"Boiler interlock" is one of those phrases that sounds like a single component but isn't — it's a result achieved by wiring the controls together correctly. Get it right and the boiler only fires when there's real demand; get it wrong and the boiler short-cycles, wasting gas. This guide explains the idea and the parts. It's study material; only a Gas Safe registered engineer may carry out the work.
What boiler interlock means
Boiler interlock means the boiler and its pump are switched off when there is no demand for heating or hot water — when every thermostat is satisfied, the boiler stops firing rather than cycling on and off against a system that doesn't need heat. It's delivered by the controls acting together so that "no demand anywhere" reaches the boiler as "stop."
Why TRVs alone don't count
A thermostatic radiator valve (TRV) throttles flow to its own radiator, but it doesn't tell the boiler to switch off. If TRVs were the only control, every TRV could close and the boiler would keep firing into a system that can't accept heat. So TRVs do not provide interlock — you need a room thermostat (in the reference room, which usually has no TRV) plus cylinder control to actually stop the boiler.
The controls that deliver interlock
- Programmer / time control — decides when heating and hot water are allowed.
- Room thermostat — calls for heat until the room reaches temperature, then drops the demand.
- Cylinder thermostat — calls for hot water until the cylinder is up to temperature (on stored-water systems).
- Motorised (zone) valve(s) — open the circuit on demand and, through their auxiliary switch, tell the boiler to fire; when all demands cease they close and the boiler stops.
S-plan and Y-plan
Two classic layouts wire these together:
- S-plan — separate 2-port zone valves for heating and for hot water, each driven by its thermostat. Flexible and easy to zone.
- Y-plan — a single 3-port mid-position valve sharing the boiler between heating and hot water.
Both give full interlock when correctly wired, because the boiler only runs while a valve is calling.
Automatic bypass and compensation
An automatic bypass valve lets a minimum flow circulate when most valves/TRVs have closed, protecting the pump and boiler from running against a closed system. For efficiency, weather or load compensation adjusts the boiler's flow temperature to conditions, keeping it modulating and condensing — these are among the measures that satisfy Boiler Plus.
- Interlock = boiler/pump switch off when there's no heat or hot-water demand.
- TRVs alone don't provide interlock — they only throttle their own radiator.
- Delivered by: programmer + room thermostat + cylinder thermostat + motorised valve(s).
- S-plan = two 2-port valves; Y-plan = one 3-port mid-position valve.
- Automatic bypass protects pump/boiler when valves close.
- Weather/load compensation improves efficiency and helps meet Boiler Plus.
- Prove interlock at commissioning: no demand → boiler and pump stop.
10-Question Mock Test
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It's a result: when every thermostat is satisfied, the boiler and pump stop rather than cycling.
A TRV throttles its own radiator but doesn't tell the boiler to stop, so TRVs alone don't give interlock.
The reference room's radiator is left without a TRV so the room thermostat controls it directly.
The programmer (time control) sets when heating and hot water can run.
Its auxiliary switch fires the boiler when open and removes the demand when it closes.
S-plan uses two 2-port valves; Y-plan uses a single 3-port mid-position valve.
Y-plan shares the boiler between heating and hot water through one 3-port mid-position valve.
It maintains a minimum circulation so the pump/boiler aren't running against a closed system.
It tunes flow temperature to weather/room conditions so the boiler keeps modulating and condensing — a Boiler Plus measure.
Satisfy all demands and check the boiler and pump switch off — if they keep running, there's no interlock.
Interlock is wiring, not a widget. Make the logic stick.
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