Domestic heat pumps: enthusiasm needs to be tempered

Mitsubishi Ecodan. Image source: Ecodan Brochure.

Mitsubishi Ecodan. Image source: Ecodan brochure.

(The information in this article has been updated by a more optimistic article that looks at the before and after experience of a ASHP installation in Oxford, Please go to

Small heat pumps are increasingly used to provide space and water heating in UK homes. This trend is strongly encouraged by policy-makers and the government’s proposed Renewable Heat Incentive will add further financial support. The enthusiasm for this expensive technology should be moderated: for a home on the mains gas network, the savings in money will be small. Carbon benefits are probable but far from guaranteed. Moreover, air source heat pumps are unlikely to be able to heat many older homes effectively. Government, manufacturers, and installers need to be very much more cautious in encouraging the use of heat pumps and should use far more conservative payback assumptions. Heat pumps will eventually be a good investment for homeowners but probably not yet.


What is a heat pump?
When a gas is compressed, it heats up. When it is uncompressed, it cools. Imagine the simplest possible compressor – a bicycle pump. Hold your finger over the exit and push the pump handle. The air inside will get very hot. Lift your finger and let the hot air out, and it cools again. Imagine that the cylinder of the bicycle pump was inside your house but the exhaust air was vented through a window. When you pumped the bicycle pump, the chamber would get hot, and this heat would heat the room. As the air left the pump and was exhausted to the outside air, it would cool, tending to reduce (to a very tiny extent of course) the external temperatures.

This is the principle of a heat pump. The heat from the compressed gas is used to increase temperatures in one place; whereas the reverse – heat loss from the decompression – decreases temperatures in another place. Think of this as taking a block of air and separating it into a hot gas and cold gas in two separate places. When temperatures are high, you can reverse the pump, putting the cold air into the house and the hot air outside. Most heat pumps transfer the heat or cold into water that is then circulated round the house. So a domestic heat pump can, under certain circumstances, use a house’s existing network of hot water pipes and radiators. Similarly, a heat pump can provide the hot water for domestic baths and showers.

How much energy do heat pumps save?
Heat pumps look like a free source of energy and good ones are indeed very efficient. But they do need compressors and other electrically powered devices to work. (Or, in the case of the bicycle pump, the compression is provided by the person pumping. He or she will be using energy to work the pump.) So heat from heat pumps is not free. The ratio of energy used to power the pump and the useful heat output is called the Coefficient of Performance, usually abbreviated to CoP. This figure is critically important when you are assessing how much money or carbon you will save. The CoP will vary according to the air temperature and the demands placed on the pump. Broadly speaking, the greater the temperature difference between the interior of the house or the hot water supply and the outside temperature, the lower the CoP of the heat pump. A poor CoP means that you will use a lot of electricity for each unit of useful heat.

Today, attention is focused on heat pumps that use the outside air for their energy. These are called air source heat pumps (ASHPs) and are relatively easy to install in domestic houses. The unit can be attached to the wall or sit on the ground, taking relatively little space. The best ones, such as Mitsubishi’s Ecodan have a CoP of about 3-3.3 in average British conditions. For every unit of electricity used, the home gets up to 3.3 units of heating, but I’ve used the average figure of 3.15 in the calculations that follow. As heat pumps improve, this number will rise, but please don’t use the manufacturers’ figures when you are assessing them. Look for real-world examples.

Ground source heat pumps can achieve better CoP figures than their air source equivalents. But they are more expensive to install and get their ‘fuel’ from small pipes that run underneath the garden, collecting and dispersing heat energy. The garden has to be dug up to install these pipes.

Radiators versus underfloor heating
Hot water from the heat pump can be circulated using a house’s existing pipework and radiators. Unfortunately, some householders will see substantial problems. The water coming out of heat pumps is usually far cooler than from conventional gas or oil boilers. Typically, the water is at 45 degrees compared to perhaps 75 degrees from an ordinary boiler. As you might imagine, this means that radiators do not get really hot, and the amount of heat that they transfer into a room is much less. The solution is either to replace all the radiators with much larger ones with a far greater surface area, or to install a dense network of hot water pipes under the floors. In a new house with a heat pump it is almost certainly best to avoid radiators and use underfloor pipes throughout the house.

Heat pumps also heat water for showers. This water needs to be hotter, which adversely affects the efficiency (the CoP) of the heat pump. Modern air source pumps take the water up to 55 or 60 degrees, which is hot enough to bathe in.

How much do heat pumps cost?
Unsurprisingly, the cost varies enormously according to the complexity of the installation and the size of the pump. In an average-sized new house, the extra cost compared to a conventional boiler is probably between £2,000 and £3,000. To replace an existing boiler in a house already standing will add slightly more, particularly if any radiators need to be replaced. One system I have recently seen cost about £6,000 compared to perhaps £2,000 for a good condensing boiler. The government is currently offering a grant of £900, which makes a real difference but still doesn’t create an overwhelming incentive.

