How to Specify Radiators for Low-Temperature Heating Systems

Low-temperature systems require the emitter and the room heat loss to be considered together. A radiator output quoted at one test condition cannot simply be assumed at another.

Start with room heat loss

Use a room-by-room design heat-loss calculation that considers external walls, glazing, ventilation, insulation and the required indoor temperature. A simple floor-area rule is useful only for an early estimate.

Compare outputs at the intended temperature

Ask for output data at the system design temperatures. The important temperature difference is between the radiator mean water temperature and the room air. As that difference falls, the output from a given radiator also falls.

Ways to recover output

• Select a longer or taller emitter where wall space permits.
• Consider additional panels, convectors or columns.
• Use more than one emitter in large or exposed rooms.
• Check that valves, pipework and balancing support the required flow.

Specification checklist

Record flow, return and room temperatures; calculated room heat loss; available wall dimensions; connection preference; and any projection limit. This gives a supplier enough information to propose a realistic starting point.