Sometimes it is said that heat pumps only work with underfloor heating systems. This is not true. Heat pumps work very well with radiators provided the system is properly designed. We have numerous examples of houses (including our Chairman’s and Managing Director’s) running very comfortably with heat pumps supplying the radiators.
What is true is that a heat pump will be at its most efficient when the temperature of the water it heats is kept as low as possible. It is also true that good underfloor heating systems are designed to run at low temperatures. But not all underfloor heating is good. A badly designed system will cut corners by using less pipe in the floor, may be poorly insulated, be in bad thermal contact with the floor, and have a thick floor covering. Such a system can easily perform worse than radiators.
Radiators that are matched to a heat pump need to be large enough to put out enough heat at the temperatures of up to about 40-50oC at which heat pumps are efficient. This is sometimes described as “over sizing” Ϯ the radiators. But since it has been the habit of the plumbing industry for many years, driven by the inadequacy of traditional controls system, to install substantially over sized radiators, most houses can be converted to heat pumps with very little need for remedial work to radiators. Over the years we also have found that traditional cast iron radiators not only look very good but perform as well and often better than modern pressed steel radiators.
So when we install a heat pump into a house, the first stage of our design process involves a survey of the house and its heat emitters to identify whether any changes may be needed.
Note Ϯ: When radiators are sold they are described as having a nominal output value in kW. This is how much heat power they will output with a flow temperature of about 70oC. If the flow temperature is reduced to about 50oC they will continue to function perfectly well but will output less heat. So to get the same output power at the lower temperature the radiator needs to be larger – hence “over sized”.
