Turning a room heat-loss figure into the number of radiators needed, and why output ratings must be corrected.
A radiator has to put back the heat a room loses to the outside, so once you know a room’s heat loss you can choose emitters to match. This example shows the method with chosen figures.
The situation. A room has a calculated heat loss of 1.8 kW. The radiators we are considering are each rated at 1.2 kW at the system’s operating conditions.
Step 1 — output needed. The emitters must supply at least the heat loss, so we need at least 1.8 kW of radiator output in that room.
Step 2 — number of radiators. Divide required output by output per radiator: 1.8 kW / 1.2 kW = 1.5. You cannot fit half a radiator, so round up to 2 radiators (or choose a single larger radiator rated 1.8 kW or more).
Step 3 — correct the rating. Quoted radiator outputs assume a specific temperature difference between the radiator and the room. If your system runs at a lower flow temperature than that test condition, each radiator gives out less than its quoted figure, so you must apply the manufacturer’s correction factor and may need more or larger emitters.
Takeaway. Emitter sizing is heat loss divided by corrected output per emitter, then rounded up. Heat loss tells you what to replace; the corrected rating tells you what a real radiator delivers in your system. Use manufacturer data and current Part L guidance for an actual design.