Elite Energy News

How Air Source Heat Pumps Work

Published: 28th May 2026

An air source heat pump heats your home and hot water by extracting thermal energy from the outside air.

Inside the unit, a refrigerant absorbs that heat, and a compressor raises it to a temperature high enough to warm your radiators, underfloor heating, and hot water cylinder. Because the system moves heat rather than burning fuel to create it, a heat pump can produce three to four units of heat for every one unit of electricity it uses.

Our guide explains how air source heat pumps work for homes, what they're made up of, how the refrigeration cycle operates, how heat reaches your radiators and hot water, and how to tell if one is right for your property.

If you're considering an air source heat pump, you can get a free quote or explore our ASHP installation service.

What Does an Air Source Heat Pump Do?

An air source heat pump replaces your current heating system, gas, oil, solid fuel & LPG boilers or electric storage heaters. It provides your central heating and domestic hot water from a single system, using electricity to move heat from the outside air into your home rather than burning fuel to create it.

The type installed in most UK homes, and the type we install at Elite Energy, is an air-to-water system. This means the heat extracted from the air is transferred into a water-based heating circuit, which feeds your radiators, underfloor heating, and a hot water cylinder. Air-to-air systems also exist, which heat room air directly rather than water, but these don't provide hot water and are less common in residential properties.

Modern air source heat pumps are rated to operate in temperatures as low as -25°C. The outside air may feel cold, but it still contains thermal energy that the system can extract and use.

How Does an Air Source Heat Pump Work?

An air source heat pump operates using a continuous process called the refrigeration cycle. This cycle moves heat from the outside air into your home's heating system through four stages: evaporation, compression, condensation, and expansion.

Step 1 – Evaporation

The outdoor unit contains a heat exchanger filled with refrigerant, a fluid with a very low boiling point (typically around -25°C to -30°C). A fan draws outside air across the heat exchanger. Even when the air temperature is low, it contains enough thermal energy to warm the refrigerant above its boiling point, causing it to evaporate from a cold liquid into a low-pressure gas.

Step 2 – Compression

The low-pressure gas is drawn into the compressor, the component that uses the system's electricity. The compressor squeezes the gas, increasing its pressure. When gas is compressed, its temperature rises. This is the same principle that makes a bicycle pump feel warm when you use it. The compressor raises the refrigerant temperature to between 50°C and 75°C, hot enough to heat your home and hot water.

Step 3 – Condensation

The hot, high-pressure gas passes through a second heat exchanger called the condenser. Here, the heat from the refrigerant transfers into the water in your central heating circuit. As the refrigerant releases its heat, it cools and condenses back into a liquid. The heated water is then circulated to your radiators, underfloor heating, or hot water cylinder.

Step 4 – Expansion

The liquid refrigerant passes through the expansion valve, which rapidly reduces its pressure. This drops the temperature back down, returning the refrigerant to its cold, low-pressure state, ready to absorb heat from the outside air again. The cycle then repeats continuously for as long as your home needs heating.

Components of an Air Source Heat Pump

An air source heat pump system is made up of several components, each with a specific role in the refrigeration cycle. Here's what each one does and where it sits in the system:

1. The Outdoor Unit
The most visible part of the system, installed outside your property. Contains the evaporator, fan, and compressor. Typically about the size of a large suitcase or washing machine. Requires adequate airflow around it, which is why positioning is assessed during the survey.

2.The Compressor
The component that uses the system's electricity. It compresses the refrigerant gas to raise its temperature from ambient air level to 50–75°C. The efficiency of the compressor directly affects the overall efficiency of the system, which is measured as COP or SCOP.

3.The Heat Exchangers
There are two in the system. The evaporator (in the outdoor unit) absorbs heat from the outside air. The condenser transfers that heat into your home's water circuit. Together, they form the two points in the cycle where heat changes hands.

