How a Hot Water or Steam Boiler Works

Hot water or steam boilers use a gas or oil fueled boiler to heat water and circulate the water or steam through pipes to radiators, radiant baseboards or piping in floors and walls to warm a home.

The system works in the following way. The thermostat in the living area is set with a temperature that is the lower limit at which the home should be maintained. When the ambient temperature at the thermostat drops below the preset, a circuit closes (effectively a switch) allowing current to flow to the hot water circulator and thus signalling it to begin heating. In Canada, the system may recirculate water continuously, in which case the thermostat signals the boiler to operate.

The water circulates through a closed system, starting from the boiler and initially passes through a device to capture and purge air from the system. Somewhere in the loop there is an expansion tank to deal with the change in volume of water that coincides with the change in the water temperature. On newer systems, this is typically adjacent to the boiler. In older systems in may be found in the attic. The water or steam then passes into the distribution system to carry the heat throughout the home to the radiators, radiant baseboards and other radiant and convective equipment. As the water circulates it releases its heat energy and cools down, in some systems gravity helps to convey the water back to the boiler. Newer systems use a pump to circulate water and pump it back through the boiler.

The boiler itself operates when the water temperature in the boiler drops below a threshold temperature, around 185° (F). A temperature sensor in the boiler signals the boiler primary control which in turn ignites the burner. Gas fired boilers have a control valve to allow gas to flow into the burner. Oil fired boilers have an oil pump which delivers oil at high pressure to an atomizing nozzle which sprays into the fire chamber. The fuel is ignited with a pilot light or electric ignitor, in the case of a gas fired unit, or a high voltage spark created by a transformer on oil fired units.

An air intake or burner blower provides combustion air for the fire chamber. Safety devices ensure that a flame is present to consume the fuel, otherwise burner operation automatically shuts down.

The combustion gases resulting from the burning of the fuel are contained by the heat exchanger, pass through tubes or between steel plates of the heat exchanger to transfer heat to the water in the boiler. The gases cool and are vented out through the exhaust flue.

The boiler stops firing when the internal temperature sensor indicates the "HI" limit temperature for the water has been reached. The circulator stops the flow of water when the living area thermostat indicates the desired temperature has been reached.

Hot water for the home's taps, shower etc. may be supplied by a boiler or by a separate water heater. When the boiler is involved, often a second tank is invloved and the water is run through the boiler to be heated. This water remains separate or isolated from the water circulated through the heating system.