What does this mean for the householder?
The typical UK house on the mains gas network uses about 15,000 kWh for room heating, and much smaller amounts for water heating and cooking. If an air source heat pump has a CoP of 3.3, this means that replacing a gas boiler should significantly reduce the amount of energy used to heat the home. Here are the figures:

Energy savings from using an air source heat pump

a) Typical gas used for heating 15,000 kWh
b) Boiler efficiency from new condensing boiler 88%
c) Total heat demand (a times b) 13,2000 kWh
d) Heat pump CoP 3.15
e) Electricity needed to drive heat pump (c divided by d) 4,190 kWh

In the example above, the electricity needed to heat the house is less than a third of a gas boiler. But electricity is far more expensive than gas for each kilowatt hour. In June 2009, the cheapest tariff on the British Gas website offers a price of just over 3p a kilowatt hour for gas and slightly less than 10p for electricity.[1] Using these rates, I calculate that an air source heat pump will save the average customer on the gas network about £50 a year. This is not a good return on the investment of several thousand pounds.

The government’s Energy Saving Trust suggests typical savings of £300 for a home with gas, but this seems unreasonably optimistic.[2] It is probably a mistake for government bodies to exaggerate the benefits of new technologies in an effort to persuade the public to adopt them.

But if you use electricity to heat your house, the savings could more impressive. Many of the five million homes off the gas network employ night storage radiators that take advantage of low overnight electricity rates. The radiators heat up at night and then give off their heat during the day. However there are two problems. First, the householder will have to put new radiators in the property, adding to the cost and disruption. Second, heat pumps usually work all the time, and not just at night. So if a householder puts in a new heat pump, she will be using both low price night electricity and very expensive daytime power. Personally, I doubt whether the savings will be much greater than £200 or £300 a year, not the £870 estimated by the Energy Saving Trust.

The CO2 savings
The CO2 savings also tend to be exaggerated. A heat pump uses electricity (largely generated from burning gas or coal) to replace a boiler that typically burns gas. The CO2 saving therefore depends on the relative efficiency of heat pumps and large scale power stations.

The amount of carbon dioxide produced by a power station depends on the fuel it burns and the quality of its generating equipment. An old coal-fired station produces a kilogramme of CO2 for each kilowatt hour. A new gas plant has carbon dioxide output of well under half this figure. The UK average varies from year to year depending on which power stations are working. As of June 2009, the most recently published figure by the Carbon Trust suggested an average figure of 0.54 kg of CO2 per kilowatt hour. This figure is derived from a five-year average of power stations supplying the National Grid, mixing coal generation with gas, nuclear, and wind.

We can easily work out the CO2 savings from running a heat pump to heat a typical house:

Carbon dioxide savings from heat pump use in the average home

a) Gas needed 15,000 kilowatt hours
b) Kilogrammes of CO2 per kilowatt hour of gas burnt 0.19kg
c) Total CO2 from house heating (a times b) 2.85 tonnes
d) Electricity needed to power heat pump 4,190 kilowatt hours
e) Kilogrammes of CO2 per kilowatt hour of electricity used 0.54kg
f) Total CO2 from heat pump for house heating (d times e) 2.26 tonnes
Total saving (c minus f) 0.59 tonnes

This is a more substantial reduction than the financial saving, cutting emissions from heating by about a fifth. Since home heating is often the single most important source of emissions, a heat pump may be worthwhile. But the cost of the pump for every tonne of CO2 saved is very high.

Cost of a heat pump per tonne of CO2 saved

a) Possible life of heat pump 20 years
b) Annual savings of CO2 0.59 tonnes
c) Total savings 11.8 tonnes
d) Possible extra cost of buying a heat pump £3,000
e) Cost per tonne of CO2 (d divided by c) At least £250

The issues with heat pumps
In some countries – such as Switzerland and Sweden – heat pumps are very common. In these places, insulation standards have been high and heat pumps can heat houses even in very cold weather. In countries with low-carbon electricity supplies, like Switzerland, which has large amounts of hydro electricity, there is a strong reason to move to using electric power rather than gas or oil for heating. For the UK, this is not the case. In recent years, we’ve actually seen a slight increase in the carbon dioxide produced in electricity generation as the nuclear power stations have become increasingly unreliable and large amounts of coal have been burnt rather than cleaner gas.

So the climate argument for using heat pumps in the Britain does rather depend on whether we do successfully develop new and low-carbon sources of electricity. The attractiveness of using heat pumps will rise as we switch to wind energy, biomass, and other low-carbon sources of power. However, I think it probably makes sense to wait for this to happen rather than buying a heat pump now.

