Air source
An outdoor unit extracts heat from outside air. It usually needs less ground work, while location, airflow and sound must be assessed.
Explore air source
Heat pump basics
How heat pumps work
A heat pump moves heat from the air, ground or water into your home rather than creating it from scratch. Electricity powers the process; most of the heat delivered comes from the surrounding environment.
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A completed Thermal Earth heat-pump plant room.
The basic cycle
Absorb heat, compress it, transfer it, then repeat.
The same refrigeration cycle sits behind air, ground and water-source heat pumps.
Scroll the picture sideways to see all of it. Every label is written out below.
How a heat pump moves energy into your home
What is in the picture
- Environmental warmth gathered from the ground Collector loop buried in the garden
- Heating water: warm out, cooler back Radiator; Underfloor heating in the screed; Hot-water cylinder; Heating coil - heating water never mixes with your tap water; Underfloor manifold; Warm water out; Cooler water back
- Electricity from the grid to the compressor Fuse board; Electricity from the grid
- Tap water, kept completely separate from the heating water Your shower and taps; Cold water mains
- Refrigerant sealed inside the heat pump
The four steps inside the heat pump
- 1 Collect evaporator Environmental energy warms the refrigerant until it boils. It leaves as a warm gas at low pressure.
- 2 Squeeze compressor Electricity drives the compressor, squeezing that gas into a much smaller space. It leaves hot and at high pressure.
- 3 Release condenser The hot gas hands its heat to your heating water and condenses. It leaves as a hot liquid, still at high pressure.
- 4 Reset expansion valve A valve drops the pressure and the liquid flashes cold. It leaves as a cold mist, ready to collect more.
Choose one energy source
Air, ground or water. A home collects from one of them, never all three. This picture shows the ground; each one has its own page.
Shown at all times: Heat pump.
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Your home, cut in half
One house, drawn straight on so nothing is hidden. The heat pump is the small cabinet on the right of the plant room, and it is the only machine in the picture.
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The warmth is already in the ground
A sealed loop of fluid runs through the soil under the garden. The soil is a little warmer than the fluid, so warmth crosses into the pipe. Nothing is taken out of the ground but heat.
- Collector loop buried in the garden
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Where the loop meets the machine
The warmed fluid comes up into the base of the heat pump and the cooled fluid goes straight back down. Warm water leaves the top of the machine for the cylinder, the floor and the radiators; cooler water comes back the same way.
- Hot-water cylinder
- Heating coil - heating water never mixes with your tap water
- Underfloor manifold
- Cold water mains
- Fuse board
- Heat pump
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Inside the machine: four steps, over and over
This is the only part of the whole picture that uses electricity. A sealed refrigerant goes round and round, and at each of the four stations it visibly changes.
- Electricity from the grid
- 1 CollectevaporatorEnvironmental energy warms the refrigerant until it boils. It leaves as a warm gas at low pressure.
- 2 SqueezecompressorElectricity drives the compressor, squeezing that gas into a much smaller space. It leaves hot and at high pressure.
- 3 ReleasecondenserThe hot gas hands its heat to your heating water and condenses. It leaves as a hot liquid, still at high pressure.
- 4 Resetexpansion valveA valve drops the pressure and the liquid flashes cold. It leaves as a cold mist, ready to collect more.
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Where the heat actually goes
Warm water runs to the pipes buried in the ground-floor screed, up to the radiator, and through a sealed coil inside the hot-water cylinder. Your tap water never touches it.
- Radiator
- Underfloor heating in the screed
- Your shower and taps
- Warm water out
- Cooler water back
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A heat pump moves heat. It does not make it.
+ =The electricity is the smaller part. Most of the heat was already outside, waiting to be gathered up.
Proportions vary with the weather, the property and the system, so no figures are shown.
1 Absorb Evaporator
Environmental energy warms the refrigerant.
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What goes in, and what comes out
The same substance, before and after. It arrives on the left, passes through the one component this step is about, and leaves on the right as something visibly different.
- From the ground, air or water outside
- Back outside, a little cooler
- going incold mistlow pressure
- coming outwarm gaslow pressure
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Evaporator - a plate heat exchanger
The component itself. Its texture, its ports and the pipe stubs are the same drawing the wide picture carries - only closer.
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What happens here
Fluid from outside arrives only slightly warm. The refrigerant inside is colder still, so heat crosses into it.
That warmth makes the refrigerant boil. It stops being a cold misty liquid and becomes a gas.
The pressure barely moves. It is the phase that changes.
- Pressure
- lowhigh
- Temperature
- lowhigh
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The two never touch
Only heat crosses the metal wall. The fluid from outside and the refrigerant stay in separate sealed circuits and never mix.
One of four steps. They repeat, in order, for as long as your home is asking for heat.
2 Compress Compressor
The compressor uses electricity to raise the temperature.
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What goes in, and what comes out
The same substance, before and after. It arrives on the left, passes through the one component this step is about, and leaves on the right as something visibly different.
- Electricity from your fuse board
- going inwarm gaslow pressure
- coming outhot gashigh pressure
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Compressor - a sealed scroll pump
The component itself. Its texture, its ports and the pipe stubs are the same drawing the wide picture carries - only closer.
