What are Heat Pumps?
Heat pumps are a relatively simple technology and one of the best known and oldest of the renewable energy technologies. Having said that, it is not a true renewable energy technology – they run on electricity, so unless you get your electricity from a renewable source it is still fossilfuel based. It is, however, a very efficient way to use electricity.
Heat pumps are a hybrid heat source. They are run entirely on electricity but they capture additional energy from the external environment, most commonly heat from the garden space around the house. They take a few degrees from this low-grade heat and transfer the energy into a smaller volume of water at higher temperatures inside the house. The efficiency of heat pump systems is measured by the coefficient of performance (CoP). This is the ratio of units of heat output for each unit of electricity used.
Typical CoPs range from 2.5 to 4. The critical factors determining the efficiency of the system are the performance of the heat pump itself and the input and output temperatures: the narrower the range of temperature between the source and the house, the more efficiently the system runs. However, heat pumps continue to work effectively even at sub-zero temperatures.
The popularity of heat pumps has increased dramatically over the last few years, especially as an option for off-mains heating. There are two factors behind this. Firstly, the technology has become far more robust: the early versions were unreliable and often failed to deliver as promised. Secondly, the cost of heating oil and liquid petroleum gas have skyrocketed, causing people to think much more seriously about other options. The heat pump, and in particular the ground source heat pump, appears to be a major beneficiary.
3 Types of Heat Pumps
Water Source Heat Pumps
Usually the most efficient, water-source heat pumps draw energy from a body of water such as a stream and often have a COP of five or more. If you have access to running water this system is probably viable; if not, at least one borehole will be needed.
Ground Source Heat Pumps
Heat pumps that use energy drawn from the ground are the most commonly installed machines, with a COP of four quite achievable. A complete system including the heat pump, ground pipes, controls etc. will cost £12,000 on average. Installation will vary with ground conditions, but allow £2-3,000.
Air Source Heat Pumps
Those heat pumps that draw energy from the air tend to be the lowest cost (around £7,000 plus installation). A COP of 3.5 is about the best that can be expected, falling in winter.
The all important coefficient
The efficiency of a heat pump is stated as the Coefficient of Productivity (COP) and there is more confusion around this figure than any other issue. The Energy Saving Trust suggests that we should look for a COP of 4 — which means that for every 1kW of electricity the machine consumes, it produces 4kW of heat. Manufacturers’ COP figures range from 2.2 to as high as 6. All of which are probably true, but not in the way we may think.
The COP is calculated on the relationship to the heat output and electricity delivered to the compressor pump. An immersion heater, or any other equipment used to increase the output temperature, is not included in the calculation. It also relates to the difference between the source temperature (that is the temperature of the air, ground or water we take heat from) and the delivered temperature (that is the temperature of the air or water used to heat the house). The closer those two temperatures are together, the less heat that needs to be extracted — and the higher the COP.
Take the Dimplex range for example. This is a reputable manufacturer that gives good, if not clear, information. Its LA-TR air-source range offers the following figures:
|Air Temp||Water Temp||Heat Output||COP|
Looking at the first line, what these figures mean is that with a heat output of 8.7kW, and outside air temperature of -7°C and a delivered water temperature of 35°C, the machine delivers a COP of just 2.5. Increase the heat output or the water temperature and the COP falls. By comparison the Dimplex WI-TE range of water source heat pumps give these figures:
|Source Water Temp||Internal Water Temp||Heat Output||COP|
Notice that the COP in this case varies from 3.6 to 5.2 and that the difference lies in the delivered water temperature. When the delivered water temperature (that is the water temperature we ask the heat pump to produce) rises to 50°C the COP falls to 3.6.
Which brings us to the problem. Heat pumps work best when they are only moving heat from one place to another. Ask them to do more than that and the efficiency drops. To put it simply, underfloor heating needs water at around 35°C and heat pumps work well at that temperature. Ask the heat pump to deliver domestic hot water at 55°C and the COP falls dramatically.
Whilst heat pumps can most definitely reach a COP of 6 plus, in reality when asked to provide water temperatures above 35, their coefficient falls sharply. As an example, a Barratts development in Chorley, Lancashire, showed that the average COP for groundsource heat pumps was just 2.6, probably because they were being asked to provide domestic hot water as well as space heating. Depending on how they are used therefore, heat pumps are not necessarily as efficient as some manufacturers suggest.
Why do manufacturers do it?
In short, because we want them to. To get good efficiency from any type of heat pump means running them with a low output temperature — below 40°C. And that means that they cannot be used for radiators or for domestic hot water, both of which have to be 50°C or above. This is inconvenient and means that the homeowner will need another heat source. The manufacturers try to be all things to all men and deliver everything we want in one package. The result is an apparently useful package that does everything, except deliver good efficiency. And heat pumps are all about efficiency.
So we have adverts that suggest a heat pump will deliver a COP of 5 and a hot water temperature of 50°C — both of which are true, but not at the same time! Suppliers advertise savings of 70% on heating bills, adjacent to an image of a beautiful young lady relaxing in a hot bath. The implication being that you can have both. Which, of course, you can, but you won’t get that 70% saving when you are heating water for the bath.
A manufacturer’s opinion
“We’ve been manufacturing heat pumps in the UK since 1999 and are well respected for providing honest advice on heat pumps. Here’s what we think your readers should consider:
- Insulation is the key to any good heat pump application. The better insulated the building, the more efficient the heat pump, hence greater cost savings.
- Low output temperatures are a key issue with all heat pumps since low temperatures result in higher efficiency, hence the reason that heat pumps are ideally suited for UFH systems.
- All heat pumps have similar components and therefore similar performances, so large variations in claimed efficiency should be looked at carefully. Beware of inbuilt immersion heaters, as these will increase running costs when in use.
- DHW and radiators can both be serviced by heat pumps; however, due to the higher temperatures required this will be at a lower efficiency.
- Heat pumps, even at 400% efficiency, do not suit every application and great care should be taken in studying the financial justification for installing one where mains gas is available.
- Manufacturer history and pedigree should always be checked.”
When are heat pumps a good idea?
Only two things bear on whether heat pumps are a good idea: the heating distribution and the heat source. If the heating distribution is anything other than underfloor heating, then maybe think of installing a different boiler. Specifically if the idea is to replace a gas boiler with an air-source heat pump running radiators, then forget it. Heat pumps can cope with radiators, and skirting heaters, but they have to be very efficient radiators, over-sized, and the COP will still fall.