There are some other issues. Experience from the first ASHPs suggests that some do not heat the house effectively in winter. A gas boiler has enough power to pump huge amounts of heat into a house in a short time. You can turn it on at five o’clock in the morning and the house will be warm for when people get up. Heat pumps aren’t like this. They are kept on constantly, but deliver heat at lower levels. This is fine if your house is well insulated because the heat will remain. But in a draughty older house the heat will leak away and the lack of warmth may be a problem, particularly when outdoor temperatures have fallen rapidly. One householder intending to install a heat pump responded to this point by saying to me that his home would also have electric immersion heaters to increase the temperature in the central heating system when necessary. This is an unusual configuration but it may work – although at the price of higher electricity bills and reduced carbon savings.

It should be stressed much more prominently in the literature advertising air source heat pumps that they are not suitable for many houses built more than ten years ago. Before this time, insulation standards were simply too low for heat pumps to maintain reasonable temperatures in the coldest weather.

As we mentioned above, in many houses the installer should also think about replacing small radiators for much larger ones with greater surface area. This would help spread the heat effectively, but because it would be costly and disruptive, most companies selling ASHPs don’t push this option. Ideally, householders should replace radiators entirely with underfloor piping.

Another disadvantage may become evident when the heat pump is heating hot water for bathing. Heating enough water for two baths will take almost an hour with a standard 8.5 kW pump. During this time, the heat flowing into the central heating system will inevitably be much colder because all the energy from the pump will be going into the bathing water. In a well insulated house, having the central heating off for an hour shouldn’t matter very much, but in older homes the impact will occasionally be unpleasantly noticeable.

A proponent of air source heat pump responds
I rang Ice Energy, one of the largest installers of domestic heat pumps, to discuss some of these concerns. Andrew Sheldon gave me his company’s response:

  • Small savings: At current gas and electricity prices, this may be the case. But, Andrew argued, gas prices are likely to rise relative to electricity prices. I think this is possible, but it is equally likely that the reverse will be true as the government forces the development of higher cost sources of electricity such as offshore wind. Andrew also said that the Renewable Heat Incentive, to be introduced in 2010 or 2011, would probably enable heat pump owners to claim money. He mentioned 10 or 12p for each kilowatt hour of heat generated, implying a payment of over £1,000 a year for an average-sized house. Even 2p would make the financing of heat pumps look more attractive.
  • Limited carbon savings: The UK is on track to decarbonise its electricity supply by 2030. Once this has happened, carbon savings will be very substantial.
  • Worries over heating on cold days: A well insulated house should not suffer from low temperatures. Newly built houses with underfloor piping should see large financial savings and high levels of comfort.
  • The high price of heat pumps: Andrew said we should take into account the longer life of heat pumps, which will last more than 20 years. They will also need lower levels of routine yearly maintenance.

He also stressed the greater safety of heat pumps. The radiators and bathing water temperatures are never dangerously hot, minimising the risk to the elderly and young children. The constant heat in the winter also means better indoor air quality because the high temperatures and powerful convection currents close to radiators in today’s homes tend to result in high levels of dust in many rooms, particularly in older houses.

When the UK has built an infrastructure of low-carbon electricity generation, we will need to find ways of reducing the carbon dioxide emitted from heating buildings. For domestic homes, heating is much more important as a source of CO2 than electricity use, so the savings could be very important. Heat pumps and domestic fuel cells (such as those in testing from Ceramic Fuel Cells) may be the most important ways of cutting emissions from houses. But at the moment, the economics of heat pumps are not overwhelming attractive. Householders in existing properties, particularly those living in older homes, should be very wary of installing an ASHP.

(This article will form part of the 2nd edition of How to Live a Low-Carbon Life to be published by Earthscan in February 2010.)

[1] These prices are ‘Tier 2’ rates which apply to all consumption of gas and electricity above a certain minimum level. They exclude some small discounts because I couldn’t understand how these reductions worked.
[2] Energy Saving Trust figures downloaded from on 10 June 2009.

Tags: , , , , , , , , , , , , , ,

  1. David MacKay’s avatar

    Hi Chris – you suggested that we should wait until the electricity supply is cleaner and greener before pushing for heat pumps. Here’s one argument for pushing for heat pumps now: we need to eventually install millions of them, and we need to have experienced installers, and low-cost suppliers – so the sooner we start training people up, the better. All those thousands of CORGI certified gas people, they need conversion courses. To get a conversion course, you need teachers. To get teachers, you need experience. We need to get a move on. Yes, the carbon savings will be negligible. But (in just the same way that we pay through the nose for solar PV panels and wind, to help the industries grow) we should think of the long game. What do you think?

  2. Will Griffiths’s avatar

    The fundamental problem here is the ability to accurately and independently assess the seasonal performance of heat pumps, particularly since with new-build dwellings the water heating load starts to overtake the space heating. As you point out, the COP drops significantly for water heating.

