Heat pumps, air conditioners and fridges all work in the same way. A fluid called a refrigerant travels round a sealed loop, picking up heat in one place and releasing it in another. Four main parts make that happen. Once you know what each one does, the whole system makes sense.
The four main parts
- The evaporator. This is a coil where the refrigerant collects heat. The refrigerant arrives cold and at low pressure, takes in heat from the air, ground or water around it, and boils into a gas.
- The compressor. This is the heart of the system and the main user of electricity. It squeezes the refrigerant gas into a smaller space. Like a bike pump getting hot, this raises the temperature of the gas a lot.
- The condenser. This is another coil, where the hot gas gives its heat away to the water or air being heated. As it loses heat, the refrigerant condenses back into a liquid.
- The expansion valve. This lets the liquid through a narrow opening into a low-pressure area. Like spray from an aerosol can, it cools down sharply, ready to go back to the evaporator.
The names can swap
Many heat pumps can heat and cool. To do this, a reversing valve changes the direction the refrigerant flows. When that happens, the two coils swap jobs.
In heating mode, the outdoor coil is the evaporator, collecting heat from outside, and the indoor coil is the condenser, releasing heat inside. In cooling mode, it is the other way round. This is why the outdoor unit of an air conditioner is often called a condensing unit: in cooling, it is where heat is released.
Split and monobloc systems
The parts can be arranged in different ways.
In a monobloc system, everything is in one outdoor unit. The heat pump heats water, and that water is piped into the building to radiators, underfloor heating, fan coil units or a hot water cylinder. The refrigerant never leaves the outdoor unit.
In a split system, the compressor and outdoor coil sit outside, while the other coil is in an indoor unit. The two are joined by refrigerant pipes. The indoor unit might heat water, or it might be a fan coil unit blowing warm or cool air straight into a room.
Some split systems connect one outdoor unit to several indoor units. These are often called multi-split or variable refrigerant flow systems, and they can serve many rooms from one outdoor unit.
Variable-speed compressors
Older compressors were either fully on or fully off. Most modern heat pumps use a variable-speed compressor, often called an inverter compressor. It speeds up and slows down to match how much heat is needed. This keeps temperatures steadier, reduces wear and saves energy.
This is also why heat pumps like to run gently for long periods, rather than in short bursts.
Why qualified engineers matter
Refrigerants are fluorinated gases, often called F-Gases. Many of them are powerful greenhouse gases if they leak. In the UK, anyone installing, servicing or decommissioning these systems must hold the right F-Gas qualifications. We are F-Gas registered, and we treat refrigerant pipework with the same care as electrical work: properly sized, neatly run, pressure-tested and leak-checked before the system is filled.