How Does a Heat Pump Work? Simple Homeowner Guide

The Simple Explanation

A heat pump works like a fridge in reverse. It draws heat from the outdoor air — even at temperatures below freezing — concentrates that heat using a compressor, and transfers it into your home via your radiators, underfloor heating, and hot water cylinder. For every 1 kWh of electricity the compressor uses, a well-installed heat pump delivers 3–4 kWh of useful heat. This ratio (the COP) is what makes heat pumps significantly more efficient than gas boilers and comparable or cheaper to run at 2026 energy prices (Ofgem: electricity 24.5p/kWh, gas 7.4p/kWh).

This is not experimental technology. Heat pumps have been the primary heating source in Scandinavia for decades, where winter temperatures regularly drop well below freezing.

The Four-Step Heating Cycle

Inside every heat pump, there is a continuous cycle that moves heat from outside to inside your home. It uses a special fluid called a refrigerant that changes between a liquid and a gas as it circulates through the system. Here is how it works, step by step.

Step 1: Absorbing heat from the air

The outdoor unit of a heat pump contains a large fan and a component called an evaporator. The fan draws outdoor air across the evaporator, which contains cold refrigerant fluid. Because the refrigerant is colder than the outside air, heat naturally flows from the air into the refrigerant — just as a cold drink absorbs warmth from a warm room.

As it absorbs heat, the refrigerant warms up and turns from a cold liquid into a cool gas.

Step 2: Compressing the gas to make it hot

The cool gas then passes into a compressor — the most important component in the entire system. The compressor squeezes the gas into a much smaller space, which increases its temperature dramatically.

Think of how a bicycle pump gets warm when you pump up a tyre. Compressing air (or any gas) makes it hotter. The compressor does the same thing to the refrigerant gas, raising its temperature from around 5–10 degrees up to 50–65 degrees — hot enough to heat your home and your hot water.

This compression step is where the heat pump uses electricity. The compressor is powered by an electric motor, and this is the main energy cost of running a heat pump.

Step 3: Releasing heat into your home

The hot, compressed gas now flows into the indoor part of the system, where it passes through a heat exchanger called a condenser. Here, the heat from the hot gas is transferred into your heating system — warming the water that circulates through your radiators or underfloor heating, and heating the water stored in your hot water cylinder.

As the refrigerant gives up its heat, it cools down and turns back into a liquid.

Step 4: Expanding and starting again

The cooled liquid refrigerant passes through an expansion valve, which reduces its pressure and temperature even further — making it cold again and ready to absorb more heat from the outdoor air.

The refrigerant then flows back to the outdoor unit, and the whole cycle starts again. This process runs continuously, with the compressor speeding up and slowing down as needed to maintain your home at the temperature you have set.

Why Does a Heat Pump Produce More Heat Than It Uses in Electricity?

A heat pump does not create heat — it moves heat that already exists in the outdoor air. The electricity powers the compressor and fans, but most of the heat delivered to your home actually comes from the air outside.

For every 1 unit of electricity a heat pump uses, it typically delivers 3 to 4 units of heat into your home. This ratio is called the Coefficient of Performance (COP).

A heat pump with a COP of 3.0 produces 3 kWh of heat for every 1 kWh of electricity
A heat pump with a COP of 4.0 produces 4 kWh of heat for every 1 kWh of electricity

By comparison, even the most efficient gas boiler converts just 0.92 kWh of heat from 1 kWh of gas (92% efficiency). An electric heater converts exactly 1 kWh of heat from 1 kWh of electricity (100% efficiency). A heat pump at COP 3.5 is effectively 350% efficient — it delivers 3.5 times more energy than it consumes.

This is why heat pumps are considered one of the most energy-efficient ways to heat a home, and why the UK government supports homeowners who switch through the £7,500 BUS Grant (subject to eligibility).

Does the COP change with the weather?

Yes. The COP is highest when the outdoor temperature is mild and drops as it gets colder. On a 10-degree day, your heat pump might achieve a COP of 4.0 or higher. On a minus 5-degree day, it might drop to 2.5.

