Heat Pumps

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4 to 239 -Years

For those interested it operates in a similar way to a refrigerator working in reverse, it works on the principle of the vapour compression cycle within the pump.  There is a volatile fluid known as a refrigerant. The refrigerant in the evaporator is heated by the heat source this causes it to turn into a gas. The gas then passes through the compressor; the compressor increases its pressure and causes its temperature to rise. The hot gas then moves to the condenser where it is condensed back into a liquid and in doing so it releases heat into the house via a distribution system. The refrigerant is then allowed to expand back to a low pressure through the expansion valve and pass back to the evaporator where it repeats the cycle in a closed loop.

What are they?

There are three main types, these are:-

  • Ground Source Heat Pump (GSHP),
  • Water Source Heat Pump (WSHP), and
  • Air Source Heat Pump (ASHP)

They function by extracting heat from the ground the water or the air respectively and releasing that heat energy at a higher temperature within a building.   All heat pumps consume energy usually in the form of electricity to operate the pump that is required to transfer the heat.  They can supply all the heating you would require, and ground source systems are the most effective.  To compare the performance of heat pump the term coefficient of performance is used (COP) and it described the ratio of heat movement to energy input.  In the table below the Best Average COP is used.  This a seasonal average, since air source and water source vary in performance with the temperature of the environment.  For ground source the underground temperature is however remarkably constant all year.

It should also be noted that you will need a reasonable amount of land for an effective GSHP, unless you drill down, and for a water source heat pump a body of water is essential!

Energetically are they worth it?

According to manufacturer information heat pumps provide between 2.5 and 5 times (or they have a COP of between 2.5 to 5) the energy they consume and by energy consumption they usually mean energy in the form of electricity.  Electrical power generation and distribution is covered in another article, however when trying to compare like for like and come to conclusions regarding total energy consumption, the inefficiency of power generation must be factored in, and it never is!  So here are the main points:-

  • 1kWh of electrical energy consumed by the pump should deliver 4kWh of heat energy, and it would have a COP of 4.
  • For a gas fired power station for example 1kWh in your house requires about 2.2kWh of gas energy to generate it, so in reality 2.2kWh is required to deliver 4kWh of heat energy, so should the COP value really be 1.8?

You decide!

Financially are they worth it?

The answer to this question is depends.   It simply depends on the form of heating you have currently, if you are mains gas heated this technology make no sense on financial grounds (without incentive schemes), since one unit of gas energy is three times cheaper than one unit of electrical energy; the system will take a lifetime to fay for itself; If your only source of alternative heating is electrically powered at the standard rate, then it could be well worth the investment and pay back in about 8 years.

Data table

Heat Pump Best Average COP

Very Rough Equipment and Installation Cost

Grant are available to reduce this cost

Annual Electrical Energy Requirement


Cost of Electrical Energy

Cost of "Replaced" Energy

Payback Time
          Gas LPG Electrical Gas LPG Electrical
Ground Source 4 £ 12000 4000 kWh £ 616 £ 768 £ 928 £ 2464 79 Years 38 Years 8 Years
Water Source 2.8 £ 8000 5700 kWh £ 877.8 £ 768 £ 928 £ 2464 Never 159 Years 5 Years
Air Source 2.8 £ 6000 5700 kWh £ 877.8 £ 768 £ 928 £ 2464 Never 120 Years 4 Years
This table gives the potential payback times for a various heat source pumps assuming that the total household heat requirement is satisfied. The requirement is assumed at slightly above the average at 16,000kWh each year.

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0 # error in data re heat pumpsmike ambrose 2014-06-21 14:21
The comment on "is it worth it to install a heat pump" are incorrect. in fact even withou the Renewable Heat Incentive it is still cheaper to run a correctly installed heat pump than a gas central heating system - main points gas is not a quarter the price of electricity your figures show it at one third. heat pumps can run at night using cheap rate electricity. A correctly installed water source heat pump will have a better cop than a grond source heat pump. The data table is also wrong - there is no adjustment for the gas or LPG efficiencies. please see our calculator at www.heatcollector.co.uk or call me on 01844 203895 for further details - mike ambrose
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0 # replyrdh 2014-06-23 14:18
Hi Mike

Can you share with us an actual case study to illustate your point and I will add it to the artice. I have never heard it claimed that replacing natural gas with a heat pump is rational without the incentive so I would be very interested in some real data, including how one sensibly applies the use of off peak electricity within the mix when most of us turn our heating off.

The renewable heat incentive makes an enormous difference, and this does make it financially viable.

At the time of writing the article it was closer to 4 times diffenent gas/electric prices, how times chanege. I have altered it to 3

Can you tell us the prices for fully installed systems now , because payback time is defined by this. Are your electricity and gas prices ex VAT? as they are very low against BEER figures.

I agree on efficency of boilers this is not factored into the table above. Modern boilers are beween 90 and 95% efficients, for those who wish to factor this into the sums. The calculations are however faily crude and intended to give people and idea of what is going on. The efficiency of a boiler is not as significant as the COP, I see you are quoting 4.3 (unless the boiler is ancient).

One thing I will never understand is why heat pumps are regarded as green renewable energy and eligible for the scheme at all. To me they generate more CO2 than burning gas in a boiler as they use grid electricity which is at best 80% fossil fuel supplied and converted at 30 to 40% efficiency. Views on this are also welcome.


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0 # MrGarethC 2014-07-03 08:50
Hi Richard,

Your figures show, electricity emits 0.54 kg of CO2 per kWh. Oil emits 0.25 kg/kWh, LPG 0.21 and gas 0.19.

