Not so long ago Ground Source Heat Pumps, (GSHP) and their more urban cousin the Air Source Heat Pump (ASHP) were being touted as part of the answer to carbon reductions in the heating of houses. All boast something called the Co-Efficient of Performance (CoP). This is a equation that states the change in kWh between the energy consumed and the energy produced. So typically for every 1 kWh of electrical energy (needed to power the GSHP / ASHP) the system can theoretically produce between 3 and 4 kWh of heat energy. This sounds great. Surely this is ‘free’ energy.
Well, yes and no.
Yes. The fact that the systems use latent heat in the ground or air to extract heat for use in the home is great. This is especially the case if you are using electricity from renewable sources (mainly this means wind and hydro power as solar is not so plentiful when powering your heating system in the winter). As our mains electricity production changes over to ever more renewable sources of energy this bodes well. The system also produces its best CoP when producing lower temperature water, so it is ideal for underfloor heating or oversized / specialist radiators systems.
No. Most people are using mains electricity to power their GSHP and this is seen as being about 30% efficient, so the CoP of around 3.5 only changes the mains power back to its source fuels original potential. I.e. Gas burned in power stations has a potential of 100%, the power station’s processes and national grid’s network reduces this to around 30% of its original potential and then the GSHP changes this back to 100%. Would it not be better to have used the oil / gas etc in the home? Arguments can rage here around figures etc, but the overall principle is true at the moment.
There are some more problems as well that you will need to be aware of:
GHSP / ASHP are great for the lower temperature systems, but you need to have one, so ideally you will need to budget for changing over to an underfloor / oversized radiator system.
This lower temperature also means that if it is driving your Domestic Hot Water (DHW) as well it will need to reach 60 degrees once during the day to combat any chances of Legionnaires Disease. For this an immersion heater is generally used. So it is not a great energy saving system for your DHW. Linking in with a solar system will help during the summer months etc.
A recent Energy Saving Trust report looked at a wide number of GSHP and ASHP data to see how they were doing. The reading was not good. Most systems in the UK were operating at around 2.5 CoP, so well down on what was being touted by suppliers. This has been put down to the infancy of the technology in the UK. However these same products are working at around 3.5 CoP on the continent, so why not here?
The answer seems to be that in order to save costs systems are being installed by different teams of people. Some designing the system, others digging the trenches, others installing the systems, another company doing the underfloor heating etc. What is being recommended is that one company does the lot. There is then no dispute if it doesn’t work properly and also they will have control over things like: Original calculations on the size of the system? How long are the underground pipes?; how deep are they?; what underfloor heating system is being used?; how are the controls set? There is no reason why the UK installed systems are not as efficient as the European ones.
In conclusion, where you have no mains gas and your oil bills are soaring (bear in mind that electricity prices are also going up!) you may wish to look at a GSHP / ASHP. With the greening of the grid slowly under way the present day high carbon associated with electricity will slowly drop and the electricity powered heating systems will become more sustainable. However, you will need to remember to look for a low heat delivery system and also reduce your demand by insulating your home as much as possible. You will also need to think about your DHW and how to get this up to temperature without breaking the bank. Also look at how you commission your system. Try to stick to a reputable company who can take control over the whole process and give a meaningful guarantee. Also, monitor your consumption to make sure that it is all working properly. Switch your electricity supplier to a Green Tariff so that you get extra piece of mind.
GSHP and ASHP all have a place in the mix to reduce carbon but they need to be designed, installed and commissioned well. All also need to think about minimising demand and maximising the capacity for low heat delivery.
