Air source system layout
An air source system has an outdoor unit containing the refrigerant circuit, connected by flow and return pipes to the wet heating system inside.
Reading the diagram
- Evaporator. Refrigerant absorbs heat from the outside air and boils into a gas, even well below freezing.
- Compressor. Compresses that gas, raising its temperature sharply. This is the part that consumes the electricity.
- Condenser. The hot gas gives up its heat to your central heating water and condenses back to liquid.
- Expansion valve. Drops the pressure, the refrigerant cools, and the cycle repeats.
- Flow and return. Heated water out to the emitters, cooled water back. Typically 35 to 50°C against a boiler's 70 to 80°C.
- Cylinder. Required, because a heat pump cannot make instant hot water. See heat pumps and hot water.
- Controls. Weather compensation varies flow temperature against outside temperature, and is the main determinant of running cost.
Ground source, for comparison
Ground source replaces the outdoor unit with buried pipework and moves the heat pump indoors. Everything after it is the same.
What a diagram cannot show you
Two things decide whether an installation works, and neither appears on any layout drawing. The first is emitter sizing: whether each radiator can heat its room at the design flow temperature. The second is the heat loss calculation behind the unit selection.
A diagram tells you what connects to what. It cannot tell you whether the system was sized correctly, and that is what determines whether you are warm and what it costs you.