    The Building Research Establishment, the author and manager of the Standard Assessment Procedure (SAP) and in conjunction with twelve industry partners and the Energy Saving Trust is currently undertaking a project within SAP Appendix Q. The project seeks to develop a methodology that can accurately devise a seasonal performance factor and should enable a far better comparison of the CO2 intensity of different heating system types (although assessment of boiler technologies within SAP 2009 has been revised and I am uncertain of the impact this may have upon the comparison).

    For reference, the latest version of SAP (SAP 2009) is currently in consultation and is recommending a CO2 emission factor of 0.591 kg/kWhr – refer here:

  3. Chris Goodall’s avatar

    Professor MacKay uses the ‘infant industry’ argument, saying that support is necessary to build the manufacturing, sales, installation and service of domestic heat pumps in the UK. Otherwise, we will not have the capacity to install millions of heat pumps a year once electricity has been decarbonised. I accept this argument completely and thank David for pointing this out.

    My concern remains that aggressively sold ASHPs are now being installed in houses which are completely inappropriate. One owner showed me her bills. June has seen substantially higher total energy use in her house (gas plus electricity) than last year because the ASHP’s efficiency is so low when delivering hot water.

    If this happens on a large scale, the risk is that the reputation of heat pumps is damaged for a decade. The same thing happened with micro-wind. Gable-end 1kW turbines are not necessarily a poor investment. But in cities, they are usually a financial disaster.

    I just wanted to suggest that the rapidly gathering enthusiasm for ASHPs didn’t translate into a gold rush for unethical installers. Heat pumps are a great idea, as is pointed out in David’s book ‘Sustainable Energy – without the hot air’.

    Will Griffiths has also correctly pointed out that CoPs are extremely difficult to measure. Mitsubishi’s careful *winter* evaluation of the latest Ecodan products also suggests high variability between installations. Summer figures may well be substantially lower and even more unpredictable.

    Thank you also for the link to the BRE document, which I have not seen before. The problem at the moment with emissions factors is that we are seeing quite large swings in the coal/gas grid balance. (Relatively low US natural gas prices means that LNG tankers are turning right at the Cape, bringing large amounts of gas, and cheaper prices, into the UK. A hard winter will change things). So I am at loss what emissions factor to use for a book to be published in February 2010!

  4. Andrew Smith-Gibbs’s avatar

    It’s worth noting that heat pumps can be very noisy, and that those sold in Britain will typically only work down to about -15°C, so when it gets very very cold, people may find that they have no hot water and no heating.

    The proposal to dash to heat pumps now is naive and unjustified: we can do far better in the short to medium term by bringing insulation in our building stock up to very high standards, and by encouraging the use of hot water storage tanks in combination with smart meters, to make the use of cheap gas when it’s available, and cheap electricity (via an immersion heater) when that’s available. That would give us a low-cost and easy transition to decarbonised water and space heating. Additionally, it gives us a huge energy buffer that will enable the grid to incorporate a far higher proportion of renewables.

  5. Chris Aasa’s avatar


    I agree with you that ASHP should not be sold to everyone by unethical installers for a quick profit. Air source and Ground source heat pumps are both a good step in the direction of building up the profile of sustainable heating systems in the UK. They are in my opinion, however, not the first step. As you correctly pointed out, it is virtually useless to install an expensive system without first addressing the issue of the energy efficiency of the property.
    Home owners should spent their first few hundred or thousand Pounds looking at their levels of insulation, windows and efficiency of their appliances. A relatively small cost (plus grants) as well as very little disturbance to your home (no digging up of floors or the back garden) can go a long way in reducing both your heating requirements and CO2 emissions. Once the energy efficiency of your home has been properly addressed you may wish to look at the various options of sustainable heating systems available today.

    Another important issue, which is only relevant to larger properties is that of single phase vs 3 phase electricity supply. Unlike most other European countries like Germany, Sweden and Switzerland, the UK does not often have 3 phase electricity supply to old residential homes. When the heating requirements for a property reaches around 20KW, there are very few products available that offer that sort of output with single phase electricity supply. Manufacturers such as IVT, Vaillant, Nibe etc manufacture products expect that your property has 3 phase electricity when you require a larger system. Worth keeping in mind before buying.

  6. Andrew Smith-Gibbs’s avatar

    Chris – might be worth looking at the price per unit energy too – see how that compares to the marginal cost of production of energy. With 3p/kWh for onshore wind, ca 10p/kWh for offshore wind, and more than enough resource between those two to meet all of Britain’s energy demands (electricity, heating and transport), the issue for us, and society as a whole is: when is the marginal cost of power savings cheaper than the marginal cost of power production?

