ZCA tech explainer series: what are heat pumps?

In our new series of blogs, we explore some of the key green technologies in the news today, giving our readers the lowdown on how these work, alongside the current state of play for their introduction and usage.
March 7, 2023

What is a heat pump?

Heat pumps are increasingly touted as an alternative to fossil fuel heating such as gas and oil. At present, gas boilers account for 74% of home heating in the UK, and the nation’s heating industry produces as much greenhouse gas emissions as all of the country's petrol and diesel vehicles combined. Comparable figures are observed in the US, where gas accounts for about half of all heating. One suggestion for reducing these high emissions is to use heat pumps.[i] Over the next 20 years, a heat pump is expected to save a UK household an average of £6,700 more than a gas boiler, all while reducing its carbon footprint by an average of 44%.

Heat pumps use electricity and work in a manner not too dissimilar to a refrigerator in reverse, rather than keeping food cool, refrigerant is used to provide heat through a process where it is evaporated into gas and condensed into a liquid. The Energy Saving Trust explains: “Everything around us contains thermal energy (heat). Heat naturally flows from a warmer place to a colder place. To provide heat energy in a home when outdoor temperatures are colder, we need heat to flow in the other direction – from a colder place to a warmer place. When the pressure of a gas increases, the temperature also increases. But how does it do it? When the pressure decreases, the temperature decreases. This relationship between pressure and temperature is the key to how a heat pump works.”[ii]

Figure 1: Diagram to show how a heat pump works

Source: Energy Saving Trust

Step by step, this process is as follows:

  1. The source of heat, which can be either air from outside or warmth from the ground, is blown or pumped over the heat exchange surface of the exterior part of the heat pump.
  2. This heat (which is cold when compared to a centrally heated home) is warm enough to cause the refrigerant liquid to evaporate and turn into a gas.
  3. This gas then moves through a compressor which increases pressure, thus causing the temperature to rise.
  4. The gas (now heated) is passed over the internal heat exchange surface. This heat can then be either blown around the interior of the building or transferred into a central heating or hot water system.
  5. The gas falls in temperature as the heat is transferred into the home and it subsequently returns to a liquid state.
  6. The cycle of reverse refrigeration repeats until the building reaches the required temperature set by the thermostat[iii].

Types of heat pump

There are two main types of heat pump, as explained below:

  • Air source heat pump (ASHP)

An air source heat pump sees the cold refrigerant begin its journey in an outside unit (known as an evaporator). Using fans to blow air across a heat exchanger, it is able to absorb heat, which can then be used to warm the building.

  • Ground source heat pump (GSHP)

In a ground source set-up, heat energy is gathered from water circulating in underground pipes, which is pumped to a heat exchanger inside the house. The cool water, mixed with antifreeze and referred to as ‘brine’, passes through the heat exchanger, transferring the heat to the refrigerant, continuing its journey around the compressor circuit[iv].

There are also several less common types of heat pump; these are air to air- which heats external air and blows it into the property (not dissimilar to what we recognise as air conditioning systems), therefore most suited to flats or small areas- water source heat pump, which uses the heat in a water source such as a lake, river, spring, well, or borehole, and hybrid (or dual energy) which uses a heat pump plus another fuel source, for example, gas or even hydrogen in the future. In the case of the hybrid model, the system monitors outside temperatures and chooses the most energy-efficient heating method.

Introduction and government support

Heat pumps are forecast to play a significant role in the future of how we heat our homes. According to the International Energy Agency, 22% of households in North America, Asia, and Europe will have air source heat pumps by 2030[v]. In the UK, a recent study by the Energy Efficiency Infrastructure Group (EEIG) found that this parliament will need public investments totalling £5.1 billion for heat pumps.[vi]

Currently, the UK government is providing the Boiler Upgrade Scheme, which offers households £5,000 to replace their gas boilers with heat pumps. As Eco Experts note, price is presently a significant barrier towards uptake, with 69% of consumers ranking cost as the most important factor when evaluating which low-carbon product to purchase, yet the average air source heat pump costs £10,000 to buy and install, a significant outlay. Further costs, where necessary, come from upgrading the heating infrastructure in the home itself. For example, homeowners may need to install underfloor heating or improv insulation in order to make the most of their heat pump system. Additionally, new radiators are often required. These are generally bigger than traditional ones, allowing heat to be efficiently transferred into the house.


[i] BBC- The enormous heat pumps warming cities

[ii] In-depth guide to heat pumps - Energy Saving Trust

[iii] How do heat pumps work? | National Grid Group

[iv] Ibid

[v] Heat Pumps – Analysis - IEA

[vi] EEIG- Better buildings investment plan

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Lauren Foye
Head of Reports

Lauren has extensive experience as an analyst and market researcher in the digital technology and travel sectors. She has a background in researching and forecasting emerging technologies, with a particular passion for the Videogames and eSports industries. She joined the Critical Information Group as Head of Reports and Market Research at GRC World Forums, and leads the content and data research team at the Zero Carbon Academy. “What drew me to the academy is the opportunity to add content and commentary around sustainability across a wealth of industries and sectors.”

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