Over the course of a full heating season (October to April), the average COP — known as the Seasonal Coefficient of Performance (SCOP) — is typically 3.0 to 3.5 for a well-installed system in the UK.

Even on the coldest days, a modern heat pump still delivers significantly more heat than the electricity it consumes.

Air Source vs Ground Source: What Is the Difference?

There are two main types of heat pump used in UK homes. Both work on the same principle — extracting heat from outside and delivering it inside — but they draw heat from different sources.

Air source heat pumps (ASHP)

An air source heat pump extracts heat from the outdoor air using a unit that sits outside your home, usually on a concrete base next to an external wall. It looks similar to an air conditioning unit.

Advantages: - Lower cost to install (£10,000–£14,000 before the BUS Grant — subject to eligibility) - Simpler installation — no digging required - Suitable for almost all property types, including terraced houses - Installation typically completed in 2–3 days

Considerations: - Efficiency drops slightly in very cold weather - The outdoor unit produces some noise (similar to a quiet conversation)

Air source heat pumps are by far the most popular choice in the UK, accounting for over 95% of domestic heat pump installations.

Ground source heat pumps (GSHP)

A ground source heat pump extracts heat from the ground via pipes buried in your garden, either in a horizontal trench (1–2 metres deep) or a vertical borehole (up to 100 metres deep). The ground temperature stays relatively constant at around 10–12 degrees year-round.

Advantages: - Slightly higher efficiency (COP 4.0–5.0) - No visible outdoor unit - No noise outside the home

Considerations: - Significantly higher cost (£20,000–£35,000 before the BUS Grant) - Requires substantial garden space for horizontal loops, or expensive borehole drilling - Installation takes 1–2 weeks - Not feasible for many urban properties

For most UK homeowners, an air source heat pump offers the best balance of performance, cost, and practicality.

The Main Components of a Heat Pump System

The outdoor unit

This is the box that sits outside your home, typically on a concrete pad next to a side or rear wall. It contains the evaporator, fan, and compressor. A typical domestic outdoor unit measures around 1 metre wide, 0.7 metres deep, and 0.8 metres tall. It needs a clearance of at least 300mm around it for airflow.

The indoor unit (or hydraulic module)

Inside your home, there is a wall-mounted unit (roughly the size of a small boiler) that contains the heat exchanger, circulation pump, and controls. Most modern domestic heat pumps are monobloc systems, meaning all the refrigerant stays in the outdoor unit and only hot water pipes come inside. This simplifies installation.

The hot water cylinder

Unlike a combination boiler, which heats water on demand, a heat pump works best with a hot water cylinder that stores a tank of hot water ready for when you need it. A typical cylinder holds 170–250 litres. If you currently have a combination boiler and no cylinder, one will need to be installed as part of the heat pump conversion.

Radiators or underfloor heating

A heat pump delivers heat at a lower temperature than a gas boiler — typically 45–55 degrees compared to 70–80 degrees for a boiler. This means some radiators may need to be slightly larger. In most homes, the existing radiators work perfectly well. Where individual radiators are too small, they can be swapped — this typically affects only one or two radiators.

Underfloor heating is the ideal partner for a heat pump because it is designed to run at low temperatures.

The controls and thermostat

Modern heat pumps come with smart controls — often including a smartphone app — that let you set temperatures for different times of day, monitor energy consumption, and receive maintenance alerts.

How a Heat Pump Heats Your Rooms

When you set your thermostat to 21 degrees and the room is currently at 18 degrees, the heat pump kicks into action:

The compressor speeds up, increasing the rate of heat transfer from outside to inside
Hot water (at around 45–55 degrees) is pumped through your radiators or underfloor heating
The radiators release heat into the room, gradually warming it
As the room approaches 21 degrees, the heat pump slows down and runs at a lower, more efficient output
Once the room reaches 21 degrees, the heat pump drops to a very low output — just enough to maintain the temperature

Unlike a gas boiler, which blasts out heat at full power and then switches off, a heat pump runs continuously at a low, steady output. This produces a more even, comfortable temperature throughout the day and is actually more efficient than cycling on and off.

