Ventilation
Ventilation and infiltration heat loss
- where
- ventilation heat loss coefficient (W/K)
- air change rate (per hour)
- internal volume (m³)
- heat loss at this temperature difference (W)
- inside to outside temperature difference (K)
- annual ventilation heat loss (kWh)
- heating degree days per year (K·day)
Explain this equation
Works out the heat lost by warming up incoming air from ventilation and draughts, per degree, at design conditions and over a year.
n V is the volume of air replaced each hour. Warming one cubic metre of air by one degree takes about 0.33 watt-hours, so 0.33 n V is the watts lost per degree. Times the temperature difference gives the loss now; times degree days and 24 hours, divided by 1000, gives kWh a year.
Heat to warm a flow of air is its volume flow times its volumetric heat capacity times the temperature rise.
Sense check: 0.6 air changes in a 250 m³ house is about 50 W/K, about 1 kW at 20 K difference and about 2,400 kWh a year at 2000 degree days.
Watch out: cutting draughts saves energy, but you still need enough planned ventilation for air quality and moisture.
Works out how much heat is lost by air leaking and being ventilated out of the dwelling, as a rate and over a heating season.
Enter your values to see the result.
Good to know
0.33 Wh/(m³·K) is the volumetric heat capacity of air (about 1.2 kg/m³ × 1005 J/(kg·K) ÷ 3600) used in SAP line (38). Use the infiltration rate from the ÷ 20 estimate plus any mechanical ventilation. Degree days for your area are published by DESNZ (Energy Trends table ET 7.1) and degree-day services; 2000 K·day is a round figure for much of England.
Common mistake
Plugging in n50 instead of the everyday air change rate, which overstates the loss about twentyfold.
Limits
Steady-state. Ignores heat recovery (see the MVHR calculator) and solar or internal gains.
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))
- 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.