Boiler interlock explained: what it is and how it works

Boiler interlock is the control arrangement that stops unnecessary boiler firing when neither heating nor hot water needs heat. The controls must communicate that demand has ended. Interlock is a result of their operation, rather than a separate component.

Which controls provide boiler interlock?

• Programmer or timer: sets the permitted operating periods.
• Room thermostat: requests space heating until its temperature setting is satisfied.
• Cylinder thermostat: requests heat for stored hot water until the cylinder reaches its setting.
• Motorised valves and their controls: coordinate the demand and water path in a system with separate heating and cylinder circuits.

See S-plan vs Y-plan: how the valves control the system for the two common valve arrangements.

Three demand examples

Assume a boiler heats radiators and an indirect cylinder, with the relevant programme enabled:

• Room cool, cylinder hot: space heating can request heat; the satisfied cylinder does not.
• Room warm, cylinder cool: the cylinder can still request heat. Turning down the room thermostat does not cancel that separate demand.
• Room and cylinder satisfied: neither circuit requests heat. Interlock prevents needless firing to keep the boiler hot.

Does the pump stop immediately?

Not necessarily. A boiler may require the pump to continue after the burner stops. This pump overrun must follow the appliance instructions; its presence alone does not show that interlock has failed. Interlock does not mean cutting every electrical supply to the boiler or disabling protective functions.

Frost protection, maintenance cycles and combi preheat can also operate separately from normal space-heating demand. Identify the active function before diagnosing a fault, using the appliance and controls instructions.

Are TRVs enough?

Ordinary mechanical TRVs restrict radiator flow as rooms warm up. They do not themselves send a boiler-off signal. Closing them all is therefore not proof of interlock. The boiler temperature control also has a different job: controlling water temperature does not establish whether the rooms need heat.

Judgement check: an ordinary TRV closes the radiator in the room containing the room thermostat before that thermostat is satisfied. The thermostat may keep calling while its room receives too little heat. Coordinate the reference-room radiator and thermostat as the controls instructions require. Integrated smart systems can use a different arrangement.

Boiler interlock and Part L

Approved Document L includes boiler interlock in its heating-control guidance for England. Use the edition and transitional arrangements applicable to the work; a newly published edition does not automatically govern every existing project. For revision, distinguish time control, temperature control and the removal of demand.