Choosing radiators from a room heat loss (worked example)

A room’s emitters must replace its calculated heat loss at the chosen design conditions. Select them using the output available at the intended water and room temperatures.

Worked example. The design heat loss is 1.8 kW. After checking the manufacturer’s data for this system’s operating temperatures, each radiator under consideration will provide 1.2 kW.

1. Check the usable output first. A catalogue rating is often for a different water-to-room temperature difference, so apply the correction factor, or use the manufacturer’s output table, before you rely on it. In this example, the 1.2 kW figure is already for the intended conditions: do not correct it again.
2. Find the quantity needed. Divide the room duty by the usable output: 1.8 kW / 1.2 kW = 1.5 radiators of this size.
3. Select a practical arrangement. Two of these radiators provide enough total output, or choose one different radiator delivering at least 1.8 kW under the same conditions. Check the space, connections, flow and room control as part of the design.

Lower water temperatures cut a radiator’s output, but an existing radiator may still have capacity to spare: calculate before deciding it must be bigger. For a new design, assess the room heat loss and the corrected outputs together.

Where the radiator goes

Purmo’s buyers guide puts a radiator on the outside wall under the window wherever possible: cold air coming in at the window is warmed by the radiator below it and carried round the room. That mattered most with single glazing; with modern glazing other positions can work, as long as air can move freely round the radiator. The guide’s minimum gaps are 10 cm between the floor and the radiator’s lower edge, and 10 cm between its top edge and the window sill. So the space between the sill and the floor sets the radiator’s height. A radiator that long curtains will cover should be noticeably bigger, because covering it reduces the heat it gives out.