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How Does a Heat Pump Work? Try Our Interactive Simulator

An air source heat pump takes heat from the outside air and transfers it into the water circulating through your radiators or underfloor heating.

The principle sounds simple, but understanding what is happening inside the heat pump is much easier when you can actually see the refrigeration cycle working.

Use our interactive heat pump simulator below to follow the refrigerant through the evaporator, compressor, plate heat exchanger and expansion valve.

Change the operating conditions, switch between radiator and underfloor heating, explore the individual components and trigger a defrost cycle to see what happens when frost builds up on the outdoor heat exchanger.

No sign-up required. Use the simulator as many times as you like.

Try the Interactive Heat Pump Simulator

Press Start to watch the heat pump operate in normal heating mode. You can explore the components, adjust operating conditions and use the Defrost control to watch the refrigeration circuit temporarily reverse.

Try this: Run the heat pump normally first, then press Defrost and watch what happens to the reversing valve, refrigerant flow and outdoor heat exchanger.

How an Air Source Heat Pump Works in Four Stages

A heat pump doesn’t create heat in the same way as a boiler.

Instead, it uses a refrigerant to collect low-temperature heat from outside, raise its temperature and transfer that energy into your heating system.

The refrigerant continuously changes pressure, temperature and physical state as it travels around the circuit.

Evaporator — heat enters the refrigerant

Outside air is drawn through the outdoor heat exchanger by the heat pump fan.

The refrigerant entering the evaporator is at a low pressure and low temperature.

Because the refrigerant is colder than the surrounding outside air, energy transfers from the air into the refrigerant.

As it absorbs this heat, the refrigerant boils and changes from a liquid/liquid-vapour mixture into vapour.

In the simulator: Watch the refrigerant change as it travels through the evaporator.

Compressor — pressure and temperature rise

The refrigerant vapour enters the compressor.

The compressor raises its pressure considerably. As the refrigerant is compressed, its temperature also rises.

The refrigerant leaving the compressor is now a hot, high-pressure vapour.

The compressor requires electricity to do this work and is one of the most important components affecting heat pump performance.

In the simulator: Follow the refrigerant from the evaporator into the compressor and watch its condition change.

Plate heat exchanger — heat enters the heating water

The hot refrigerant passes through the refrigerant side of the plate heat exchanger.

Heating water circulates through separate passages on the other side.

The two fluids never mix, but heat passes through the metal plates from the hotter refrigerant into the cooler heating water.

As the refrigerant gives up energy it condenses from vapour back into liquid.

The heated water then leaves the heat pump and travels towards your radiators or underfloor heating.

Expansion valve — pressure falls

The liquid refrigerant then passes through the expansion valve.

Its pressure falls rapidly, which also lowers its temperature.

The refrigerant is now cold enough to absorb heat from the outside air again.

It returns to the evaporator and the cycle repeats.

The refrigeration cycle at a glance

Step 1
Evaporate
Absorb heat from outside air.
Step 2
Compress
Raise the refrigerant pressure and temperature.
Step 3
Condense
Transfer heat into the heating water.
Step 4
Expand
Reduce refrigerant pressure and temperature.

Then the cycle starts again.

What Happens When a Heat Pump Defrosts?

One of the most interesting parts of the simulator is the Defrost button.

In cold and damp weather, moisture from the outside air can freeze onto the outdoor heat exchanger.

This is normal.

As frost builds up between the fins it begins to restrict airflow and insulate the heat exchanger, making it harder for the heat pump to collect heat from the air.

The system therefore needs to remove the frost.

How the defrost cycle works

During a typical reverse-cycle defrost, the heat pump temporarily changes the direction of the refrigeration cycle.

The reversing valve changes position and hot refrigerant is directed towards the outdoor heat exchanger.

Instead of absorbing heat, the outdoor heat exchanger temporarily becomes the part of the refrigeration circuit where heat is released.

