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What is a heat pump?

What are Heat Pumps, are Heat Pumps a good investment and will a Heat Pump heat my house?

Heat pumps work by extracting small amounts of energy from a low level heat source in the same way that a fridge extracts heat from its inside. There are two types - ground source heat pumps (GSHPs) that use pipes filled with a heat exchanger fluid to extract energy from the ground or water, and air source heat pumps (ASHPs) that absorb energy from the outside air. This energy, converted to heat, can then be stored in a thermal store and used to run underfloor heating systems, radiators and to contribute to domestic hot water. Broadly speaking GSHPs are more expensive to install but work more efficiently, ASHPs are cheaper to put in but work less efficiently when it gets cold outside.

I met an engineer at a show recently who told me that he reckoned 90% of heat pumps that have been put in have either been incorrectly spec'ed or badly installed. I trust that that is an exaggeration but from the number of people who have heat pumps and tell us of having very high electricity bills, I suspect that there is certainly some truth in it.

Let's make one thing clear first - when people say that they are heating their homes with a heat pump that isn't so. They are heating their home with electricity and the heat pump is simply (or should be) making that process more efficient. A Heat Pump uses electricity to run but it should be producing more kW of heat energy than it has used kW of electricity.

The limitation of Heat Pumps is that they are very efficient at producing warm water but become less efficient as they are required to produce higher temperature water. For this reason they work well with highly insulated houses but not with dwellings that have a higher heat demand. They work well with underfloor heating, which runs at much lower temperatures, but don't work so well with radiators (or the radiators will need to be a great deal bigger to compensate for the lower temperature of the circulating water). If they are used to try and produce a tank of domestic hot water then they can become very inefficient and it is usually better that the water temperature is boosted by a separate energy source. The gain with a heat pump is in raising the temperature of mains cold water from 10° up to 40°. Even approaching 50° the efficiency of the heat pump will be falling fast.

A domestic hot water tank would normally be at around 60° to kill any water-borne bacteria and so that it can give sufficient hot water for long enough to power a shower, run a bath etc. A thermal store should have the water at the top of the tank at around 80° since the heat for the domestic hot water (DHW) is largely going to be taken from above the baffle (i.e. the top third of the tank) and of course it must be borne in mind that the heating will also be being taken from the same tank. It does depend on the size of the thermal store, since a larger thermal store can obviously maintain a DHW flow for longer before the heat in the tank is depleted and so a lower tank temperature is sufficient.

In conclusion, heat pumps are generally only worth it on well insulated houses that don't have a high heat demand. A large thermal store is essential to give enough buffer for the heating and to provide plenty of domestic hot water. If the DHW section is too small then higher temperature water will be needed to compensate for that, leading to very inefficient use of the heat pump. So heat pumps are a clever invention and can save a lot in heating and hot water costs, but they are not a quick fix and only work if they are specified and installed correctly on the right type of house.

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