Boiler interlock: why the boiler fires only when a thermostat calls

What the rule says

In England, Approved Document L, Volume 1: Dwellings (2021 edition) puts it in two paragraphs.

• Paragraph 5.15: “System controls should be wired so that when there is no demand for space heating or hot water the heating appliance and pump are switched off.”
• Paragraph 5.25: “If work is carried out on a system that includes a boiler, a boiler interlock should be installed.”

So the interlock is not an extra. A job on a system with a boiler should leave it with one.

The 2026 edition takes effect in England on 24 March 2027, with later dates for some buildings. It keeps the interlock (paragraph 4.27). With no demand, it lets the controls either switch the boiler and pump off, or set them back to a reduced output temperature (paragraph 4.17). Wales, Scotland and Northern Ireland have their own rules.

Why a boiler needs one

A boiler that fires with nothing calling heats water nobody asked for. With every valve shut, it heats only itself, and fuel is wasted every day it is left like that.

A thermostat on the wall is not an interlock by itself. What matters is where its call goes. The boiler must be wired so that a call is the only thing that can fire it.

How an S-Plan does it

On an S-Plan, the boiler’s switched live comes only from the zone valves’ orange cores. On a wiring centre numbered the common way, they meet on block 10. The chain runs in this order:

1. A thermostat (stat) calls. Its live drives its valve’s brown core, and the motor opens the valve.
2. At the end of its travel, the valve’s END SWITCH closes. The permanent live on its grey core comes out on its orange.
3. The orange puts a live on block 10, and the boiler fires.

No call, no open valve, no live on block 10: the boiler cannot fire. On a boiler without a pump overrun, the pump’s live usually joins block 10 as well, so the pump stops with the boiler. See pump overrun.

The end switch does a second job. The boiler fires only once a valve is fully open, so its water has somewhere to go.

How a Y-Plan and a combi do it

On a Y-Plan, the boiler’s switched live is on block 8. Two things feed it: the cylinder thermostat’s call, and the 3-port valve’s orange, live only once the valve reaches the heating port. So with no call from either thermostat, nothing can fire the boiler, wherever the valve sits. See what white, grey and orange do.

On a combi, the room thermostat’s call goes to the boiler’s room-thermostat terminals, in place of the link fitted at the factory. With that link left in, the boiler ignores the thermostat, and there is no interlock for the heating. The hot water needs no wiring: the boiler starts when a tap runs.

What does not count as an interlock

• Radiator valves alone. THERMOSTATIC RADIATOR VALVES (TRVs) shut their own radiators as the room warms. They cannot switch the boiler off. The industry’s guidance on Part L says it plainly: TRVs alone do not provide interlock.
• A programmer with no thermostat after it. The programmer says when heat is allowed. Without a thermostat, the boiler fires all through each timed period, whether the home needs heat or not.
• A thermostat beside a link that is still in. On a combi, the link must come out for the thermostat’s call to count.

Proving it when you commission

Prove the interlock row by row, as part of commissioning. See what a truth table proves.

The row that proves it: the programmer allows both heating and hot water, and neither thermostat wants heat. Both valves shut and the burner stops. The pump stops too, after its overrun where the boiler has one.

Prove that row with the meter as well as your eyes. These are live tests: only for a competent person, or a learner under the supervision of one, with test leads made to the HSE’s guidance note GS38. On an S-Plan, block 10 reads about 230 volts (230 V) to neutral while a zone calls, and 0 V once both are satisfied.

Any work on the wiring is done dead: isolate, lock off and prove dead first. See safe isolation in five steps. Controls work stops at the terminals the boiler’s manual gives for the controls. Anything inside the boiler’s sealed case is a job for a Gas Safe registered engineer.

Sources

• Approved Document L: conservation of fuel and power, Volume 1: Dwellings, the 2021 edition (paragraphs 5.15 and 5.25) and the 2026 edition (paragraphs 4.17 and 4.27).
• BEAMA, Installing heating controls with gas or oil boilers in dwellings (2022): guidance on Part L that says TRVs alone do not provide interlock.
• HSE guidance note GS38, Electrical test equipment for use on low voltage electrical systems.