This warms the coil and melts the accumulated frost.

During this period, heat being supplied to the central heating system can reduce temporarily.

Once the outdoor heat exchanger has cleared, the reversing valve changes back, the normal refrigeration cycle resumes and the heat pump continues heating the property.

Why does steam sometimes come from a heat pump?

Seeing what looks like a cloud of steam around an outdoor heat pump can initially look alarming.

During defrost, however, the outdoor heat exchanger becomes warm very quickly.

Water produced as the frost melts can evaporate into the cold surrounding air, creating a visible cloud of water vapour.

This can look dramatic, particularly on a cold day, but during a normal defrost cycle it does not necessarily indicate a fault.

Why Does Our Simulator Show a Monobloc Heat Pump?

You may notice that some diagrams online show an outdoor heat pump connected by refrigerant pipes to separate indoor refrigeration equipment.

Our simulator is deliberately different.

It represents a monobloc air-to-water heat pump, a common arrangement for UK domestic heating installations.

In a monobloc heat pump, the main refrigeration circuit is contained within the outdoor unit.

The evaporator, compressor, expansion device, reversing valve and refrigerant-to-water heat exchanger are therefore shown within the heat pump casing.

The pipes leaving the unit in our simulator are not refrigerant pipes.

They contain heating water.

One pipe carries heated water from the heat pump towards the property and the other returns cooler water ready to be reheated.

Heat pump → heating flow → radiator or underfloor heating → heating return → heat pump

What Happens to the Refrigerant Inside a Heat Pump?

The refrigerant is the working fluid that allows the heat pump to move heat.

It repeatedly changes pressure, temperature and physical state as it circulates around the sealed refrigeration circuit.

Position in circuit	Refrigerant condition	What is happening
After expansion valve	Low pressure and cold	Ready to absorb heat
Through evaporator	Evaporating	Absorbs energy from outside air
Compressor outlet	Hot, high-pressure vapour	Pressure and temperature have increased
Through condenser / plate heat exchanger	Condensing	Releases heat into the heating water
Condenser outlet	High-pressure liquid	Ready to return towards the expansion valve

The exact refrigerant temperatures and pressures vary according to the refrigerant used, outside conditions, heating-water temperature and how the heat pump is operating.

The simulator is designed to explain the principle and relationship between the different parts of the cycle, rather than replace manufacturer-specific engineering data.

How Does the Heat Reach Your Radiators or Underfloor Heating?

The refrigerant remains inside the heat pump.

The energy it collects is transferred into the property’s heating water through the plate heat exchanger.

Hot water then leaves the heat pump through the heating flow.

It travels around the heating system and gives up energy through radiators, underfloor heating or another suitable emitter.

As energy is released into the rooms, the water temperature falls.

The cooler water returns to the heat pump through the heating return, where it is heated again.

Heating flow → emitters → heating return

The amount of heat transported by the water is closely related to:

water flow rate
flow temperature
return temperature
the temperature difference between flow and return
the heat required by the property

Want to explore the water side?

Understanding water flow becomes important when designing a heat pump system. Use our free Mass Flow Rate Calculator to explore the relationship between heat output, water flow and temperature difference.

Open the Mass Flow Rate Calculator →

How Do Radiators Work With a Heat Pump?

Heat pumps can work extremely well with radiators, but radiator output depends heavily on water temperature.

A radiator’s advertised output is normally based on a particular set of test temperatures.

When it operates at the lower flow temperatures commonly associated with heat pumps, its actual output will be lower.

That does not automatically mean every radiator needs replacing.

The important question is whether each radiator can provide enough heat to match the calculated heat loss of the room at the chosen system temperatures.

This is why proper heat loss calculation and emitter design matter.

See what happens to radiator output

Use the free Heat Loss Hub Radiator Output Calculator to see how radiator output changes as heating-water temperatures change.

Open the Radiator Output Calculator →

Why Does Flow Temperature Matter to Heat Pump Efficiency?

