Air Source Heat Pumps vs. Gas Boilers


Air Source Heat Pumps

Air source heat pumps take energy from the air outside of a buildings through a process using a refrigerant fluid. The measure of performance of a heat pump is called the COP, which is the comparison of the amount of energy used in the system (compressor, pumps etc) against how much energy it takes out of the air. The higher the COP, the better the system. This blog looks at the values for the COP which are required for an air source heat pump to perform better than a gas condensing boiler system in terms of reducing greenhouse gas emissions and fuel costs.

Greenhouse Gas Emissions

Air source heat pumps run on electricity. According to the UK Government’s “2012 Guidelines to Defra / DECC’s GHG Conversion Factors for Company Reporting” the average value for greenhouse gasses emitted per kWh of electricity is 0.54702 kgCO2eq/kWh, whereas for gas this is 0.20435 kgCO2eq/kWh. If we assume that the alternative to a heat pump would be a new gas boiler at 90% efficiency (e.g. then in order for an air source heat pump to have a lower level of greenhouse gas emissions, it would need to have a COP of over 2.41 in the UK.

In Europe, the carbon intensity of electricity varies. Using information from the GaBi Professional Database, it can be seen than the levels of CO2eq/kWh vary greatly, within this database the UK has a value of 0.597 kgCO2eq/kWh, whereas some countries have far higher values such as the US with 0.64012 or Greece with 1.1012. Alternatively, many countries (especially in Europe) have far lower global warming potentials than the UK, such as Spain with 0.4131 kgCO2/kWh, Latvia with 0.1794 kgCO2eq/kWh, Sweden with 0.16331 kgCO2eq/kWhh or even Iceland with 0.0194 kgCO2eq/kWh. These figures mean that heat pumps will be far better with carbon performance in some countries.

The UK’s emissions will hopefully decrease in the future, however the trend over the past ten years has been for the kgCO2eq/kWh value of electricity to remain fairly stable.

Financial Costs

Financial costs of electricity and gas are complex to work out, with numerous different tariffs. However, using figures from the website, the price of domestic electricity is given as 0.17078 €/kWh. Whilst domestic gas is given as 0.0445 €/kWh. As before it can be assumed a gas boiler will run at 90% efficiency, so the cost per kWh of heating would be 0.0494 €/kWh. This would suggest that in order for the heat pump heating cost to be the same or better than gas, the COP would be need to be 3.45. If we look to the industry prices for electricity and natural gas, the necessary COP for a heat pump to be more economical than gas would be 3.05.

Real Life COP values

The Energy Saving Trust carried out on site testing of 83 heat pumps, published in the report “Getting warmer: a field trial of heat pumps”. This showed for air source heat pumps the ‘mid-range’ of measured COPs was near 2.2 and the highest figures in excess of 3.0. The test also included ground source heat pumps, which has slightly higher measured system efficiencies than the air source heat pumps. The ‘mid-range’ ground source system efficiencies were between 2.3 and 2.5, with the highest figures above 3.0.

Further analysis of the EST trial by DECC in March 2012 (Detailed analysis from the first phase of the Energy Saving Trust’s heat pump field trial) examined a number of these heat pump installations in more detail, paying particular attention to the factors that influence system performance. It appears some sites had anomalies in the data, for example boost heater energy consumption not measured, the contribution of an oil boiler being included in system heat and some data provided from a manufacturer rather than the EST monitoring system. Consequently, some of the EST report data has been revised and Table 1 of the DECC report shows the mean system efficiency to be 1.82 for ASHP and 2.39 for GSHP. The ‘good performance’ GSHPs achieved 2.98 and 3.04 system efficiencies.

Narec Distributed Energy has carried out detailed monitoring of numerous air source heat pumps, and our results tally with those of the Energy Savings Trust, with values between 2.0 and 3.0, although the upper end is rare.

It is important to note that both the Energy Savings Trust report and Narec Distributed Energy’s own work has shown that heat pumps tend not to perform in real life with as high a COP as might be expected from manufacturer’s literature. This is due to a high number of reasons, including installation, the buildings the system is in, and the way it is operated.


