Hi boiler interlock is a strange name for it , a bit misleading I believe, but it refers to Part L of bldg regs 'energy efficiency'.
In an attempt to give greater control over C.H. systems plumbers/heating Engs. began fitting TRV's to every rad. this gives rise to boilers constantly 'cycling' on/off as so little water flowing in a circuit that is up to temp. the boiler/pump cool down a bit , fire up and straight away the returning water is at cut off temp so boiler/pump stop, within a very short time water cools & starts cycle again. This is wasteful of gas & wearing on components so a 'boiler interlock' is introduced, sounds technical but is in fact just a room thermostat fitted in an area which has a radiator without a TRV fitted. This acts as a final cut off so is usually in an area such as a hallway that would cut back in if say a cold draught from an open door cooled the hall. The capacity of water in the rad and the volume off air around the stat have a 'damping' effect on the system.
There is a statement in Bldg Regs which says 'fitting of all TRV's in a system is unlikely to comply with Part-L requirements'
hope this is helpfull,
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