Heat loss
Ventilation heat loss from air change rate (0.33 n V ΔT)
- where
- ventilation heat loss (W)
- volumetric heat capacity of air (W h/m³·K)
- air change rate (air changes per hour)
- room volume (m³)
- internal design temperature (°C)
- external design temperature (°C)
Explain this equation
Works out the heat needed to warm the outside air that leaks or is ventilated into a room. Use it alongside fabric loss for room heat-loss calculations.
n is how many times per hour the room's air is replaced, V is the room volume, so n V is cubic metres of air per hour. t_i − t_e is how much that air must be warmed. 0.33 is the heat needed, in watt-hours, to warm one cubic metre of air by one degree.
The 0.33 comes from air density (about 1.2 kg/m³) times specific heat (about 1.0 kJ/kg·K) divided by 3600 to change kJ per hour into watts.
Sense check: a 50 m³ room at one air change per hour and a 24 K difference loses about 400 W, often similar to its fabric loss in an older house.
Watch out: with heat recovery, multiply by (1 − efficiency); do not double count infiltration and mechanical extract.
Heat needed to warm incoming outdoor air from infiltration or ventilation, from the room’s air change rate. Add it to the fabric loss to get the room heat loss.
Enter your values to see the result.
Good to know
0.33 W h/(m³·K) is the volumetric heat capacity of air (1.2 kg/m³ × 1.0 kJ/(kg·K) ÷ 3600). Typical design air change rates are given in CIBSE Guide A; a well-sealed modern room may be 0.5 ach, an older draughty room 1.5–2 ach or more. Heat recovery reduces the effective rate by its efficiency.
Common mistake
Using a mechanical extract rate on top of infiltration as if both brought in separate outdoor air, or forgetting to reduce for MVHR heat recovery.
Limits
Assumes all incoming air is at the external temperature. For the full method and minimum air flows use BS EN 12831-1; for ventilation rates see Approved Document F.
Sources
- The Open University, OpenLearn: Energy in buildings, section 3.3 (ventilation heat loss Qv = 0.33 n V ΔT; 0.33 Wh/m³K is the volumetric heat capacity of air)
- CIBSE Guide A: Environmental design (2015), chapters 5 and 6 (thermal design, internal heat gains from people, lighting and equipment)
- BS EN 12831-1:2017 Energy performance of buildings — Method for calculation of the design heat load (transmission heat loss H = ΣU·A, ventilation heat loss from air volume flow)
- Approved Document F: Ventilation, Volume 1 Dwellings and Volume 2 Buildings other than dwellings (2021 edition incorporating 2022 amendments)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.