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Radiator Heat Outputs Explained

01 July 2022

Calculating correct radiator panel heat output is dependent on using industry standard inlet and outlet temperatures

Modern boilers operate more efficiently at lower temperatures. By modulating their output they achieve optimum efficiency at a lower max temperature than older style boilers. Hence when a panel is being sized up to meet a room’s heating requirements the mean temperature used in calculations is of significant importance.

Old atmospheric type boilers heated water to a high temperature as they were unable to self modulate and constantly needed to on-off cycle. Calculating with a high average ‘mean’ temperature only applies to these older, inefficient type boilers, giving a false impression of heat outputs; both unachievable and undesirable using today’s boiler technology.

Rads Output

The Anatomy of Heat Output

  • The mean temperature is the average water temperature within the radiator, based on the inflow and outflow water temps. The output of the radiator is the amount of heat emitted in Watts (W)
  • If radiator sizing is based on a high inlet temperature, modern day boilers will not supply water at that temperature and the radiators won’t provide enough heat for the room areas
  • The kW output of the boiler should be matched to the system requirements
  • Using an oversized boiler does not improve heating, it simply results in the boiler constantly on-off cycling as its output is too great for the size of the system
  • A correctly sized boiler maximises it’s modulation ‘turn down’ capacity to maintain constant temperature without off cycling, thus consuming less energy and providing more efficient heating
  • Optimal heating is achieved at the lowest possible temperature. Using constant modulated heat, condensing boilers provide the most effective heating at a low mean temp of 60Cº

Different Panel Size Outputs

Convection fins in a radiator act as the heat dissipating mechanism, with each layer of fins being connected to a panel through which hot water flows. Heat is transferred from the panel to the fins and then into the air, known as ‘thermal convection’ heating. Single panel radiators are ideal for heating small spaces or rooms, Double and Triple panel types have greater heat output to suit larger rooms or areas with high ceilings.

Heat Loss

Output Calculators

A heat load calculator is used to assess heat output requirements. By inputting the panel type and correct inlet/outlet temperatures the output calculator displays the corresponding radiator heat output to suit room size.

View the Henrad online heat load calculator: Henrad Radiator Heat Load Calculator

System Components

System components such as well insulated pipe, correct pipe diameter, correctly sized modulating pumps and external temperature sensor to compensate boiler operation for ambient temperature also play a role in heat output.

Heat Pump Systems

If you intend to use a heat pump instead of a boiler as the heat source, note that heat pumps typically only achieve a maximum water temperature of 55°C. This is ideal for low temperature systems such as floor heating but is not suited to high temp panel radiator systems. Heat pump boosters or oversized radiators can be used to achieve the required temperature but consume greater energy resource and additional cost.


Always use heat outputs calculated at the EN442 standard of 70°C mean water temperature (80°C inlet, 60°C outlet, 20°C room). Outputs based on higher mean temperatures of 80°C or more artificially create higher output figures that are typically unattainable in real world installations. Distorted heat outputs give a false impression of radiator sizing for room areas and can lead to incorrect installation and radiators that do not adequately heat rooms. Different boiler types, radiator panel types and other system components also greatly impact heat output.

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