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. If multiple zones are desired, a circulator is needed for each loop, or one circulator can be used with zone valves. Zone valves will open and close with the call from the thermostat, as well as turn the circulator on. If each zone has its own circulator, a flow check valve is required to prevent the flow of water by convection or by another circulator.