An air source heat pump takes heat from the outside air and moves it into your home.
It uses electricity to do this. However, it can deliver more heat energy than the electricity it uses because some of the heat comes from the outside air.
So, it works differently from a gas boiler or standard electric heater.
How Does It Work?
An air source heat pump has an outdoor unit.
This takes heat from the outside air.
Then, the heat pump raises the temperature of that heat and moves it into your home.
In simple terms:
Outside air → Heat pump → Heating → Home
The process uses a special fluid called a refrigerant.
However, you do not need to understand the full refrigerant cycle to use a heat pump.
The important point is simple:
A heat pump moves heat rather than making all its heat directly from electricity.
Does It Work in Winter?
Yes.
Outside air still contains heat when the weather is cold.
So, an air source heat pump can continue to work when the temperature falls below 0°C.
However, it usually has to work harder as the outside temperature falls.
As a result, the amount of electricity it uses can change with the weather.
How Does It Heat Your Home?
Most home air source heat pumps in the UK heat water for the heating system.
The warm water can then flow through:
- radiators
- underfloor heating
- a hot-water cylinder
However, heat pumps often work with lower water temperatures than traditional boilers.
So, the heating system needs to be designed around this.
For example, some homes may need larger radiators.
Others may already have suitable radiators.
Will I Need New Radiators?
Not always.
It depends on how much heat each room needs and how much heat the existing radiators can provide.
So, a proper heat-loss check should be carried out first.
The installer can then work out whether each radiator is suitable.
Some may be large enough already.
Others may need to be changed.
Learn more: Insulation and Heat Loss →
What Is Heat Loss?
Your home is always losing some heat.
For example, heat can escape through:
- walls
- the roof
- windows
- doors
- floors
- gaps
Therefore, the heating system needs to replace the heat that is being lost.
A home that loses a lot of heat will need more heating.
So, before choosing a heat pump, it is important to know how much heat your home needs.
Does My Home Need Perfect Insulation?
No.
A home does not have to be perfectly insulated before a heat pump can work.
However, reducing heat loss can make heating easier and cheaper.
For example, loft insulation or draught proofing may reduce the amount of heat your home needs.
So, it often makes sense to look at sensible energy improvements before or alongside a new heating system.
Learn more: Insulation and Heat Loss →
What Is COP?
You may see COP when looking at heat pumps.
COP means Coefficient of Performance.
It compares the heat produced with the electricity used.
For example, if a heat pump uses:
1 kWh of electricity
and supplies:
3 kWh of heat
its COP is:
3
However, COP is not fixed.
It changes with things such as:
- outside temperature
- heating water temperature
- system design
- how the heat pump is being used
So, do not assume one COP figure will apply all year.
What Is SCOP?
You may also see SCOP.
This means Seasonal Coefficient of Performance.
It gives a wider view of performance across a heating season.
So, it can be more useful than looking at one COP figure from one set of conditions.
However, it is still an estimate based on set test conditions.
Your own results will depend on your home and how the system is used.
Can It Heat Hot Water?
Yes, many air source heat pump systems can also heat water for taps, baths and showers.
However, they usually work differently from a gas combi boiler.
A heat pump system will often use a hot-water cylinder to store hot water.
So, if you currently have a combi boiler and no cylinder, space may be needed for one.
Some systems may also use an immersion heater when needed.
Learn more: Your Hot Water System →
How Should a Heat Pump Be Used?
Heat pumps often work best when they provide steady heat over longer periods.
This is different from turning a powerful boiler on for a short burst.
So, you may need to use your heating controls differently.
For example, you may find that:
- heating runs for longer
- radiators feel warm rather than very hot
- room temperatures change more slowly
However, the exact settings depend on the system and the home.
So, follow the settings designed for your installation rather than copying someone else’s.
How Much Electricity Will It Use?
That depends mainly on how much heat your home needs and how well the heat pump performs.
For example, imagine your home needs 9,000 kWh of heat over a period.
If the heat pump delivers an average of three units of heat for every unit of electricity, it would use about:
9,000 ÷ 3 = 3,000 kWh of electricity
This is only a simple example.
However, it shows why both heat demand and heat-pump performance matter.
