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 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.
A frost thermostat is one of those separate protective functions: it senses when the system is close to freezing — typically in an unheated loft, garage or empty property — and fires the boiler to protect the pipework even outside the normal programmed periods. It overrides the programmer and room thermostat precisely because the risk it guards against arises when they have already switched the heating off.
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.
Siting the room thermostat
A room thermostat only works well if it senses a temperature that represents the home generally. Fit it on an inside wall in a normally occupied room, clear of direct sunlight, draughts from doors or windows, and other heat sources such as a radiator, lamp or fire. Sited somewhere cold and draughty it reads low, so it keeps calling for heat and the home overheats; sited somewhere warm, such as in sunlight or beside a radiator, it reads high, so it switches off early and the home underheats.
Filling a sealed system
A sealed system has no feed and expansion cistern. It is filled through its specified connection to the water supply, with backflow protection appropriate to the heating fluid. A common arrangement is a removable filling-loop hose: open the manual valves while watching the gauge, stop at the manufacturer’s cold-fill pressure, close the valves and disconnect the hose as instructed. Many domestic systems use about 1 to 1.5 bar cold, but the appliance’s figure takes priority.
An integral filling device may use a key, a lever or an automatic control, with no removable hose. Follow its own procedure. Unwanted filling through open or passing valves can raise pressure or conceal a leak; a closed, attached hose does not by itself prove water is entering. The filling arrangement is separate from boiler interlock and from the pressure relief valve.