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SERIES 23 / Module 03 Heat Pumps
Greening gas (biomethane or
hydrogen) is a possibility but there are challenges in its implementation as it requires significant infrastructure work. In the UK the government has deferred its decision on the role of hydrogen in home heating until 2026.
Calculating performance When looking at boilers, we rate the performance in terms of efficiency the ratio of heat output to heat input e.g. 95% efficient. However, when we look at heat pumps the input energy (purchased) is significantly less than the output energy. Accordingly, performance is expressed using the Coefficient of Performance (COP). For example, a heat pump that delivers 6kW of heat with an electricity input of 2kW would have a COP of 3.0. The theoretical maximum COP
of a system operating between two temperatures Ts (the temperature of the heat source) and Th (the heating supply temperature) is given by:
Theoretical Coefficient of Performance = Th/(Th-Ts)
Where Th and Ts are measured in Kelvin In practice the measured COP
might only be 60% of the theoretical COP. What this equation does tell us is the criticality of the temperature difference. The smaller the temperature difference, the higher the COP. It is for this reason that heat pumps
work best with ‘low temperature’ heating systems. For example, warm air, underfloor heating, fan coil systems. Standard heat pumps operate most efficiently in the range 35-55ºC. (Gas fired heating systems typically operate at 60-80ºC). Because of this heat pumps should not be seen as a direct replacement for gas boilers. Also, domestic hot water (DHW) systems ideally need to achieve a temperature of 60ºC for legionella control – this can be achieved with an additional direct electric boost. With boilers we also consider
seasonal efficiency. For heat pumps we use the Seasonal Coefficient of Performance (SCOP) for heating and the Seasonal Energy Efficiency Ratio (SEER) for cooling. These methodologies derive from the EU Energy Related Products Directive (ErP). Under EU labelling there is the familiar rating system – which currently runs from D to A+++. For a typical air source heat pump
System Type Gas boiler ASHP
GSHP – open loop GSHP – closed loop
system delivering water at 35ºC water the COP could range from 2.0 at minus 15ºC rising to 5.5 at plus 15ºC. In contrast, if it were delivering water at 55ºC the COP would range from 1.8 to 3.0. High temperature heat pumps
are defined as pumps capable of delivering output temperatures over 55ºC. Higher temperatures are possible and can be achieved by some specialist heat pumps. One approach is a cascade heat pump system which is in effect one heat pump coupled to another through a heat exchanger. Absorption heat pumps (gas-fired) can also reach the higher temperatures, but the carbon benefits are compromised by using natural gas. The typical COP for a gas-fired heat pump is around 1.3 to 1.5. The relevant standard for heat pump
testing is: BS EN 14825:2022 - Air conditioners, liquid chilling packages and heat pumps, with electrically driven compressors, for space heating and cooling. Testing and rating at part load conditions and calculation of seasonal performance.
Installation costs The cost of an installation will depend on the system selected and site specifics. However, a useful rule of thumb guide for initial appraisal
Cost per kW heating output £70 to £150
£250 to £1,500 £1,000 to £2,000 £1,500 to £3,500
purposes is provided by the Carbon Trust in CTV 072. The figures are based on 2018 data. Whilst dated, this data does show the relative cost of the various types of system. According to the National Audit
Office (NAO) as of December 2023 the average market rate for replacing a (home) gas boiler with a heat pump was found to be around four times higher than a like for like (boiler with boiler) replacement – with an average installation cost of £11,287. Maintenance cost — studies
suggest that the cost of maintaining a heat pump is similar to that for an equivalent gas or oil boiler. The Carbon Trust notes that a well- maintained system will potentially be 10-25% more efficient than a poorly maintained equivalent. The expected life of an ASHP is between 10 and 15 years – arguably a little less than a boiler system.
The fi nancial case for heat pumps For a heat pump to be cost effective, the cost of the electricity to drive the system must be lower than the cost of the gas it displaces. This means that the relationship between gas and electricity unit costs is a critical economic factor. Typically, a kWh of electricity costs about four times that of a kWh of natural gas. As wholesale electricity prices are currently linked
to the price of gas, and not the cost of electricity generation this arguably inflates the cost of electricity above its real cost. As a retrofit technology heat pump
technology works best where the heat pump is replacing a high-cost existing heating system, such as direct electricity, and a low temperature heating system can be used. The financial case for a heat pump, as a boiler replacement, is improved where the existing boiler is at the end of its life – in that the avoided replacement cost of the boiler can be used to offset the cost of the heat pump. When looking at the financial case the availability of grants should be investigated. Currently in England & Wales there is a Boiler Upgrade Scheme, that provides a grant to cover part of the cost of replacing fossil fuel heating systems with a heat pump or biomass boiler.
Summary The heat pump is a key technology in the provision of decarbonised heating. It is proven technology that also benefits from the developments in refrigeration and air conditioning technology. The most effective deployment of heat pumps is in new build as all systems can be optimised to maximise the impact of the technology. The next cost- effective level is the replacement of high-cost heating systems – such as direct electric. Replacement of good condition, efficient gas-fired systems will give extended paybacks but can deliver carbon savings. When specifying a heat pump, it is important to take a complete system view that also includes the building/process that the heat is required for. Future regulation (e.g. zero carbon buildings) and changes to electricity pricing could positively impact on the uptake of heat pumps.
Sources of further information ● Heat Pumps CTV072, Carbon Trust, 2018
● Domestic Heat Pumps – A Best Practice Guide, MCS,2018
●
https://www.seai.ie/sites/default/ files/publications/Heat-Pump- Technology-Guide.pdf
●
https://es.catapult.org.uk/guide/ electrification-of-heat-heat-pump- user-guides/
●
https://www.quietmark.com/ products/certified-products/ centralheating/heatpumps
Produced in Association with
EIBI | SEPTEMBER 2025
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