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How do heat pumps actually work?

Heat pumps are a clean alternative to traditional heating systems. They are four times more efficient than a gas or oil boiler and slash energy costs by up to 25%.

In this article, we'll dissect the anatomy of heat pumps, examining their essential parts such as the heat exchanger and expansion valve. We'll also tackle the burning question on everyone's mind: how do heat pumps actually work?

How much could your heat pump cost?
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What is a heat pump?

Heat pumps efficiently transfer heat energy from one location to another, providing both heating and cooling solutions for homes and buildings. Brought into residential use in the 1960s, heat pumps have become a sustainable alternative to conventional gas boilers and heating systems.

Heat pumps use renewable sources to minimise energy consumption and carbon emissions. Countries like Sweden have embraced this technology, witnessing a significant reduction in CO₂ output from home heating compared to regions like the UK, which rely more heavily on traditional heating systems.

Not only are heat pumps built for maximum efficiency, even in cold weather, but they also contribute to lower energy bills.

The Aira Heat Pump
Aira Heat Pump outdoor unit against a grey wall covered in snow

How do heat pumps work?

Heat pumps work by transferring thermal energy from a cooler environment to a warmer one. Unlike traditional heating methods that generate warmth by combusting fuels or using electrical resistance, heat pumps move existing heat from one place to another. This is why they are more efficient and environmentally friendly than traditional fossil fuel boilers.

Central to the operation of a heat pump is the refrigerant (our Aira Heat Pump uses R290) which circulates within the system, undergoing pressure changes that cause it to heat up or cool down. This enables the heat pump to move thermal energy, which can then be used to heat a property, without the need to produce new heat energy through combustion.

A step-by-step guide to how a heat pump works

Heat pump infographic showing the refrigerant evaporating and being compressed into the condenser

Step 1

The outdoor unit of the heat pump system draws in the surrounding air using a fan. This air passes over an evaporator, where the air's natural heat is absorbed by a refrigerant. The refrigerant, now in a gaseous state, is then compressed by a compressor, raising its temperature.

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Step 2

The hot gas from the compressor travels into a condenser, where it transfers its heat to the water in the heating and hot water system. As it releases its heat, the refrigerant condenses back into a liquid. 

Heat pump infographic showing the now cold refrigerant passing through the expansion valve where it loses pressure and temperature

Step 3

The now-liquid refrigerant passes through an expansion valve, where its pressure is reduced. This causes the refrigerant to cool down before it re-enters the evaporator to repeat the cycle. And that's it! Your home and hot water are heated using free, fresh air. 

Types of heat pumps

There are three common types of heat pumps: air source heat pumps, ground source heat pumps (otherwise known as geothermal heat pumps), and water source heat pumps. Each type extracts heat energy from different elements of the environment. 

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Air source heat pumps

The most common form of heat pump, air source heat pumps absorb heat from the air via an outdoor unit and transfer it indoors to raise the temperature.

They come in two forms: air-to-water systems, which supply hot water and central heating via an indoor unit and buffer tank, and air-to-air systems, which directly warm the air in a room, akin to air conditioning units.

While the upfront installation cost can be higher than traditional boilers, they are typically cheaper than ground or water source alternatives and the long-term savings on energy bills justify the investment. Additionally, there are a range of government grants in the UK to help offset the upfront costs.

Learn more about air source heat pumps

See how an air to water heat pump works

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Close up of the ground with the sky out of focus

Ground source heat pumps

Taking advantage of the relatively constant temperatures beneath the Earth's surface, ground source heat pumps extract heat from the ground via water or refrigerant circulating in loops of buried pipes.

The consistent ground temperature ensures a regular supply of heat, which is captured by the pipes under the ground, passing through a heat exchanger in the heat pump on the surface before supplying the home with heating or hot water.

Ground source heat pumps require a large amount of outdoor space and are typically more expensive to install than an air source heat pump.

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Water source heat pumps

Like their ground counterpart, water source heat pumps harness naturally occurring heat but, in this case, from bodies of water such as lakes, rivers or ponds. They operate by drawing heat from these water sources, compressing it to increase the temperature and then distributing it to heat living spaces or provide hot water.

A water source heat pump's energy efficiency is attributed to the fact that water maintains a constant temperature, even in colder seasons. However, it's important to note that a water source heat pump installation may be more complex compared to an air source heat pump unit.

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Components of a heat pump system

Heat pumps are complex central heating systems. To understand how they function, here are the key components that enable their operation.

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Evaporator coil
An evaporator coil is a network of tubes filled with cool refrigerant. It passes the cool refrigerant across the first heat exchanger, allowing it to evaporate from a liquid into a gas.
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Evaporator coil

An evaporator coil is a network of tubes filled with cool refrigerant. It passes the cool refrigerant across the first heat exchanger, allowing it to evaporate from a liquid into a gas.

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First heat exchanger
The first heat exchanger is where heat from the external environment (air, ground or water) is efficiently transferred to the refrigerant inside the system.
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First heat exchanger

The first heat exchanger is where heat from the external environment (air, ground or water) is efficiently transferred to the refrigerant inside the system.

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Compressor
The compressor's job is to compress the refrigerant gas, boosting its pressure and temperature. The efficiency of this process is crucial for the heat pump's overall efficiency, as it determines how much heat the pump can generate from each unit of electricity.
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Compressor

The compressor's job is to compress the refrigerant gas, boosting its pressure and temperature. The efficiency of this process is crucial for the heat pump's overall efficiency, as it determines how much heat the pump can generate from each unit of electricity.

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Condenser coil
The compressed gas enters the condenser coil, before passing across a second heat exchanger to heat the home's heating system.
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Condenser coil

The compressed gas enters the condenser coil, before passing across a second heat exchanger to heat the home's heating system.

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Second heat exchanger
The second heat exchanger is where the hot refrigerant heats cold water to warm your home and hot water. As it heats the cold water, its temperature reduces, condensing back into a liquid.
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Second heat exchanger

The second heat exchanger is where the hot refrigerant heats cold water to warm your home and hot water. As it heats the cold water, its temperature reduces, condensing back into a liquid.

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Expansion valve
The expansion valve reduces the refrigerant's pressure, cooling it down further, and passes it into the evaporator coil for the process to begin again.
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Expansion valve

The expansion valve reduces the refrigerant's pressure, cooling it down further, and passes it into the evaporator coil for the process to begin again.

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Does a heat pump work in winter?

A quality heat pump is remarkably efficient in the cold, even outperforming fossil-fuel heating systems in terms of energy use.

Ground and water-source heat pumps are especially reliable during winter. So too are modern air source heat pumps, like the Aira Heat Pump, which can maintain a cosy indoor environment even if the outdoor temperature reaches -25°C.

Proper installation and additional measures like larger radiators and insulation are important to help mitigate heat loss during colder weather.

Do heat pumps work in cold weather?

Learn more about heat pumps

Aira Heat Pump outdoor unit at a slight angle

The Aira Heat Pump

Some say intelligent, affordable and carbon emission-free heating sounds too good to be true. We say it sounds like Aira.

Aira heat pump outdoor unit face on against a wall with some potted plants either side

How much are air source heat pumps?

Air source heat pumps have the power to slash your heating bills by 25%. And they cost less than you think.

Aira Heat Pump outdoor unit against a grey wall covered in snow

What are air to water heat pumps?

Air to water heat pumps use thermal energy to heat your home and water. But how do they work and how are they installed?

How much will your heat pump cost?

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