Boiler interlock is the control arrangement that stops the boiler firing when nothing is calling for heat — not a part you fit. What delivers it, and why Part L expects it.
Boiler interlock is a control arrangement that stops the boiler firing, and the pump running, when no thermostat is calling for heat. It is not a single part you fit — it is the result of wiring the heating controls together so the boiler switches fully off instead of cycling needlessly.
The controls that deliver it each have a job. A programmer or timer decides the times of day that heating and hot water are allowed to operate, so the system is not running 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.
Wire these together and interlock follows: when neither the room thermostat nor the cylinder thermostat is calling for heat, the boiler and pump are switched off rather than left to cycle on closed valves. Effective controls and interlock are part of the efficiency expectations in Part L, so confirm the current control requirements against that guidance. For revision, be able to name each control, say what it senses, and explain how they combine to give interlock.