    It’s worth noting also that an immersion heater and hot water storage tank is probably the heating of the future, in the decarbonised world, for most homes. And you’d need those in a system with a heat pump anyway, so the whole heat pump cost (£6k-9k for air-source, more for ground-source,) every 10-12 years is the marginal cost.

    Now put in all the usual cost-effective energy efficiency measures, and look at the energy consumption for all heating: it will be less half of the average home now, maybe a third. After all that, the energy savings from a heat pump are going to cost 12-20p/kWh. That’s more than the marginal cost of generation from renewables, and even getting towards the cost of energy from new nuclear! Compare this with a solar thermal system, which will save about as much energy per year as a heat pump, costs less than a heat pump, has twice the lifespan, operates silently, and gives some energy independence in the case of a power cut.

    So it’s going to be cheaper to forget heat pumps, in general, except for some specialist applications. If the noise isn’t going to be a problem (so not high-density housing), if it’s a new build, if there’s sufficient space for a ground-source heat pump, and if heating demand in winter is going to be so high that solar thermal won’t be anywhere near enough to cope, and if the building would require a lot of cooling in summer anyway, and if it’s possible to use the heat pump for dual-purpose, as heating in winter and cooling in summer, then it might, might, just make sense.

  7. Prashant Vaze’s avatar

    It might be worth having a more nuanced view of which locations will ultimately be best suited to ASHP and GSHP to ensure they are not fitted in the wrong locations, especially given the very long pay-back periods. In inner urban areas with high density homes with single brick walls it will always be expensive to adequately and cheaply insulate the walls to the standard needed to make heat pumps either cost or carbon effective until grid electricity is much lower carbon than today. Lack o access to back gardens often make GSHPs inappropriate too.

    The best option here must be install centralised heating like district heating systems initially fuelled by gas central heating and progressibly moving to low carbon heat sources (waste heat from large thermal power stations, biomass, even concentarted solar).

    The cost of installing district heating systems is presently prohibitive but as analysis by Decc / Office of Climate Change in 2008 showed might still be the best solution.


  8. ralph perry robinson’s avatar

    A very nicely nuanced article; thank you. I was about to forward to Professor Mckay when I saw his input already.

    I work designing and specifying (until I find better employ) on old and listed buildings belonging to the sort of people who until recently never bothered to look at their electricity bills. We have been latched onto by what I consider to be a dodgy heat pump firm but who talk the talk.

    I have attempted for my current project (extending and updating a draughty old country house without oil or gas) to engage other firms but the data that they work from is very sketchy, as have been their pitching quotations of £9000 to 24000.

    The capital costs are hopelessly unrecoverable. If the Grid is to be decarbonised by 2030, why not simply install efficient immersion heaters and use inexpensive convection or storage systems as before?

  9. Heat Pump Reviews’s avatar

    Do you have a cost difference in the air source vs ground source heat pumps. From my understanding the ground source is a much sounder heat transfer design. Thanks

  10. Heat Pump Reviews’s avatar

    I was also wondering if people have tied in other appliances to there heat pump systems. IE hot water, dryer.

  11. Anthony Walsh’s avatar

    Very interesting blog with a balanced viewpoint!

    However I think the ‘elephant in the room’ is the figure which is being used for Grid Carbon Intensity. In the UK this figure is based on the current grid intensity , which is a poor way of evaluating long term investments! It should be based on the prospective intensity in 2020. This will be a much lower figure than at present and will provide correct signals as to the benefit to the UK of a reduction in overall carbon emissions from the use of Heat Pumps.

    Like other UK countries the UK must meet EU emissions targets as well as Renewables targets by 2020, and a Heat Pump counts as 100% renewable, as well as providing a reduction in CO2.

    I also wonder whether there is not a dysfunctionality built into UK SAP calculations where the CO2 emissions imputed to electricity usage in the house are related to the Carbon targets for the house – which are in turn related to the EU targets for Heating/Transport/Agriculture.

    Electricity is not included in the Heating /Transport/Agriculture sectors but is instead in the Emissions Trading Sector, where the carbon emissions are measured by generator using either the national grid intensity or the EU Average Grid intensity. This provides the driver to reduce CO2 emissions in the electricity sector.

    The problem of including CO2 emissions from electricity usage in the target for household CO2 emissions is that it leads to sub-optimal investments.

    Firstly it allows other countries to increase coal burning plant as the overall target is for the whole of the EU, secondly it does not provide the greatest reduction in CO2 per £ spent – using Night Storage Heaters, Immersion heaters and Control system such as a Home Area Network would provide low CO2 emissions at low capital cost provided that the grid CO2 intensity decreases as planned. The money saved could then be used for other investments which had a greater return in terms of CO2 saved.

