Design Temperature Calculator — UK

Use the CIBSE Guide A 2015 (Table 2.5) external design temperature for your nearest weather station. The 99.6% (design) figures used by MCS for heat-pump sizing include London (Heathrow) -3.0°C, Birmingham (Coleshill) -5.1°C, Manchester (Woodford) -4.5°C, Newcastle -3.7°C, Cardiff -3.1°C, Belfast -3.2°C, Edinburgh -5.4°C, Glasgow -5.6°C and Plymouth -1.5°C. A milder 99% percentile is also tabulated. The 99.6% value is exceeded for only about 0.4% of the year.

For newer (post-2000) dwellings the CIBSE Domestic Heating Design Guide (2026, Table 2-2) uses 21°C throughout, with bathrooms at 22°C — so living room 21°C, bedroom 21°C, kitchen 21°C, hallway 21°C, office/study 21°C, bathroom 22°C. Older age bands sit halls, kitchens and WCs at 18°C. These are the temperatures the heating system should achieve at external design conditions.

CIBSE Guide A and the MCS method reduce the design temperature by 0.6°C for every whole 100m the property altitude exceeds the reference weather station. A property at 300m above sea level near Birmingham (Coleshill -5.1°C) would use -5.1°C minus 1.8°C = -6.9°C. This correction matters for properties in hilly or upland areas.

ΔT (delta-T) is the temperature difference between inside and outside design conditions. It drives the heat loss through the building fabric and ventilation. For example: 21°C internal minus -5.1°C external = 26.1°C ΔT. The higher the ΔT, the greater the heat loss and the larger the heating system needed.

Total heat loss = fabric loss + ventilation loss. Fabric loss = U-value × area × ΔT for each building element (walls, floor, ceiling, windows). Ventilation loss = 0.33 × room volume × air changes per hour × ΔT. The total determines the radiator/heat emitter size needed. Use our Radiator Sizing Calculator or Radiator BTU Calculator to size emitters for each room.