Heat Loss Calculator — Room-by-Room Heating Requirements

UK heat loss calculations typically use an internal design temperature of 21°C and an external design temperature of -1°C (for most of England and Wales), giving a ΔT of 22K. Scotland and exposed locations may use -3°C or -4°C externally. These are standard conditions from CIBSE Guide A.

The total heat loss in Watts tells you the minimum radiator output needed. Always round up to the next available radiator size. For example, if the heat loss is 1,450W, choose a radiator rated at 1,500W or more. Note that radiator output is usually quoted at ΔT50 — check the manufacturer's data for your specific flow temperature.

Modern well-sealed homes with mechanical ventilation (MVHR) typically have 0.5 air changes per hour (ACH). Average homes are around 0.75-1.0 ACH. Older draughty homes can be 1.5 ACH or more. Note that Approved Document F 2021 sets whole-dwelling ventilation in litres per second (13 l/s plus 6 l/s per extra bedroom, or 0.3 l/s/m² of internal floor area, whichever is greater) and purge ventilation of 4 ACH — it does not set a 0.5 ACH minimum. The 0.5 ACH figure used here is a typical infiltration assumption for heat-loss modelling, not a regulatory limit.

High heat loss is usually caused by poor wall insulation (especially uninsulated solid walls at 2.1 W/m²K), single glazing (4.8 W/m²K), uninsulated lofts, or high ventilation rates from draughts. Improving wall and loft insulation, upgrading windows, and draught-proofing are the most effective ways to reduce heat loss. Use our Cavity Wall Insulation Calculator or Draught-Proofing Calculator to plan improvements.

This calculator gives the steady-state heat loss, which is suitable for continuous heating. If you heat your home intermittently (e.g., timer-controlled), you may need to add 10-20% extra capacity for the warm-up period. Consult a heating engineer for precise sizing of intermittent heating systems.