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What happens here
The compressor is the only part of the cycle that uses power. It squeezes the gas into a much smaller space.
Squeezing a gas raises its pressure, and raising its pressure raises its temperature.
The gas leaves hotter than anything in your home needs to be - and it leaves higher up.
- Pressure
- lowhigh
- Temperature
- lowhigh
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Electricity drives it, it does not become it
The power goes into the squeeze, not into the heat. Most of the warmth you end up with was already outside before the compressor started.
One of four steps. They repeat, in order, for as long as your home is asking for heat.
3 Transfer Condenser
Heat is transferred into the home's heating water.
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What goes in, and what comes out
The same substance, before and after. It arrives on the left, passes through the one component this step is about, and leaves on the right as something visibly different.
- Warm water out to your home
- Cooler water back to be warmed again
- going inhot gashigh pressure
- coming outhot liquidhigh pressure
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Condenser - a plate heat exchanger
The component itself. Its texture, its ports and the pipe stubs are the same drawing the wide picture carries - only closer.
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What happens here
The hot gas meets water that is cooler than it is, so the heat crosses the wall the other way.
Giving up that heat makes the gas condense. The bubbles disappear and it runs as a hot liquid.
The water leaves warm enough for radiators, underfloor pipes and the hot-water cylinder.
- Pressure
- lowhigh
- Temperature
- lowhigh
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The two never touch
Only heat crosses the metal wall. Your heating water and the refrigerant stay in separate sealed circuits and never mix.
One of four steps. They repeat, in order, for as long as your home is asking for heat.
4 Expand Expansion valve
The refrigerant cools and the cycle begins again.
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What goes in, and what comes out
The same substance, before and after. It arrives on the left, passes through the one component this step is about, and leaves on the right as something visibly different.
- straight on to Step 1 - Collect
- going inhot liquidhigh pressure
- coming outcold mistlow pressure
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Expansion valve - a machined restriction
The component itself. Its texture, its ports and the pipe stubs are the same drawing the wide picture carries - only closer.
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What happens here
The valve is a deliberate bottleneck. High-pressure liquid is forced through a tiny machined opening.
On the far side there is far less pressure, so the liquid drops in temperature and part of it flashes back to mist.
It leaves colder than the ground outside - which is exactly what Step 1 needs.
- Pressure
- lowhigh
- Temperature
- lowhigh
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Why a restriction cools things
Squeezing a gas heats it, so letting a liquid out into space cools it. The valve does the compressor's job in reverse, and it needs no power at all.
One of four steps. They repeat, in order, for as long as your home is asking for heat.
Where the heat comes from
Three sources, one underlying principle.
Site conditions, available space and the building's needs determine which route is worth assessing.
Ground source
Horizontal collectors or boreholes absorb heat from the ground. The ground array is designed around the site's conditions and heat demand.
Explore ground sourceWater source
A suitable water source can provide heat where permissions, environmental considerations and site-specific design allow.
Explore water sourcePerformance terms
Heat pump performance, in plain English.
Useful comparison figures—but not a promise of one home's running cost.
Coefficient of Performance (COP)
Compares heat output with electricity input at a stated operating point. A COP of 3 means three units of heat for each unit of electricity under those test conditions.
Seasonal Coefficient of Performance (SCOP)
A standardised product rating representative of a reference heating season. It compares heat output with electricity use across that season, rather than at just one operating point.
Your completed system: performance also depends on source temperature, heating-water temperature, sizing, emitters, controls, commissioning and how the home is used.
Neither COP nor SCOP guarantees an individual home's running cost.
System design
Why the whole heating system matters.
A heat pump cannot be assessed in isolation from the building, emitters and controls around it.
Heat loss and sizing
Room-by-room heat loss helps establish the output the building needs at design conditions.
Emitters and temperature
Radiators or underfloor heating must provide the required heat at the selected heating-water temperature.
Controls and commissioning
Settings, weather compensation and commissioning influence how the completed system operates.
Inside the building
From the heat pump to rooms and hot water.
Many heat-pump systems supply heating water to underfloor heating or radiators and can also provide domestic hot water when designed with suitable storage and controls.
Heat pumps often operate most efficiently at lower heating-water temperatures, so emitter sizes, pipework and controls should be considered as part of the design.
Common questions
Heat pumps without the myths.
Yes. A correctly selected heat pump is designed to operate in local winter conditions. Available output and efficiency vary with outdoor or source temperature, which is why heat-loss calculations, sizing and system design matter.
Many systems can provide space heating and domestic hot water when designed with suitable storage and controls.
Not necessarily. Suitable radiators can also work. Required emitter sizes depend on room heat loss and the design heating-water temperature.
Heat pumps are not silent, but a correctly selected and positioned system should not be intrusive. Product sound levels, location and any required sound calculation or acoustic assessment are considered during design.
A well-designed heat pump can reduce heating bills. The result for your home depends on its heat demand, the completed system's efficiency, your electricity tariff and the heating system being replaced, so savings should be assessed for your property rather than assumed.
Ready to explore what could suit your home?
Start with a home estimate or ask the Thermal Earth team about the next step.