4.The Expansion Valve
A small but critical component on the return side of the cycle. It reduces the pressure of the refrigerant after it has released its heat, cooling it back down so it can absorb heat from the air again.

5.The Hot Water Cylinder
Located inside your home. This stores heated water for your taps, showers, and baths. Unlike a combi boiler which heats water on demand, a heat pump heats water to a set temperature and stores it in the cylinder.

6. The Controls
Modern ASHP systems include smart controls that allow you to set heating schedules, monitor performance, and adjust temperatures room by room. Some systems offer weather compensation, which automatically adjusts the output based on the outside temperature to maintain efficiency without you needing to intervene.

Monobloc vs Split Systems

There are two configurations of air-to-water heat pump.

Monobloc systems house all the refrigerant components in the outdoor unit. Water pipes run from the outdoor unit into the house. This is the simpler installation and the most common configuration for domestic properties.

Split systems have the outdoor unit connected to a separate indoor unit via refrigerant lines. The heat transfer to water happens inside the house, which can offer more flexibility where space or pipework routing is a consideration.

We install both. Which one we recommend is determined by the suitability of your property, assessed during the free home survey.

How Does a Heat Pump Heat Your Home?

Once the refrigeration cycle has produced heated water, it needs to reach your rooms. Here's how that works depending on your heating setup.

Radiators

Heated water from the heat pump circulates through your radiators in the same way it would from a boiler. The key difference is the flow temperature; a heat pump typically operates at 35–55°C, compared to 60–80°C for a gas boiler.

This means the system runs for longer periods at a lower, steadier temperature rather than short bursts of intense heat. In some homes, existing radiators work well at these temperatures. In others, larger radiators may be needed to deliver the same level of warmth.

Underfloor Heating

Underfloor heating is one of the most efficient pairings with a heat pump. It covers a much larger surface area than radiators, so it can heat a room effectively at flow temperatures of around 30–35°C, which is where a heat pump runs most efficiently. If you're planning a renovation or extension, underfloor heating is worth considering alongside an ASHP.

Hot Water

The heat pump heats water in your cylinder to around 50–55°C, slightly lower than a typical gas boiler at 60–65°C. In practice, this is more than adequate for showers, taps, and everyday use. The system heats the cylinder automatically based on your schedule.

How It Differs from a Gas Boiler in Use

A gas boiler fires up, heats the house quickly, then switches off. A heat pump runs at a lower output for longer periods, maintaining a consistent temperature rather than cycling on and off. This is more efficient, but it means the system works best when allowed to run steadily. Most homeowners find the warmth more even and comfortable once they adjust to this.

Do Air Source Heat Pumps Work in Winter?

Yes. Air source heat pumps are rated to operate in temperatures as low as -25°C. The refrigerant inside the system has a boiling point well below freezing, which means it can absorb heat from the air even when it feels cold to us.

Efficiency does decrease as temperatures drop, the system has to work harder to extract heat from colder air, which uses more electricity. But UK temperatures rarely fall below -5°C for sustained periods, and systems are sized for your home's peak heat demand rather than average conditions. A correctly sized system will maintain consistent warmth through the coldest months.

It's worth noting that some of the biggest adopters of heat pumps in Europe are in Scandinavia – Sweden, Norway, and Finland – where winter temperatures regularly fall well below anything the UK experiences.

Are Air Source Heat Pumps Efficient

Heat pump efficiency is measured in two ways.

COP (Coefficient of Performance) measures efficiency at a single point in time under specific conditions. A COP of 4.0 means the system is producing 4 kWh of heat for every 1 kWh of electricity it uses.

SCOP (Seasonal Coefficient of Performance) averages efficiency across a full heating season, accounting for the range of temperatures the system operates in throughout the year. This gives a more realistic picture of what you'll actually experience. Most modern ASHPs achieve a SCOP of 3.5–5.0.