    In Ireland the Grid CO2 is decreasing because of an increase in gas coupled with a much larger increase in large wind farms – currently the day proportion of wind is 1,000MW which is about 20% peak, but at night can rise to 40% of generation. A further 2,000MW have connection agreements and a further 3,000MW are in the process of getting connection agreements. So Grid intensity will be low by 2020, in which case Direct Electric heating in some form (heat pump, Storage heaters etc) will have less CO2 emissions than a condensing gas boiler.

    Most analysis in the UK seems to be about large amounts of microgeneration etc yet this will be ‘crowded out’ if large windfarms arrive.

    Am I correct in my comments or is the situation in the UK somehow different that the rest of the EU? e.g. ROC Certs to Domestic customers etc.

  12. Gavin Lowe’s avatar

    Having read your article ‘Domestic heat pumps: enthusiasm needs to be tempered’and the comments from other readers, it now puts me in a very confused position, not knowing which way to turn!

    We are just about to demolish our house to rebuild and 6 months ago, we decided to go for a GSHP solution, then, (and I work in construction of hotels) after discussion with various clients of mine, I decided that as we were going to insulate highly, there was no point to a GSHP and just to go for the cheaper option and install a good condensing boiler. This weekend, we attended a trade show at Earls Court, got talking to some providers of GSHP and suddenly I saw the light and decided yes we should ‘do our bit’ and install one —- now I read your article and have changed again!!!! — is there anywhere that I can get a definitive answer to the question – “is it economical to install a GSHP now? put infrastructure in to support a heat pump in years to come or just forget it completely and focus on the insulation?”

    We are on mains gas, we have the space to install (3/4 acre) and we have a ‘clean sheet’ to carry out installation, I guess I’m looking for a steer on where to go or who to talk to!

  13. Chris Goodall’s avatar

    To Gavin Lowe from Chris Goodall

    Dear Gavin,

    Thank you for your question.

    There isn’t a definitive answer to your question without detailed analysis of your particular circumstances.


    * Ground source heat pumps are slightly more ‘efficient’ than air source ones. In other words, they get a better Coefficient of Performance (energy in versus energy out).
    * If you are on gas, the cash savings today are likely to be quite small because gas is only about one quarter the price of electricity per kWh.
    In my view, electricity is likely to rise in price in relation to gas over the next decade. (Why? Because all the costs of decarbonisation – such as the subsidy for wind – are going to be loaded onto electricity).
    * Carbon savings from using a heat pump will, by contrast, rise as CO2 is driven out of electricity generation.
    * If you are doing a full refurbishment that provides excellent insulation and will allow you to run underfloor heating then a heat pump may be OK.
    You need to be assured that your hot water supply will deliver water at a high enough temperature. If necessary, this needs to be topped up with an immersion heater, or similar.
    * Ground source heat pumps are expensive compared to a good condensing boiler.

    From a long term point of view, David MacKay’s comments on this web site are right – we need to switch to heat pumps for space heating. But you will be a pioneer and may not save any money for some years. And the installation cost is high.

    Nevertheless, I hope you do put in a heat pump, because we need pioneers. But because you are on the gas network you may find it doesn’t save much, if any, money.

    Chris Goodall

  14. bil bailey’s avatar

    I’m interested – and so is my village. We have a fairly substantial river flowing by, with a redundant ’2m. head’ mill dam, and we are beginning to explore microgeneration of ‘lecky by means of Archimedean screw techniques.

    Could the same source of energy also provide us with a ‘hybrid’ heat pump?

  15. John Brooke’s avatar

    What are your thoughts on using a mixed source heat pump (MSHP)? i.e. Ground-source for winter, solar panels for sunny days (when the temp is higher than the ground source), air-source in the spring and fall, and heat-recovery from dryers, ovens, grey-water, etc.? It seems if a person is building a new house, squeezing all the energy out of every facet of the home is the most efficient, rather than flushing it all down the drain or up the stack. Using that in tandem with the new CO2 compressor systems would seem to give a higer COP, and therefore a quicker return on investment. Any thoughts on this? Thanks!

  16. Bill’s avatar

    I have read the article with interest. We have just built a 6 bedroom house, 380sq metres in size and have a ground source heat pump to provide underfloor heating throughout the house and provide hot water. The house is insulated to current requirements.
    We are only in the first month of it working but already have concerns. The contractor who installed it does not understand how we can control the temperature within the house. There are automatic settings for the ground floor and thermostats on the first and second floors. The house is currently too warm, the pump is quite noisy and the appearance of the pipework in the garage is not great. The pipes that carry the cold water back out of the system do freeze overnight and thaw onto our garage floor each morning.
    We are currently consuming about 80kwh per day of electricity. In our old house where we had gas central heating and hot water we consumed about 18kwh per day on fridges, TV’s etc. In simple terms therefore we are now consuming 62kwh per day on running our underfloor heating and hot water. It is probably too simplistic to use this as an average for the year, but clearly we will use more electricity in winter and less. If we were to use 62kwh as a daily average however this equates to 22,600kwh per year, way, way above the figure of 5,000 suggested in the article. Now the installers need to come back and we need to resolve problems of excessive heat in the building, but at this level of consumption we are going to be losing money compared with gas, even if you ignore the capital cost.