Heat pumps must manage SPFs of 1.0+, 2.2+, 2.5+ and 2.9+ to be greener than electricity, oil, LPG and gas respectively (actually less due to boiler inefficiencies) . i.e. a heat pump with an SPF of 4.0 emits 28.5% less CO2 per kWh than even a natural gas boiler.

The best data on UK heat pump performance is DEFRA's latest field trial. It showed well installed, new Ground Source Heat Pumps achieve SPFs of around 3.8, making them cheaper to run than natural gas, but with extremely long (unsubsidied) paybacks. Air source (to water) pumps managed SPFs around 2.8 (some better), so were marginally more expensive than natural gas. Even for those that were better, payback vs gas is again very long.

No data on water source heat pumps to my knowledge.
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0 # replyrdh 2014-07-03 10:05
yup, thanks Gareth
and a good way to put it. All of that seems correct to me. It is the install cost that is the issue especially for gas as you point out.
One should also point out that if you use heat source you should also negociate an electricity price each year to get the best deal, since you will use a lot, savings are likely to be substantial and a better pay back picture than is portrayed above will materialise.
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0 # You should probably include air to air heat pumps in the tableGareth 2013-11-20 11:56
These claim COPs of over 5. Two of these 4.7kW units tinyurl.com/kj64dv7 (no connection - available elsewhere) to provide your space heating would cost only £1900 to buy and install. Existing/altern ative system needed for hot water.

Assum e they only actually achieve 4.25 COP (20% below claimed 5.26) and that space heating is 75% of heating costs (common assumption). These would pay for themselves in 11 years vs gas, 3 years vs LPG and 1 year vs electricity.

Incidentally, you need a COP of 2.4 to equal carbon emissions from using a gas boiler. This system would reduce space heating carbon emissions (assuming marginal electricity has the same carbon intensity as the average) by over 40%. A way to reduce carbon emissions vs gas boilers substantially without it being financially completely silly.
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0 # I wish you could install my heat pumpsSkin Tightening Treatment 2013-11-12 17:22
Good work on these instructions... .helped me!
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0 # Why no oil comparison?max 2012-12-23 09:53
OK, I'm numerate, I CAN work it out for myself, but it would be nice to simply see the trade off of heat pumps against oil fired heating.
Surely the calculation should also consider the efficiency of and at what point in the replaced appliance lifecycle the change takes place. If you replace an inefficient oil boiler at the end of it's expected life span the the cost of replacing like for like (a new boiler) should be deducted from the cost of the heat pump, and the efficiency gains of a like for like replacement should be factored in to the pay back time.
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0 # Wot no oil comparison?Richard 2012-01-23 20:53
re. oil, I can put oil in easily if useful on the heat pump page. The point was to just put a few indicative option in, to give people a feel for what they are doing to make sensible choices. There are lots of fuel types in use.

re. not replacing an old boiler, you are correct of course, the cost should be deducted. So if you get a quote and you intend not to replace an old boiler, then yes use this number will be the total investment cost, I think this is a very useful point. And I will try to make the point clearer on the site, particularly true for GSHPs.

re. relative efficiency gains, there it goes away from the spirit of simple digestible data, for example we could include heat pump life-cycle, relative maintenance costs (boiler, vs pump) and indeed if its better to invest the money in Greek bonds (I jest). How complicated should the comparison be, and at what point does it become over complicated is the problem here. Since every situation will be very different, it is difficult for a general site like this to cover all eventualities.


Ri chard
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0 # ASHPGrant 2012-12-06 13:52
Putting Oil into the data table would be useful.
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0 # EcoDan Airflow heat pumpsMark Ellis 2011-01-29 19:32
We've got air heat pumps here - for hot water and central heating wet radiators . We find that the radiators take a very long time to warm up compared to say a gas boilered central heating system.
We also find the system regularly flushes itself with freezing cold water, thus cooling the hot water and radiators.
And the Mitsubishi fans are noisy, especially when mounted next to the building - as vibration sound comes through the foundation, and where it's bolted to the wall.
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0 # Airflow not good for the homeMark Ellis 2011-04-07 11:22
Forgot to mention.
The air flow forces cold air out of the system.
If you have a small garden and the washing line is anywhere near in line with the fan - then your wet washing will be froozen like a board.
Any plants in line with the air being forced out of the fan (which is through a wide angle) will be killed by frostbite.
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0 # Ecodan Heat PumpsMichael Smith 2012-04-25 16:01
More likley to be poor installation/co mmissioning. The outside unit should be installed on semi flexible feet and attached, if necessary, to the wall with a non rigid connection.

Yes the flow temperature from a Heat Pump is lower than from a conventional boiler and this should have been made clear to you and factored in to the sizing calculation by your installer. Sometimes this means that some radiators may have to be increased in size and usually the pump is operated for longer periods than conventional high temperaturel fuel boilers. This increases effiiciency. Lastly, the initial setup of the flow controller is critical and if the installer has just left it at factory settings without taking into account your requirements it is unlikely to be working correctly.
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0 # Slow heating radsJonathan 2012-09-24 20:59
Of course the radiators take longer to heat up than a gas boiler, thats why a heat pump is more efficient than a gas boiler, its like putting your foot on the accelerator pedal of a car, you shouldnt be switching it off at all they run on a constant trickle of heat controlled by your thermostat, and the rads sit at a constant temp.
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