Recommended best practice for heating engineers is to install central heating systems with the following controls:
Programmable room thermostat - This is an electronic combined time switch and room thermostat which allows the user to set a number of different periods with different target temperatures for space heating, usually in a weekly cycle.
Boiler interlock - is not a physical device but an arrangement of the central heating system controls so as to ensure that the boiler does not fire when there is no demand for heat. In a system with a combi boiler it can be achieved by fitting a room thermostat. In a system with a regular boiler it can be achieved by correct wiring interconnection of the room thermostat, cylinder thermostat, and motorised valve(s). It may also be achieved by more advanced controls, such as a boiler energy manager. TRVs alone are not sufficient for boiler interlock.
Thermostatic Radiator Valves (TRVs) - should be fitted on all radiators, except in rooms with a room thermostat. They have an air temperature sensor which is used to control the heat output from the radiator by adjusting the water flow. They are not adequate as the sole heating control, because they do not turn off the boiler.
Automatic bypass valve - used to maintain a minimum flow rate through the boiler and to limit circulation pressure when alternative water paths are closed, ie when all the TRVs have closed down. Sometimes a radiator without a TRV is used as a bypass circuit.
Other controls include:
Zone control – particularly useful for larger dwellings, this is where two or more room thermostats are linked to a programmer capable of independently controlling two or more space heating zones.
Weather compensator- an external temperature sensor allows a control to reduce or increase the water temperature to suit the weather. It also senses the return water temperature and switches off the boiler when it rises above a certain level.