What each common heating control does and why interlock stops the boiler firing when no heat is needed.
Heating controls exist to make sure energy is only used when and where it is wanted. For revision, learn what each control does and how they work together to give boiler interlock.
A programmer or timer decides the times of day that heating and hot water are allowed to operate, so the system is not running needlessly overnight or when the house is empty.
A room thermostat senses the air temperature in a main living space and switches the heating demand off once that space is warm enough. Thermostatic radiator valves (TRVs) then fine-tune individual rooms by throttling flow to each radiator as its room warms.
A cylinder thermostat does the same job for stored hot water, switching off the demand once the cylinder reaches its set temperature.
Boiler interlock is the important outcome of wiring these together: when no thermostat is calling for heat, the boiler is prevented from firing and the pump from running pointlessly. In other words, the controls can fully switch the boiler off, not just close valves while it keeps cycling.
Effective controls and interlock are part of the efficiency expectations in Part L, so confirm the current control requirements there.