The core part of boilers is the fire tube, which allows heat to travel through the sealed water tube. In hot water boilers, heated water is sent through pipes, while steam boilers instead allow steam to form, which then spreads on its own to radiators.
As with almost all central heating and cooling systems, boilers are generally connected to a thermostat, which uses a thermometer to measure the ambient temperature and tells the boiler when to turn on and off. The gas valve on a boiler opens when the system lights. In some system, a pilot light, which constantly burns a bit of gas, starts the ignition. Other systems use a spark igniter, which ignites the flow of gas with an electrical spark.
Boilers also have a number of parts to ensure safe operation. The pressure relief valve triggers if the pressure inside the boiler is too high to prevent the unit from failing catastrophically. Boilers with pilot lights also have a temperature sensor to shut off the flow of gas if the light goes off or if temperatures are too high. Newer boilers often have a digital display that allows users change operating parameters. These displays also provide feedback about system operation and may show the results of various diagnostic tests.