Inside the boiler, the gas in a sealed combustion chamber ignites and sends heat out through a heat exchanger. The heat exchanger transfers the heat energy to cold water in a pipe surrounding it.
Boilers are generally manually operated, meaning an electrical switch must be turned on to start the heating process. This is different from hot-water heaters, which heat and store hot water for instant use. Flipping the switch opens a valve in the boiler that releases the natural gas into the combustion chamber. Simultaneously, the water enters the tank through a pipe. A fan keeps the natural gas flames burning to transfer the heat to the heat exchanger. The heat exchanger quickly transfers heat to the water in the pipes surrounding it. The water typically reaches 140 degrees Fahrenheit.
The heated water continues its circuit through the boiler and back out. An electrical pump moves the hot water out to radiators, providing central heating.
Boilers typically come in one of three systems: combi, heat-only and system boilers. Heat-only boilers are the general type described above. Combi boilers provide not only the hot water for radiators but also for faucets. The boiler still functions in the same way except the hot water cylinder and cold water tank are part of the set up. System boilers store hot water, similar to hot-water tanks.