Ventilation
MVHR heat recovery saving and fan energy
- where
- heat recovered per degree (W/K)
- continuous ventilation rate (l/s)
- heat recovery efficiency (%)
- annual heat recovered (kWh)
- heating degree days per year (K·day)
- annual heating fuel saved (kWh)
- heating system efficiency (%)
- annual fan electricity (kWh)
- specific fan power (W per l/s)
Explain this equation
Estimates how much heat an MVHR unit recovers in a year, the heating fuel it saves, and the electricity its fans use.
Litres per second times 3.6 gives m³/h of ventilation air, and 0.33 converts that into watts per degree. The heat exchanger returns η percent of it. Over the year that becomes kWh via degree days and 24 hours. Fuel saved is that heat divided by the boiler or heat pump efficiency. Fans run all year: specific fan power in watts per l/s times the flow gives watts; times 8760 hours, divided by 1000, gives kWh.
Ventilation heat loss formula with a recovery fraction, plus fan power times running hours.
Sense check: 31 l/s at 85% recovers about 31 W/K, about 1,500 kWh a year at 2000 degree days; the fans use about 160 kWh.
Watch out: MVHR only pays off in an airtight home. In a leaky house most heat escapes around the system, not through it.
Estimates the heat an MVHR unit recovers compared with extracting the same air without recovery, and the electricity its fans use running all year.
Enter your values to see the result.
Good to know
Recovered heat is the efficiency share of the mechanical ventilation loss, which is how SAP treats MVHR (effective air change = n_adj + n_mech × (1 − η)). Use the in-use efficiency, not the lab figure: SAP multiplies the PCDB efficiency by an in-use factor that allows for duct losses. qv is typically the Approved Document F whole dwelling rate. Fan power runs 8760 hours a year. Compare the fuel saved (at gas price) with the fan electricity (at electricity price).
Common mistake
Claiming the saving against natural ventilation in a leaky house. MVHR only pays back where the dwelling is airtight (roughly q50 of 3 or less); otherwise most air bypasses the unit.
Limits
Simple annual degree-day estimate. Ignores summer bypass, frost protection and duct losses beyond the in-use efficiency.
Sources
- BRE, SAP 10.2 The Government’s Standard Assessment Procedure for Energy Rating of Dwellings (AP50 ÷ 20 infiltration estimate; shelter factor (20) = 1 − 0.075 × sheltered sides; (38) = 0.33 × n × V; (36) = 0.20 × exposed area where thermal bridging details are unknown; (39) HTC; (40) HLP = HTC ÷ floor area; MVHR effective air change n_adj + n_mech × (1 − η))
- HM Government, Approved Document F Volume 1: Dwellings, 2021 edition (para 1.24 and Table 1.3 whole dwelling ventilation rates; Table 1.7 background ventilators; para 1.64 continuous extract; paras 1.45–1.46 airtightness; Section 3 existing dwellings)
- DESNZ, Energy Trends section 7: weather (heating degree days, table ET 7.1)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.