How Air Source Heat Pumps Work

A Simple Guide to Heating Your Home Using Energy from the Air

An air source heat pump takes heat from the outside air and moves it into your home.

That may sound strange, especially on a cold winter day.

However, even cold air contains thermal energy.

A heat pump can collect some of that energy and raise its temperature so it can be used for heating and hot water.

Unlike a gas boiler, it does not create most of its heat by burning fuel.

Instead:

It uses electricity to move heat from one place to another.

Understanding that simple idea makes the rest much easier.

Think About Your Fridge

You probably already have a type of heat pump in your home.

Your fridge takes heat from inside the fridge and releases it into the kitchen.

That is why the back of a fridge can feel warm.

An air source heat pump uses a similar principle.

However, instead of removing heat from your fridge, it collects heat from the outside air and transfers it into your heating system.

The direction is:

Outside air → Heat pump → Heating system → Home

How Can Cold Air Contain Heat?

We often describe winter air as cold.

However, cold does not mean there is no thermal energy present.

Even when the outside temperature falls below 0°C, there is still energy in the air.

An air source heat pump can collect some of this energy.

The challenge is getting that low-temperature heat to a temperature useful for your home.

This is where the refrigerant cycle comes in.

The Refrigerant Cycle

Inside an air source heat pump, a special fluid called a refrigerant moves through a continuous cycle.

The process has four main stages.

1. The Refrigerant Collects Heat

Outside air passes across a heat exchanger.

The refrigerant inside can absorb heat from that air.

Because the refrigerant has suitable properties and is at low pressure at this stage, it can evaporate at a low temperature.

It changes from a liquid into a gas.

2. The Compressor Raises the Temperature

The refrigerant gas then enters a compressor.

The compressor uses electricity to increase the pressure of the refrigerant.

As its pressure rises, its temperature rises too.

We now have a much hotter refrigerant.

3. Heat Moves Into Your Heating System

The hot refrigerant passes through another heat exchanger.

Here, heat transfers into the water used by your heating system.

That heated water can then travel to:

• Radiators
• Underfloor heating
• A suitable hot-water system

As the refrigerant gives up its heat, it cools and condenses back towards a liquid.

4. The Pressure Drops Again

The refrigerant then passes through an expansion device.

Its pressure falls.

This lowers its temperature so it can once again absorb heat from the outside air.

The cycle begins again.

So, in simple terms:

Collect heat → Compress → Transfer heat → Reduce pressure → Repeat

Where Does the Electricity Go?

The heat pump needs electricity to operate.

In particular, electricity powers components such as the compressor, fans, pumps and controls.

However, the electricity is not the only source of the heat delivered to your home.

Some of the energy comes from the outside air.

This is why a heat pump can deliver more heat energy than the electrical energy it consumes.

That brings us to one of the most important heat-pump numbers.

What Is COP?

COP means:

Coefficient of Performance

It compares the amount of heat delivered with the electricity used by the heat pump under particular conditions.

For example, suppose a heat pump uses:

1 kWh of electricity

and delivers:

3 kWh of heat

The COP would be:

3 ÷ 1 = 3

So:

COP = Heat delivered ÷ Electricity used

The extra heat has not appeared from nowhere.

Much of it has been collected from the outside environment.

COP Changes

A heat pump does not have one fixed COP in all conditions.

Its performance changes.

For example, the heat pump generally has to work harder when:

• Outside air is colder
• Heating water needs to be hotter
• Hot-water temperatures are required
• Operating conditions are less favourable

Therefore, seeing a single COP figure does not tell you exactly how the heat pump will perform throughout an entire year.

For that, another figure can be more useful.

What Is SCOP?

SCOP means:

Seasonal Coefficient of Performance

Rather than looking at one particular operating condition, SCOP represents performance across a heating season under defined conditions.

Suppose a system delivers:

10,000 kWh of heat

over a period while using:

2,857 kWh of electricity

The simplified calculation is:

10,000 ÷ 2,857 ≈ 3.5

That gives a seasonal performance of around:

3.5

This does not tell us the actual running cost yet.

However, it gives us a much better starting point.

Why Flow Temperature Matters

A gas boiler can normally send quite hot water around a heating system.

Heat pumps usually work more efficiently when they do not have to raise the water temperature as high.

The temperature of water leaving the heat pump for the heating system is known as the flow temperature.

Generally:

Lower flow temperature can help a heat pump operate more efficiently.

However, the home still needs enough heat to stay comfortable.

Therefore, the heating system has to be designed so it can deliver enough warmth at suitable water temperatures.

What About Radiators?

This does not mean every home with a heat pump needs underfloor heating.

Radiators can work perfectly well with heat pumps.