Will It Reduce My Bills?
Not necessarily.
A heat pump can use less energy than direct electric heating because it moves heat rather than producing all of it directly.
However, whether it costs less than another heating system depends on several things.
These include:
- how much heat your home needs
- heat-pump performance
- electricity prices
- your old heating fuel
- your old system efficiency
- how the new system is designed
- how you use the heating
So, do not judge a heat pump only by its efficiency figure.
Instead, compare the expected running cost for your home.
Where Does the Outdoor Unit Go?
The outdoor unit needs access to outside air.
It also needs enough clear space for air to move through it.
So, its position matters.
You should also think about:
- distance from windows and doors
- nearby walls
- neighbours
- access for servicing
- noise
- drainage
A good installer should consider these points when choosing the location.
Are Heat Pumps Noisy?
Air source heat pumps contain a fan and compressor.
So, they do make some noise while running.
However, the amount and type of sound vary between models and operating conditions.
Position also matters.
Government research has found that noise complaints are relatively uncommon, with poor installation and location contributing to some complaints.
So, check the sound information for the exact model and make sure the outdoor unit is placed carefully.
Do I Need Planning Permission?
The rules depend on where you live and on the installation.
In England, many air source heat pumps can be installed under permitted development rights if the conditions are met. The rules were made more flexible in May 2025.
However, planning rules can change and different rules apply across the UK.
So, check the current rules for your property before work starts.
Before You Buy
First, find out how much heat your home needs.
Then, ask how the system has been sized.
Also, check:
- expected yearly electricity use
- expected seasonal performance
- radiator sizes
- hot-water arrangements
- outdoor unit location
- noise
- controls
- installation cost
- warranty
- servicing needs
Finally, ask for the expected running cost and the assumptions used to calculate it.
Do not rely only on the size of your old boiler.
A heat pump should be designed around the heat your home actually needs.
What About Grants?
Government support for heat pumps can change.
So, rather than putting a grant amount permanently into this page, Energility should link to a separate current support section.
Check current heat-pump grants and support →
This keeps the main guide useful even when schemes or payment levels change.
Key Points
- An air source heat pump takes heat from the outside air and moves it into your home.
- It uses electricity, but it can provide more heat energy than the electricity it uses because some of the heat comes from the outside air.
- Heat pumps can work in cold weather. However, their performance can change as the outside temperature changes.
- Heat pumps often use lower heating-water temperatures than traditional boilers. So, the whole heating system needs to work well at these temperatures.
- You may not need to replace every radiator. A proper heat-loss check can show whether your existing radiators are suitable.
- Your home does not need perfect insulation. However, reducing heat loss can make the home easier and cheaper to heat.
- COP shows heat-pump performance at particular conditions, while SCOP gives a wider view across a heating season.
- Many heat pumps can also provide hot water. However, you may need a hot-water cylinder, especially if you are replacing a combi boiler.
- Heat pumps often work best by providing steady heat for longer periods. Your radiators may feel warm rather than very hot.
- Running costs depend on more than efficiency. Heat demand, electricity prices, system design, controls and how you use the heating all matter.
- The outdoor unit needs a suitable location. Airflow, noise, drainage, neighbours and access for servicing should all be considered.
- Check current planning rules and available support before going ahead, as these can change.
- Don’t size a heat pump simply from the size of your old boiler. It should be designed around how much heat your home actually needs.
- Ask for expected electricity use and running costs and check the assumptions behind the figures.
- Most importantly, a heat pump is part of a complete heating system. Good design, correct sizing and suitable controls are just as important as the heat pump itself.
Important Information
Energility provides general information to help you understand energy, household and business costs, services and ways to save money. It is not financial, legal, tax, technical or professional advice.
Prices, tariffs, grants, government schemes, regulations and other information can change. Costs, savings and calculator results are estimates and your actual results may be different.
Before making an important financial decision, signing a contract or carrying out significant work, check the latest information and consider getting advice from a suitably qualified professional where needed.
Energility may receive a commission or other benefit from some links or services. Where this applies, we aim to make it clear. This does not increase the price you pay unless stated otherwise.
Next: Ground Source Heat Pumps →
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