In summer, when the air is warmer and the system is mainly heating water, a heat pump can achieve a COP of 5.0 or higher. In winter, when the air is colder and the system is working harder to extract heat, COP may drop to around 2.5–3.5. SCOP accounts for this full range and gives you the average across the year, which is why it's the more useful figure when comparing systems.

To put that in context, a new A-rated gas boiler operates at around 90% efficiency – producing 0.9 kWh of heat for every 1 kWh of gas it burns. Even at the lower end of winter performance, a heat pump is still producing over two and a half times more heat per unit of energy. Across the full year at SCOP 4.0, it's producing four times more.

For a detailed breakdown of how this affects what you'll pay to heat your home, see our air source heat pump cost guide.

Is an Air Source Heat Pump Right for Your Home?

Air source heat pumps are suitable for a wide range of UK property types, from terraced houses to detached rural homes. Whether one is right for your property depends on a few factors.

If you're unsure, a free home survey can confirm whether an ASHP is suitable for your property, with no obligation to proceed.

Outdoor Space

You need room for the outdoor unit with adequate airflow around it. A rear garden, side passage, or yard is usually sufficient.

Insulation

A well-insulated home gets the most from a heat pump. If your home loses heat quickly, the system has to work harder, which reduces efficiency. Where insulation improvements are needed, these can sometimes be included through ECO4.

Existing Heating System

Heat pumps can replace gas, oil, or LPG boilers. If your home currently uses a combi boiler, a hot water cylinder will need to be added. Existing radiators may work at lower flow temperatures or may need upsizing.

Electrical Capacity

A heat pump requires a dedicated electrical supply. Some older properties may need a consumer unit upgrade, which is identified during the survey.

Noise and Positioning

While modern units operate at around 40–50 decibels (similar to a fridge), positioning matters. The outdoor unit needs to be placed where airflow is unobstructed and where noise won't be an issue for you or your neighbours.

How Is an Air Source Heat Pump Installed?

Installation typically takes 3-5 days. This includes positioning and mounting the outdoor unit, running pipework to your indoor heating system, connecting the hot water cylinder, and integrating with your existing radiators or underfloor heating. Once fitted, the system is commissioned, tested, and the controls are set up.

At Elite Energy, every installation is handled by our in-house MCS-certified engineers. For a full breakdown of the process, see our ASHP installation service.

Considering an Air Source Heat Pump?

If you're thinking about replacing your gas, oil, solid fuel, LPG boiler or electric storage heaters with a heat pump, we can help you understand exactly what's involved for your property. Our free home survey covers suitability, system sizing, and a fixed-price quote with no obligation. We also handle BUS and ECO4 grant applications on your behalf, so if funding is available for your home, we'll find it.

Get Your Free Quote Explore ASHP Installation

Frequently Asked Questions

What is the difference between COP and SCOP?

COP measures efficiency at a single point in time under specific conditions. SCOP averages efficiency across a full heating season, accounting for temperature variation throughout the year. SCOP gives the more realistic picture of what you'll experience.

How much does an air source heat pump cost?

A typical installation costs between £8,000 and £18,000. The Boiler Upgrade Scheme can reduce this by up to £7,500 when replacing gas, or up to £9,000 when replacing oil or LPG. ECO4 can cover the full cost for eligible households. For a full breakdown, see our air source heat pump cost guide.

Can a heat pump work with my existing radiators?

In many cases, yes. Some radiators may need upsizing to work effectively at the lower flow temperatures a heat pump operates at. We assess this during the survey and include any changes in the quote.

How noisy is an air source heat pump?

Most modern units operate at around 40–50 decibels, roughly equivalent to a fridge. Unit positioning and noise impact are factored into every site survey.

How long does a heat pump last?

15–25 years with annual servicing, compared to 10–15 years for a gas boiler.

Do I need planning permission?

In most cases, no. Domestic ASHP installations fall under permitted development rights in England. Exceptions can apply for listed buildings and conservation areas. We confirm this during the survey.

Published: 28th May 2026