  17. Fergal McEntee’s avatar

    From a supplier/installers point it is very interesting reading people’s opinions on heat pumps. With cheap gas prices and high co2 ratings for electricity it is hard to compete against conventional gas boilers. Off gas sites offer the best in terms of payback. When all fuel price increase then heat pumps become more financially attractive. In 2007, the global heat pump market reached nearly 477,000 unit sales and the UK accounted for about 3,000. The UK market is ‘new’ to heat pump technology and will have a teething period. Once the Heating Feed in Tariffs is introduced in July 2011 then we will see a significant increase in the use of heat pumps. With the increase generation of renewable electricity will see heat pumps become more carbon efficient. We are at the start of a transition period of technologies and there will be problems but they are needed in order to progress. We have recently sent up an office in the UK and are having our own teething problems and are looking for a test site. If you are interested then get in touch.

  18. Helen Day’s avatar

    Hi Chris, thanks for this, it’s one of the most in-depth, unbiased articles I’ve read on ASHPs.

    We were considering installing an Air Source heat pump as we are not on mains gas and have been using storage heaters since we moved into our 2 bed, 1920′s detached house last year.
    We’ve carried out the initial steps with cavity wall insulation and loft insulation and are now looking to install central heating. Which is where we’ve reached an impasse.

    We couldn’t decide between installing an oil central heating system with quotes coming in around £7,000 or an ASHP with a quote for £9,000 post grant. It’s been difficult to find information on the cost/CO2 savings between the two, especially for our situation, but this article has helped – unfortunately, oil it is!

  19. Chris Goodall’s avatar

    To Helen Day

    Thank you very much for your comments. Very useful to have your numbers.

    If anybody else has got cost or savings estimates for using air source heat pumps, please do note them down here.

    ASHPs are potentially a very important technology but more information about how they actually perform and the likely costs will be very useful to everybody.

    Thanks again

    Chris Goodall

  20. Martin Pollentine’s avatar

    We are currently assessing several installations of Air to Water Heat Pumps for a Housing association. We currently upgrade / replace existing gas boilers with modern high efficiency condensing boilers. However those remote properties without a mains gas supply are currently heated by oil or electric storage heaters. These properties will be able to benefit from an Air Source Heat Pump installation. The issue of increased size radiators can be addressed during the design stage. Coupled with Solar Panels to pre heat the domestic hot water, such an installation would benefit the end user enormously by way of reduced energy savings and significant grants are currently available to the landlord for each section of these works.

  21. Tom Bowen’s avatar

    We moved into our house last July and have lived through a, so far, rather cold winter. We have LPG central heating but can only use cylinders, which is expensive. The cost of LPG means we can only use the central heating for 1 hour in the morning and 1 hour before we go to bed.
    Our village is not on mains gas. Fortunately we also have a wood burner so part of the house is kept warm by that.
    We recentyl retired and would like the house to be warm during the day.
    We are considering whether to have an air-source heat pump fitted or to have two electric storage heaters in the part of the house that is not warmed by the wood burner. The storage heaters would not be in bedrooms and would be use to heat the ‘other part’ of the house during the day. We are on Economy 7 metering and the hot water is heated by this at night and during the time when a decent amount of sun shines by solar panels
    So – the ASHP would not need to heat the water at all. We have an out-building which is suitable to have the ASHP etc and would require a trench to be dug – approx 15 to 20 m.
    I feel that 45degrees C all day and night would be plenty warm enough.
    Clearly, the elec storage heaters would be cheaper to install. I cannot get any idea yet as to how much electricity would cost us daily for the ASHP.
    Any comments would be appreciated.

  22. Tony Evanson’s avatar

    Most of the negative issues raised in the original article relate to air source heat pumps using HFC refrigerants which can only condense at 55°C and therefore oversized radiators are required or under floor heating can be utilized. There is an alternative, Sanyo manufacture an air to water heat pump that uses CO2 as a refrigerant. These units can provide water at 65°C for the heating circuit and they also have a Domestic hot water circuit within the thermal store.

    CO2 has a global warming potential of 1 compared to a GWP of 1800 for R410A. These units were developed for the Scandinavian market so they operate efficiently in very low temperatures unlike HFC products. The SANYO heat pump can maintain 65°C on the inverter driven heat pump alone without having to use electric heaters. One of the problems with HFC heat pumps is that they have to raise the temperature of the storage tank to 65°C to sterilize against Legionalla and the only way they can do this is by utilizing electric immersion heaters which is very inefficient.