Top tip: For the best efficiency and comfort, set your heat pump to a consistent temperature and let it run gently throughout the day. Avoid turning it up and down repeatedly — that forces the compressor to work harder and uses more electricity.

How a Heat Pump Heats Your Hot Water

The heat pump heats water in the cylinder to around 48–52 degrees Celsius, which is perfectly adequate for showers, baths, and washing up.

Once a week, the heat pump automatically raises the cylinder temperature to 60 degrees for a few hours. This is called a legionella pasteurisation cycle, and it kills any bacteria that could build up in stored water. This happens automatically — you do not need to do anything.

A 200-litre cylinder heated to 50 degrees holds enough hot water for two standard showers, one bath, and multiple hand washes and washing-up sessions. The heat pump will reheat the cylinder as the water is used, typically recovering fully within 2–3 hours.

How Electromatic Can Help

Electromatic M&E Ltd offers free home surveys across London and Surrey (TW, KT, SW postcodes). We handle BUS Grant applications (subject to eligibility), install ASHP and solar PV from a single contractor, and deliver within 2–4 weeks of survey confirmation. All work is carried out under MCS certification via our accredited umbrella partner. We will explain exactly how a heat pump would work in your specific home — including expected running costs and the BUS Grant deduction.

Book your free home survey →

Call us: 07718 059 284 | Email: admin@electromatic.uk

Frequently Asked Questions

How does a heat pump work in simple terms?

A heat pump works like a fridge in reverse. It draws outdoor air across a cold refrigerant, extracts heat from that air, compresses the refrigerant to raise its temperature, and transfers that heat into your home via radiators or underfloor heating. For every 1 kWh of electricity it uses, a well-installed heat pump delivers 3–4 kWh of useful heat.

Can a heat pump heat a whole house?

Yes. A properly sized air source heat pump is designed to be the sole heating source for your entire home — every room and your hot water. The system is sized based on a detailed heat loss calculation of your property, ensuring it can maintain comfortable temperatures even on the coldest winter days. Millions of homes across Scandinavia and the UK are heated entirely by heat pumps.

Does a heat pump use a lot of electricity?

A heat pump uses significantly less electricity than most people expect. A typical three-bedroom home requires around 3,000–3,500 kWh of electricity per year to run the heat pump — costing roughly £735–£858 at 2026 Ofgem rates (24.5p/kWh). This is comparable to or slightly less than running a gas boiler on a standard tariff, and the gap widens further on smart electricity tariffs or with solar panels.

Will my radiators need replacing?

In most cases, no. A proper heat loss survey will identify whether any individual radiators are undersized for lower-temperature operation. Typically 80–90% of existing radiators work perfectly well with a heat pump. Only the one or two smallest radiators in the house may need to be swapped for slightly larger ones, typically at a cost of £200–£400 each.

How long does a heat pump last?

A well-installed air source heat pump has an expected lifespan of 20–25 years, significantly longer than a gas boiler (12–15 years). The outdoor unit requires minimal maintenance — typically an annual check and occasional cleaning. There are no annual gas safety checks required, and there is no risk of carbon monoxide.

The information in this article is for general guidance only and does not constitute financial, legal, or technical advice. Energy savings estimates are based on typical UK household data from the Energy Saving Trust and Ofgem (April 2026 price cap). Actual savings depend on your property type, insulation levels, energy usage patterns, and electricity tariff. The Boiler Upgrade Scheme (BUS) grant of £7,500 is subject to eligibility criteria set by Ofgem — not all properties qualify. Electromatic M&E Ltd operates under MCS certification via our accredited umbrella partner. All installations comply with Building Regulations Part L and MCS standards. E&OE.

Written by Electromatic M&E Ltd — ASHP & Solar installer, London & Surrey (electromatic.uk)

Last updated: April 2026 | Electromatic M&E Ltd, Company No. 13837345

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