Ground source heat pump systems can be used with radiators which are typically oversized to deliver the appropriate flow temperatures to ensure the heat pump’s efficiency.
To get the most efficient operation from a heat pump it is important that the outlet temperature is kept as low as possible.
Heat pumps can be effectively used with radiators, however they are certain considerations that need to be taken into account.
45-50ºC flow temperature
Heat pumps with radiators are required to increase their flow temperatures to 45-50ºC, which reduces the Co-efficient of Performance (COP) and hence efficiency.
Oversized hot water coils are required
The larger the size of the coil within the tank, the better the heat transfer area and hence the better the DHW performance will be. Due to the low flow temperatures generated by the heat pump the hot water tank must have an oversized coil to provide the correct heat transfer.
Due to the low water content of radiator systems and hence low thermal storage, off-peak tariffs such as Economy 10 can not be effectively used and a flat rate tariff might be more effective.
Radiators in retrofit
Most retrofit applications will have a radiator heating system. In order to obtain heat from a radiator the outlet temperature of the heat pump needs to be increased to approximately 45°C to 50°C. As the outlet temperature of the heat pump increases the efficiency decreases; at 50°C the coefficient of performance is approximately 3, i.e. for every one unit of electricity used three units of thermal heat are produced. The reduction in efficiency will reduce the running cost benefit of the heat pump.
As radiators in retrofit properties are typically sized for a flow temperature of 71°C -82°C, at 50°C they may well be undersized and hence might need replacing. To understand what the maximum output of the radiators are at the reduced temperature and whether they are large enough, Kensa recommend you consult a heating engineer.
Things to note:
- Any microbore pipe to the radiators would need to be replaced.
- Off peak tariffs cannot generally be fully utilised as radiators do not have any storage capabilities.
While heat pumps benefit from a well insulated property, this isn’t always possible. Guy Cashmore, Kensa’s Technical Director, outlines alternative options to cope with higher heat losses.
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