    HFC heat pumps also lose efficiency rapidly when you get outside air temperatures below zero as they struggle to extract heat from the atmosphere, they also require frequent defrosts of the outdoor unit which means they cannot provide heat and in some cases use the energy that has already been generated to assist defrost. The Sanyo system can operate down to -25°C.

    You can get more information from this source

    Or see and hear a unit in operation at this website

    Alternatively email me at and I will be happy to help.

  23. lee Ellerby’s avatar

    Hi All

    Its very interesting the manufacturer stating he gets a COP of 4.2, I guess this is at an ambient temperature of 7 degrees c and a water temperature of 35 degrees c thats only any good for under floor heating great for new build but not for retrofit. So that blows his COP of 4.2 out of the water for most of the country’s housing stock. Wow! how we bang on about our COP which is stated differently on every manufacturers literiture confusing the customer and installers into believing its the higher COP that matters, Some standard has to be put in place that all manufacturers have to comply with.

    When using the new SAP2009 calculations ASHP get a big kicking there stated COP gets reduced to a much less desirable figure of 2.5 Unless there Appendix Q rated which means you can use the manufacturers stated COP otherwise there not worth touching for your new build that will be having to comply with the code for sustainable homes. Have a look theres not many!.

    I design these and installers fit them which brings me on to my next gripe the industry has to be trained better, a lot better infact to reduce them energy bills and not increase them by lack of knowledge. Go on a heat pump course and its pritty much a sales pitch with not much emphasis on actual installation and theory training. Can we not have a governing body that relates to governing individuals as well as firms and not the laughable MCS accreditation thats availiable to any one thats got 500 quid to pay an internet company to get it for them

    Oh Tom Bowen if your having trouble paying for your LPG don’t bother with an ASHP you’ll be remortgaging

    If you are thinking of getting one ask the firm to supply 3 property’s were they have installed them for references. Do your homework make sure its a plumbing and heating firm and not an insulation/double glazing/facsia company thats doing it as lots of cowboys are trying to get in on the renewable revolution.

    Tony if I wanted to use an air source heat pump for -25, that would mean my ASHP would have to deliver 23kw of heat if my room temp was 21 thats with a coe efficent of .5 on your SAP reort. Thats probably 2 machines which would be ridiculous. Thats just a sales pitch it works down to minus 25 yeah they do, but they have to be sized right and there bloody expensive to run at -25. I,d be flicking my oil boiler back on at minus 5 “Fact”

    They do work but you have to remember you have to be very well insulated, use a well trained installer and be ready for your electric bills increasing as there not that wonder machine you think they are, there carbon savers not money savers, well until the RHI

    Roll on de-carbonisation 2030!

    Lee Ellerby

  24. Helen Elliott’s avatar

    Can I ask your advice?

    New build 100 sq m, two occupants, highly insulated, right on the coast in Northern Ireland, plan to live there a long time. We have roof room for 1kWh peak PV or equivalent solar water heating. Available heating options electricity and oil. No piped gas but tank gas possible.

    You seemed in 2009-10 to have been very much in favour of heat pump technology. Is this still the case?

    Many thanks

  25. Chris Goodall’s avatar


    It’s a bit difficult to give any response without seeing your house but….

    Air source heat pumps don’t seem to work well in the UK. ( I think that was what I said in 2009 as well). But if they are going to work, your house sounds an ideal location – relatively low heat need because of insulation, not over-large and with no mains gas.Also, you probably don’t get deep frosts because of the closeness to the Atlantic. My guess is you’ll save money and carbon.

    This is only a guess. Get three installers out and ask them a specific question (inter alia) about whether their recommended machines ice up in below zero temperatures. If they don’t know, or haver, throw them out. If the response seems convincing, ask for contact details of their three most recent customers. (Most problems seem either to be caused by ice or by poor installation, or both).

    Please tell me how you get on..

  26. Helen Elliott’s avatar

    Certainly will.

    Thank you.

  27. David Boswell’s avatar

    I had a 55 deg 16 Kw Daikin hest pump for hot water and heating installed two years ago. My house walls (200 years old) are 18 ins thick, solid, made of cob. The house is barely warm enough in winter. We are 90 and 86. Old radiators and 15 mm copper underfloor piping used. The ground floors are solid concrete and the pipes embedded. The circulatory water is sealed and pressurised to 1 bar. The water is among the hardest in UK – over 200 on the scale used. I find no record of any corrosion protection system and am concerned that my old pipe bores may be slowly constricting. Air collects in the upstairs radiators and roughly every 6 months needs bleeding and the sealed system filled up directly with hard mains water. What are my risks at 55 deg and should the installer have avoided a heat pump system due to this problem? Advice very welcome. David


Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>