Some gas burners have a blast tube, similar to an oil burner, but most use atmospheric burners. This type of burner uses the pressure of the gas forced through an orifice to mix with air and create rows or fire, similar to the burner in a gas oven or grill. An aquastat serves two purposes. It controls the temperature the boiler operates at, and provides a low voltage transformer and relay to control the circulator. More than one circulator requires additional relays or a multiple control panel.
Water pressure is limited by a valve known as a regulator.
A check valve or backflow preventer stops water from migrating into the domestic supply should the pressure inside the boiler become greater than the domestic water pressure. A temperature-pressure relief valve vents the system if the water pressure or the temperature exceeds the designs of the boiler. Water expands and contracts as it is heated and cooled, necessitating an expansion tank. Since it is impossible to hermetically seal the heating system, automatic or manual air bleeders must be integrated into the circulating loop. A circulating pump moves the hot water through the heat loop.