BurnerBurns the fuel, gas or oil, to make the fire. The fire heats the water. A modulating burner can turn its flame up and down.
Combustion chamberThe space where the fire burns, with the water passages above it. Its exhaust goes up the flue.
FlueCarries the hot exhaust gases away. In a chimney the warm gas rises and draws fresh air into the fire.
Supply pipeCarries hot water from the boiler out to the radiators.
RadiatorGives the water's heat to the room.
Circulating pumpPushes the water round the loop. It goes back to the boiler once it has given up its heat.
The boiler 3
Pressure relief valveLets water or steam out if the pressure gets too high. It is a safety valve to prevent the boiler bursting.
Pressure gaugeShows the pressure of the water in the boiler and its loop.
Low-water cutoffShuts the burner off if the water falls too low. The boiler is then never fired dry. It is a float or electrode switch.
The heating loop 2
Return pipeBrings the cooled water back to the boiler to be heated again.
Expansion tankA small tank with a cushion of gas behind a diaphragm. It takes up the water's expansion as it heats and protects the system from too much pressure.
How this list was compiled: Read 2026-09-27. (1) Wikipedia, Boiler, https://en.wikipedia.org/wiki/Boiler : fittings and accessories including a modulating burner, the safety valve that relieves pressure to prevent a boiler exploding, water level indicators, the low-water cutoff (a float switch or electrode that turns off the burner or fuel when the water falls below a set point, as dry-firing can rupture the boiler), the circulating pump that returns water to the boiler after it has given up heat, and pressure gauges; the chimney joined to the combustion chamber's exhaust, where the rising flue gas draws in air. (2) Wikipedia, Central heating, https://en.wikipedia.org/wiki/Central_heating : pumps that send hot water round the room radiators; an expansion tank of compressed gas behind a diaphragm allowing for pressure changes in a sealed system. (3) Wikipedia, Expansion tank, https://en.wikipedia.org/wiki/Expansion_tank : a small tank that protects closed water heating systems from excessive pressure. (4) Wikipedia, Relief valve, https://en.wikipedia.org/wiki/Relief_valve : a safety valve that limits the pressure in a system. Licence: own work
The drawing: Drawn for this site by script, 2026-09-27. Own work, nothing traced. Licence: own work
How it works
The parts of a home hot-water heating boiler, with a window cut in it, and the loop that carries its heat to a radiator. Start with the 6 main parts, then the boiler and the loop.
Read the full guide
Inside the boiler
The burner makes a fire in the combustion chamber, which heats the water passing over it; the exhaust goes up the flue. The relief valve lets pressure out if it climbs too high, the gauge shows it, and the low-water cutoff stops the burner if the water runs low.
Round the loop
Hot water leaves by the supply pipe for the radiators, which give its heat to the rooms. The cooled water comes back by the return pipe, pushed by the circulating pump. The expansion tank takes up the water's swelling as it heats.
Supply is the pipe leaving the boiler hot; return is the one coming back cooler.
Questions and answers
What are the main parts of a boiler?
The burner and combustion chamber that heat the water, the flue that carries the exhaust away, and the safety fittings: the pressure relief valve, the pressure gauge and the low-water cutoff.
What does the expansion tank on a boiler do?
It holds a cushion of air behind a rubber diaphragm. As the water heats and expands, the air compresses, which keeps the pressure in the sealed system from climbing too high.
What is a low-water cutoff on a boiler?
A float or electrode switch that turns the burner off if the water falls below a set level, because firing a boiler dry can rupture it.