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 way the controls stop the boiler firing when neither the heating nor the hot water needs heat. The controls must tell the boiler that the demand has ended. Interlock is something the controls do together, not a separate part.
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 always. A boiler may need the pump to keep running after the burner stops. This pump overrun follows the appliance instructions, and it does not mean interlock has failed. Interlock does not mean cutting every electrical supply to the boiler or disabling its protective functions.
Frost protection, maintenance cycles and combi preheat can also operate separately from normal space-heating demand. Work out which function is running, using the appliance and controls instructions, before you diagnose a fault.
A frost thermostat senses low air temperature where it is fitted and calls for heat to protect the system, through the specified controls. It can override the normal timer and room controls. A pipe thermostat may end that call once the return water has warmed. What it protects depends on where the sensors are, the wiring, the power and the circulation: a sensor in one place does not protect every pipe elsewhere.
Are TRVs enough?
Ordinary mechanical TRVs cut radiator flow as rooms warm up. They cannot tell the boiler to switch off, so closing them all is not interlock. The boiler’s temperature control has a different job too: it controls the water temperature, not 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 for heat while its room gets too little. Set up that room’s radiator and thermostat together, as the controls instructions require. Integrated smart systems can work differently.
Boiler interlock and Part L
Approved Document L includes boiler interlock in its heating-control guidance for England. Use the edition, and the transitional arrangements, that apply to the work: a new edition does not cover every project already under way. For revision, keep time control, temperature control and removing demand apart.
The 2021 edition sets out what this means for a wet heating system:
- a new dwelling with a floor area of 150 m² or more: at least two independently controlled heating circuits;
- a new dwelling, or an existing one whose boiler is replaced: thermostatic room controls in each heated room, such as a room thermostat with TRVs on every radiator outside its room, a thermostat in each room or zone, or networked controls on each emitter;
- one control for a zone instead of each room in a single-storey open-plan home whose living area is more than 70% of its floor area, or for two adjacent rooms with a similar function and heating needs, such as a kitchen and utility room;
- stored hot water: time control independent of the space heating, and electronic temperature control; a replacement cylinder gets an electronic temperature control such as a cylinder thermostat;
- any work on a system that includes a boiler: a boiler interlock.
The 2026 edition, which takes effect for most building work on 24 March 2027, keeps each of these.
On/off and OpenTherm controls
A basic room thermostat switches the boiler off at its set point and on again below it, while the boiler heats the water to one temperature. An on/off modulating control varies the average water temperature by running the boiler in longer or shorter bursts; the boiler’s own water temperature stays fixed. An OpenTherm control talks to the boiler instead: it requests a water temperature, high when the room is cold and lower as the room nears its set point. Drayton explains that the boiler then runs for longer at lower temperatures, which uses less energy, keeps a condensing boiler condensing for longer and holds the room temperature steadier.
Siting the room thermostat
Fit the thermostat as its instructions say, where air can move round it and its reading matches the part of the zone people use. Avoid draughts, sunlight, nearby heat sources and hidden positions. A cold spot keeps it calling for heat too long; a warm spot ends the call too soon. Compare its reading with the room temperature before blaming its position or its sensor. If the call has ended but heat keeps coming, check the valve, the wiring and the other ways a demand can reach the boiler.
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. Water let in through open or passing valves can raise the pressure or hide a leak, but a hose left attached with its valves closed is not, on its own, letting water in. The filling arrangement is separate from boiler interlock and from the pressure relief valve.