The basic loop
A central heating system is a closed loop of water. The boiler heats it, the pump pushes it out along the 'flow' pipe to each radiator, the radiators give off heat to the room, and the cooler water travels back along the 'return' pipe to be heated again.
The same water goes round and round for years. That's why its condition matters: if it becomes dirty with sludge, the whole system works less well.
The main parts
Whatever the type of boiler, most systems include:
- A boiler, which heats the water using gas, oil, LPG or electricity
- A pump, usually built into modern boilers, which circulates the water
- Flow and return pipes running to every radiator
- Radiators, each with a valve at either end
- Controls – a timer or programmer, a room thermostat and radiator valves
- On some systems, a hot water cylinder, loft tanks or an expansion vessel
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How hot water fits in
Combi boilers heat tap water directly as you use it, separately from the radiators. System and regular boilers heat a hot water cylinder through a coil inside it, and valves switch the hot water between the radiators and the cylinder as needed.