As long as an air source heat pump has a COP of higher than 1 then it should be cheaper than electricity and reduce carbon emissions. Compared with higher carbon fuels such as oil burners or coal fires heat pumps perform well, reducing costs and carbon.

However, the above calculations show that a boiler running on natural gas will be cheaper to run than an air source heat pump, and also will probably have lower carbon emissions. This situation may change in the future if the carbon intensity of the UK grid decreases, or heat pump efficiencies increase.

To summarise, at the present time (2013), on a cost and greenhouse gas basis, air source heat pumps should only be installed in off-gas properties. In off gas properties they can make a significant contribution towards reducing energy costs, lowering fuel poverty and lowering greenhouse gas emissions from heating.


Further reading:

For a detailed look at the COPs of solar thermal, thermodynamic panels and Air Source Heat Pumps in relation to domestic hot water, please see  our discussion paper “COP Water Heating Technologies”

If you are interested in lab or in-situ monitoring of the COP of heat pumps, please email or call 01670 543 009

12 responses to “Air Source Heat Pumps vs. Gas Boilers”

  1. Anthony Webber says:

    Hi Is this report still technically correct as it was compiled 6 years ago and I think ASHP may have gotten better and raised their advertised COP to around 4.

    I am currently on gas in a city with radiators that are large enough I think and I am looking to move to a ASHP can you tell me which the best manufacturers are and which models do what they say on the tin.

    Thinks Anthony

    • tombradley says:

      We’ve just updated the blog with up to date values Heat pumps verses Gas Boilers (Part II)

      Whilst the advertised COP is increasing, there needs to be more independent research to measure the COP of heat pumps installed across the UK. Hopefully this will happen in the future. In terms of manufacturers, make sure it is something on

      • Alasdair says:

        Hi Tom,

        As all the comments on this blog post are from 2019, demonstrating a very recent interest in information about ASHPs, and this post ranks high on Google… can I suggest you please edit this post to add something really clear at the top such as:


        and link to the post there.

  2. Roy Howat says:

    At present started new build house .
    Heating planned is LPG as natural gas unavailable. Considering changing plans to Air sourced. Other options are Oil . Ground sourced not possible.
    Can you offer any advice as to change plans will incur extra costs from the planning authority.
    Roy Howat

  3. Steve says:

    Thankyou for the interesting article.
    It would have been good if reference to the impact on the calculation that a PV array would make. So many people have them. When we buy our next house we intend to take the boilers out and use a combination of PV and ASHPs along with solar hot water…….
    Thanks again

  4. Thomas Lankester says:

    As well as being dated (UK electricity carbon intensity has halved since 2013) the report also compares gas prices to standard rate electricity. Using economy 7 rates or even half hourly rates and timed heating, ASHPs can be run cheaper than gas.

  5. Going back to anthony’s original question

    with ASHP’s now using R32 refrigerant reducing global warming producing COP of over 3 they are still not favored under EPC’s as they still use electricity and so have a high carbon footprint.

    so has anyone updated the original NAREC report

  6. Craig Limbert says:

    Hi Tom, advertising aside, do you have one,? We have a rep coming round on Thursday and I’ve seen ground pump system and it was okay, so why is this better?

  7. Gary NICHOLLS says:

    Gas central heating boilers have additional costs that electric heaters do not have: annual service charge [typically £350] and service provider Standing Charge [typically £80/year] for gas. So, if a new boiler costs £3,000 to supply & fit with a life expectancy of fifteen years, the total cost of maintenance/supply is £5,250 + £1,200 = £6,450. Add this to the cost of gas for 15 years [say £600/year] then the total cost of gas heating is £15,450 +£3,000 [boiler supply&fit] = £18,450. Which is £1,230/year or, £100/month.
    I am considering going all electric and one option is under sink water heaters, especially in rooms that have plumbing but low usage [eg. spare bedroom].

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