Rate

Total supply air through MVHR unit

Heat Recovery Efficiency

Manufacturer-stated thermal efficiency

Indoor Temperature

Outdoor Winter Design Temp

Average winter outdoor temperature

How We Calculate This

This calculator estimates the energy recovered by a Mechanical Ventilation with Heat Recovery (MVHR) system by calculating the heat transfer from extract air to supply air through the heat exchanger.

Calculation method

Heat recovered (W) = air density (1.2 kg/m³) × specific heat (1005 J/kgK) × flow rate (m³/s) × temperature rise. The temperature rise equals the heat recovery efficiency × the indoor-outdoor temperature difference.

Annual energy

The annual energy recovered is the peak heat recovery multiplied by the heating season hours. Fan energy consumption is subtracted to give the net energy saving. Gas cost saved minus electricity cost for fans gives the net financial saving.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.