How to calculate the carbon dioxide emissions of a new heat pump

Here we present the simplest calculation for working out the environmental benefit of a heat pump. Change the numbers to suit your case.

Annual Running Carbon Dioxide Emissions

The EPC (Energy Performance Certificate) should tell us what the approximate heat demand is for the property in kWh (space heating and water heating). If gas is used only for heating and hot water then the annual energy usage statement from the gas supplier will show the actual usage. Using the average from multiple years removes the weather effects (e.g. mild winter). Multiplying that by the boiler efficiency will give the actual heat demand (if the annual gas statement was 17500 kWh and the boiler efficiency is 80% then the heat demand is 17500 * 0.80 = 14000 kWh). If you use gas for cooking too then you’ll only be able to make an estimate of your heat demand. It may be easiest to use your EPC unless you know it is far off.

Let’s say we use 12000 kWh for space heating and 2000 kWh for hot water.

If we purchased a new condensing type boiler we might get an efficiency of 95%. So the kWh of gas we need to purchase to deliver 14000 kWh of heat is 14000 / 0.95 = 14737 kWh.

We use published figures for the carbon dioxide equivalent emissions for UK grid natural gas [1]. This is made up from carbon dioxide equivalent emissions due to combustion of the gas (0.18316 kgCO₂e/kWh) and emissions associated with extraction, refinement and transportation of the gas before it is ready to use, also called Well-to-tank or WTT emissions (0.03135 kgCO₂e/kWh). The emissions are (0.18616 + 0.03135) * 14737 = 3161 kg per year = 3.2 tonnes per year.

To calculate the equivalent emissions for a heat pump we need to know the Seasonally adjusted Coefficient of Performance (SCOP) from the technical data sheet. If you can’t find SCOP then look for SPF, Seasonal Performance Factor. This number varies based on the local outside temperatures, so it is important to use one for your region, or one with very similar climate.

Let’s say ours will be 3.8.

The kWh of electricity we need to purchase to deliver 14000 kWh of heat is 14000 / 3.8 = 3684 kWh.

We use published figures for the carbon dioxide equivalent emissions for UK electricity grid [1]. This is made up from the carbon intensity of average UK grid electricity supplied (0.21233 kgCO₂e/kWh), the transmission and distribution factors for UK grid electricity (0.01879 kgCO₂e/kWh, to cover losses between the generator and your home as a result of supplying your power), the Well-to-tank emissions for the primary fuels used to generate your electricity (0.05529 kgCO₂e/kWh) and the Well-to-tank emissions for the primary fuels used to generate the electricity lost in transmission and distribution (0.00489 kgCO₂e/kWh). The emissions are (0.21233 + 0.01879 + 0.05529 + 0.00489) * 3684 = 1073 kg per year = 1.1 tonnes per year.

Ok so that is emissions for running the equipment, but what of you include manufacture of equipment and so on?

Lifetime Carbon Dioxide Emissions Averaged per Year

The simplest way of estimating this is just to use cost. The ratio of total UK emissions in 2019 to the total UK GDP in 2019 was 0.16179 kgCO₂e/£. Now we use the cost to purchase and install the equipment (ignoring any grants), the cost to service the equipment and the expected lifetime of the equipment. A basic internet search gives us 15 years for a gas boiler and 20 years for a domestic heat pump.

Let’s say we get the lowest quote for a new gas boiler plus installation is £3500. Let’s also say the annual gas service is £50. The lifetime carbon emission is (£3500 + 15 * £50) * 0.16179 kgCO₂e/£ + 15 * 3161 kgCO₂e = 48 103 kg. Divide by 15 to get 3207 kg per year = 3.2 tonnes per year.

For a heat pump we can do a very similar calculation (the carbon emissions will be no different if you have a grant or not). Let’s say the cost of the new heat pump is £7500 and the annual service is £100. The lifetime carbon emission is (£7500 + 20 * £100) * 0.16179 kgCO₂e/£ + 20 * 1073 kgCO₂e = 22 997 kg. Divide by 20 to get 1150 kg per year = 1.15 tonnes per year.

But what if I use Green Gas or Green Electricity?

Isn’t it zero if I buy green gas? So what’s the benefit of a heat pump then?

Ok so let’s assume you buy 100% green gas and compare that to 100% green electricity for a heat pump.

For green gas we will use the published biogas figures [1]. This is made up from carbon dioxide equivalent emissions due to combustion of the gas (0.00022 kgCO₂e/kWh) because of methane and nitrous oxide emissions on combustion. But we also have to consider the upstream emissions for the production of biogas, also called Well-to-tank or WTT emissions (0.02405 kgCO₂e/kWh). The emissions are (0.00022 + 0.02405) * 14737 = 358 kg per year = 0.36 tonnes per year.

For electricity we will take account of lifecycle carbon emissions from the generator and add to that the transmission and distribution factors for UK grid electricity. The Intergovernmental Panel on Climate Change (IPCC) have previously published carbon dioxide emission intensity data [2]. The WTT for the primary fuel for renewables that use the sun, the wind or the waves is zero. If we purchased offshore wind (median lifecycle emissions of 12 gCO₂e/kWh) then the emissions are (0.012 + 0.01879 + 0 + 0.00489) * 3684 = 131 kg per year = 0.13 tonnes per year.

It is worth noting that the UK electricity grid’s carbon intensity is reducing at the moment and the associated transmission and distribution carbon intensities are expected to reduce as well.

The best way of minimizing the carbon emissions associated with heating the home is to use a heat pump powered by renewable electricity.

Other Emissions Considerations

1. Global Warming Potential (GWP) of refrigerants. Many heat pumps today use a refrigerant with a high Global Warming Potential (GWP). The number indicates the equivalent global warming damage done for a kg of gas in terms of kg of carbon dioxide (refrigerants can be around the 2000 mark, so an escape of 1 kg would be equivalent to a colossal 2 tonnes of CO₂). If maintained correctly there is no reason why this should be an issue as the charge should never be released into the atmosphere. For this reason only approved and trained F-Gas registered businesses are allowed by law to service or repair heat pumps.
2. Lifetime embedded emissions. The emissions from manufacture will depend of the materials and processes used. Manufacture in a country with a low carbon electricity grid is a way of reducing this source of emissions. Reducing lifetime emissions also occurs when we purchase a heat pump with a longer life. Look out for established brands with long warranties (such as here).
3. Gas boiler pollution. Gas boilers release small quantities of other pollutants both in and out of the home. The dangerous one is carbon monoxide from faulty equipment (incomplete combustion). Pollutants released into the outside environment include small quantities of methane (GWP of 25) and nitrous oxide (GWP of 298) as well as nitrogen oxide and nitrogen dioxide (together known as NOx), a serious air quality issue for the local environment and human health.

Click here to see how to calculate the cost of a heat pump.

Click here for an overview of some useful heat pump knowledge.

[1] UK Government GHG Conversion Factors for Company Reporting, June 2021, https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2021

[2] Intergovernmental Panel on Climate Change (IPCC) 2014 carbon intensities: https://www.ipcc.ch/site/assets/uploads/2018/02/ipcc_wg3_ar5_annex-iii.pdf, p.7