Inside an Air Source Heat Pump — A Component-by-Component Tour
Last reviewed: 2 October 2026
Take a tour of the main parts of an air source heat pump, what each one does, and what it means for a Cambridge home — from the fan that keeps working on frosty nights to the cylinder coil that has to cope with chalk water.
The principle
A heat pump moves heat from a cold place to a warm one, using electricity to do the work. The “cold place” is the outdoor air; the “warm place” is your heating system. A refrigerant circulates in a sealed loop, absorbing heat outside and releasing it inside.
Here are the parts that make it happen.
1. The fan and evaporator coil
What it is: the large finned coil you can see behind the outdoor unit’s grille, with a fan that pulls air across it.
What it does: the refrigerant inside the coil is colder than the outside air, even on a frosty night, so it absorbs warmth from the air and evaporates into a gas.
Cambridge note: on cold, damp nights, moisture freezes onto the fins. The unit runs a short defrost cycle to clear it, which is routine in a city with around 52 frosty nights a year. More in defrost cycles.
2. The compressor
What it is: the heart of the unit, and the main user of electricity.
What it does: compresses the refrigerant gas, which raises its temperature high enough to heat water. Modern compressors are inverter-driven, speeding up and slowing down to match demand rather than switching on and off.
Cambridge note: on the coldest mornings the compressor works hardest. A correctly sized system copes without an electric backup heater.
3. The condenser (heat exchanger)
What it is: a plate heat exchanger where hot refrigerant meets your central heating water.
What it does: passes the heat from the refrigerant to the water. The refrigerant cools and condenses back to liquid.
In a monobloc heat pump, the usual type in UK homes, this is inside the outdoor unit and only water pipes enter the house. In a split system it’s indoors. See monobloc vs split.
4. The expansion valve
What it is: a precisely controlled valve in the refrigerant loop.
What it does: lowers the pressure and temperature of the liquid refrigerant so it’s cold enough to absorb heat from the air again. Then the loop repeats.
5. Controls and weather compensation
What it is: the controller on your wall or phone, plus an outdoor temperature sensor.
What it does: sets the water temperature according to the weather. On a mild day the water runs cooler; on a frosty one, warmer. This weather compensation is one of the biggest influences on running costs. See flow temperature.
6. The hot water cylinder
What it is: an insulated tank, typically 180–250 litres, with a coil inside that the heat pump’s water flows through.
What it does: stores hot water for taps and showers, reheated between heating cycles.
Cambridge note: the city’s water comes mainly from the chalk aquifer and runs at around 286–295 mg/l of calcium carbonate. Scale on the coil slows reheating, so ask for a large, heat pump-rated coil and consider a scale reducer.
7. Radiators
What they are: the same radiators you have now, or larger replacements in some rooms.
What they do: release heat into each room. Because the water is cooler than with a gas boiler, rooms that lose heat fastest — common in Cambridge’s solid-walled gault-brick terraces — may need bigger radiators. See radiator sizing.
Putting it together
Air → evaporator → compressor → condenser → your heating water → back through the expansion valve. Each unit of electricity drives that loop and brings in two to four units of free heat from the air, depending on the weather. Over a Cambridge year, that averages around 3–3.5. See SCOP and COP explained.
Start from the beginning: air source heat pumps in five minutes.
See what a heat pump would look like in your Cambridge home
The mechanism is the same in every UK home; the design is specific to yours. Our team carries out a free in-home survey including a full room-by-room heat-loss calculation, identifies the system size and configuration that fits your property, and provides a written quote with the £7,500 BUS grant already deducted.
You'll see the actual figure you'd pay — not an estimate — and the projected efficiency for your specific install.