A heat pump has to raise the temperature of the heat it collects outside to a level useful for the heating system.

The greater this required temperature lift becomes, the harder the refrigeration circuit generally has to work.

This is one reason good heat pump design aims to operate the heating system at the lowest practical flow temperature while still keeping every room comfortable.

Underfloor heating can be particularly well suited to this because it uses a large heated surface area.

Radiators can also operate successfully at lower temperatures when they are appropriately sized.

However, there is no single flow temperature that is correct for every property.

The appropriate design temperature depends on the heat loss of the rooms, available emitter output, heating system arrangement and the output characteristics of the selected heat pump.

Main Components of an Air Source Heat Pump

Evaporator

Collects heat from the outside air and transfers it into the refrigerant.

Fan

Moves outside air through the outdoor heat exchanger.

Compressor

Raises the pressure and temperature of refrigerant vapour.

Plate Heat Exchanger / Condenser

Transfers heat from the refrigerant into the heating water while keeping the two circuits separate.

Expansion Valve

Reduces refrigerant pressure and controls refrigerant entering the evaporator.

Reversing Valve

Changes the route of refrigerant when the system needs to reverse the refrigeration cycle, including during defrost operation on suitable systems.

Circulation Pump

Moves heating water through the heat pump and central-heating circuit.

DESIGN FIRST

Understanding the heat pump is only part of the story

A heat pump can only perform as part of the heating system around it.

The equipment needs to be matched to the property rather than selected from a headline kilowatt figure alone.

A good design considers:

room-by-room heat loss
design outside temperature
required flow temperature
radiator or underfloor heating output
heating-water flow rate
pipework
heat pump output at the actual design conditions
controls
hot-water requirements

This is why Heat Loss Hub approaches heat pumps from a design-first perspective.

The aim isn’t simply to find a heat pump.

It is to understand what the property actually needs and design the complete system around it.

Frequently Asked Questions About How Heat Pumps Work

How does a heat pump get heat from cold air?

Air can contain useful thermal energy even when it feels cold.

The refrigerant inside the heat pump evaporator is maintained at an even lower temperature than the outside air, allowing energy to move from the warmer outside air into the colder refrigerant.

What does the compressor do in a heat pump?

The compressor raises the pressure of refrigerant vapour.

Compressing the refrigerant also raises its temperature, allowing it to become hot enough to transfer heat into the heating-water circuit.

What does the expansion valve do?

The expansion valve reduces the pressure of the liquid refrigerant before it enters the evaporator.

This causes its temperature to fall so it can once again absorb heat from the outside air.

Why does a heat pump need to defrost?

Cold and humid weather can cause moisture to freeze on the outdoor heat exchanger.

Too much frost restricts airflow and reduces heat transfer, so the heat pump periodically removes it using a defrost cycle.

Is steam coming from a heat pump normal?

It can be.

During a defrost cycle, frost and ice melt rapidly from the outdoor heat exchanger. Water vapour can then become visible in the cold outside air and look like steam.

What is a monobloc heat pump?

A monobloc air-to-water heat pump contains the main refrigeration circuit within the outdoor unit.

Heating water flows between the outdoor unit and the property’s heating system.

Can a heat pump work with radiators?

Yes.

The important factor is whether the radiators can provide enough output to match the room heat losses at the intended heating-water temperatures. Our guide on existing radiators and heat pumps covers how to check.

Is underfloor heating better for heat pumps?

Underfloor heating can work particularly well with heat pumps because its large surface area allows useful amounts of heat to be delivered at relatively low water temperatures.

However, correctly sized radiators can also work very effectively.

A good heat pump system starts with understanding the numbers

Whether you are researching a future installation or trying to understand an existing system, Heat Loss Hub is designed to make the technical side easier to understand.

Explore our free tools, guides and independent heating-design resources to learn what your property actually needs.