​​Sizing radiators for a heat pump: why correction factors matter

Every radiator comes with a heat output figure, quoted in watts, BTUs (British Thermal Units), or both. Heat output is mostly determined by surface area rather than overall size, so you’ll find large radiators with a modest output and smaller ones, packed with fins, tubes and convectors, that produce substantially more heat. The best way to check whether a radiator meets the requirements of a room is to use a heat calculator.

In 2013, UK industry standards for calculating heat outputs were brought in line with European regulations, specifically BS EN 442, with the aim to reflect newer, modern boilers that were able to work more efficiently at lower temperatures. This means all radiators should now be tested and their heat output displayed using a uniform set of rules, making it easier to compare one radiator with another.

Delta T equals (the flow temperature of the water leaving the boiler minus the expected return temperature of the water returning to the boiler) divided by the desired room temperature.

As an example: if hot water leaves your boiler at 75°C, and returns at 65°C, and you want to achieve 20°C in your room, then:

Find the average radiator temperature = (75°C + 65°C)/2 = 70°C

Delta T = 70°C – 20°C = 50°C

As a result, we have our Delta T 50, and it should be on this basis that all radiator heat outputs are expressed at. If they are not, you need to be wary. A higher Delta T, such as at ‘T 60’ is suggesting you will get more heat than the boiler is set up to provide. This might result in a room being underheated or energy bills not being managed efficiently.

To allow a modern gas-fired boiler to operate at its most efficient, the flow temperature of the water leaving it will be set between 60°C and 70°C. This allows the boiler to be in ‘condensing mode’, reusing some of the heat in the system rather than venting it, which you might recognise as that hot haze effect on cold mornings by the side of the house. Getting the boiler temperature settings right helps bring down energy costs by reducing wasted heat.

If the heating system is operating at a flow temperature of 55°C and a return temperature of 45°C to achieve a room temperature of 20°C (a Delta T of 30°), the Strada Efficiency correction factor is 0.50. This means a Strada radiator producing 1,000 watts on a normal heating system is likely to provide only 500 watts at the lower temperature. That might mean a radiator twice the size is needed, or perhaps two radiators in different parts of the room.

With this in mind, a Type 22 steel panel radiator 500mm x 1000mm would produce around 750 watts at Delta T 30°. A similar output on the Strada Efficiency would require a radiator 500mm x 900mm. Or, a Strada Hybrid, with its built-in fan system, can produce this amount of heat from a radiator just 500mm x 600mm.

Understanding how to get the best from your heating system can be tricky, especially if it’s based on an air source or ground source heat pump. Getting the radiator sizing right from the start makes a real difference, and our team are used to working through exactly this kind of thing.

Visit your nearest The Radiator Centre showroom to talk it through, bring your plans and let us help you find the right radiators for your heating system and room.