Radiator sizing is based on the heat loss of each room — a calculation involving room volume, insulation level, window area, and heat loss through walls. Getting it wrong means rooms that never reach temperature regardless of boiler size.
Signs your radiators are undersized
- Rooms that never get fully warm on cold days
- Boiler running continuously without reaching thermostat temperature
- Cold spots in rooms with large glass areas
Why radiators become undersized
Extensions add new rooms with small radiators. Single glazing replaced with double glazing (reduces heat loss — usually increases radiator capacity). Old radiators replaced like-for-like without sizing check. Properties with poor insulation that has been addressed post-install.
Basic radiator sizing
A standard double panel radiator (600mm × 1000mm) produces approximately 1,800 BTU/hr (530 watts) at standard conditions. A 3m × 4m × 2.4m living room in a well-insulated property typically needs 1,800–2,400W of radiator output. Older poorly-insulated homes need more.
Getting it properly checked
We include a room-by-room heat loss calculation and radiator check as part of all new boiler surveys. If we identify undersized radiators, we'll quote to replace them as part of the installation. Book a free survey.