However, some existing radiators may need to be larger if they were designed around much hotter boiler water.

A larger radiator has more surface area.

Therefore, it can transfer enough heat into the room even when the water passing through it is cooler.

Whether radiators need changing depends on the property and heating design.

So, it should be calculated rather than assumed.

Underfloor Heating

Underfloor heating can work particularly well with heat pumps because it uses a large surface area.

This allows useful amounts of heat to enter the room while using relatively low water temperatures.

However, underfloor heating is not required for an air source heat pump.

A properly designed radiator system can also work effectively.

The important issue is whether the heating system can meet the home’s heat demand at suitable flow temperatures.

Heat Loss Matters

Before choosing a heat pump, it is important to understand how much heat the property loses.

Heat escapes through areas such as:

• Walls
• Roof
• Windows
• Doors
• Floors
• Unwanted air leakage

A home that loses heat quickly needs the heating system to replace that heat quickly.

Therefore, a proper heat-loss calculation is important when designing a heat-pump system.

This helps determine:

• Required heat-pump output
• Radiator sizes
• Room heating requirements
• Suitable flow temperatures

Simply replacing a boiler with a heat pump of an apparently similar size is not the right approach.

Does the Home Need to Be Perfectly Insulated?

No.

A home does not have to reach some imaginary level of perfect insulation before a heat pump can work.

However, reducing unnecessary heat loss can help.

A better-insulated home generally needs less heat to maintain a comfortable temperature.

That can reduce the size of the heating requirement and may improve overall running costs.

Therefore, it makes sense to consider reasonable energy-efficiency improvements alongside heating-system design.

Learn more about Insulation and Heat Loss →

How Does an Air Source Heat Pump Produce Hot Water?

Many air source heat-pump systems can also heat domestic hot water.

Unlike a combi boiler, this will commonly involve a hot-water cylinder.

The heat pump heats water stored in the cylinder for later use at:

• Taps
• Baths
• Showers

Hot-water production can require higher temperatures than space heating.

Therefore, the system needs to be designed and controlled appropriately.

Some systems may also use an immersion heater or another heat source when required.

Heat Pumps Often Run Differently from Boilers

People used to gas boilers may expect heating to work in short, very hot bursts.

Heat pumps can work differently.

They often perform well by providing heat more steadily and at lower water temperatures.

Therefore, the aim is not necessarily:

Get the radiators as hot as possible.

Instead, it is:

Provide enough heat to keep the home comfortable efficiently.

A radiator can feel cooler than one heated by a boiler while still providing useful heat to the room.

What Happens in Very Cold Weather?

Air source heat pumps can operate in cold weather.

However, as the outside temperature falls, there is less useful heat available and the heat pump may have to work harder.

Its performance can therefore change.

The outdoor unit may also develop frost under some conditions.

When necessary, the heat pump can temporarily run a defrost cycle.

This removes ice from the outdoor heat exchanger so the system can continue working properly.

Defrosting is a normal part of operation.

Does a Heat Pump Save Money?

Not automatically.

A heat pump can move heat very efficiently.

However, running costs depend on several factors, including:

• Electricity price
• Seasonal performance
• Heat demand
• Flow temperature
• Heating controls
• Property heat loss
• Hot-water demand

This is why statements such as “heat pumps are cheaper to run” or “heat pumps are expensive to run” are too simplistic.

The numbers for the actual property matter.

We will look more closely at costs and suitability later in Is Renewable Energy Right for Your Home?

Solar PV and Heat Pumps

Solar PV can contribute electricity towards running a heat pump.

For example, when solar panels are generating and the heat pump is operating, some of that solar electricity may be used directly.

However, there is an important seasonal difference.

Heating demand is normally highest during winter.

Meanwhile, solar generation is generally much higher during the brighter months.

Therefore, solar PV can support a heat pump, but it should not normally be assumed that solar panels will provide all the electricity needed for heating.

Timing matters.

In Short

An air source heat pump takes heat from the outside air and moves it into your home.

A refrigerant collects heat from outside.

The compressor then raises its pressure and temperature.

That heat is transferred into the home’s heating system before the refrigerant cools and begins the cycle again.

The important principle is:

A heat pump moves heat rather than simply creating it from electricity.

This is why it can deliver several units of heat for each unit of electricity used.

COP describes performance at particular conditions.

Meanwhile, SCOP gives a broader picture across a heating season under defined conditions.

However, good performance also depends on the home.

Heat loss, radiator sizes, flow temperature, controls and system design all matter.

Once you understand those relationships, an air source heat pump becomes much less mysterious.

It is simply a machine that collects low-temperature heat, raises its temperature and moves that heat to where you need it.

Next: How Ground Source Heat Pumps Work →

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