Boiler Keeps Switching Off

A boiler that starts, runs for a short while and then shuts down — sometimes over and over — is one of the most common faults we're called to. The good news is that the boiler is usually switching off deliberately: one of its safety or control systems has decided something isn't right. The job is finding out which one, and why.

Common causes

• Low system pressure. Most modern boilers shut down below a set pressure. Check the gauge — below 1 bar is your likely answer, and our losing pressure guide covers what to do next.
• A blocked or frozen condensate pipe. Very common in freezing weather — the boiler can't drain the acidic condensate it produces, so it locks out. Often comes with a gurgling noise. See our full frozen condensate pipe guide.
• Circulation faults. A failing pump, a stuck valve or an airlock means heat isn't being carried away from the boiler, so it reaches temperature almost instantly and switches off — then cools, fires, and repeats.
• A faulty thermistor. If the temperature sensor feeds the control board wrong readings, the boiler can believe it's overheating when it isn't, and shuts down.
• Scale and sludge. Debris in the system restricts flow through the heat exchanger, causing localised overheating and short-cycling. Our power flush guide explains how a badly sludged system gets cleaned.
• Flue or overheat safety lockouts. If the boiler's overheat protection or flue monitoring is tripping, the boiler is shutting down to protect you — which brings us to the safety point below.

What's safe to check yourself

• The pressure gauge. Below 1 bar? Top up via the filling loop to around 1.2–1.5 bar if you're comfortable doing so.
• The condensate pipe, in freezing weather. If you can see an external condensate pipe and it's visibly frozen, thawing the accessible section with warm — never boiling — water can get you going again. Full steps in our condensate guide.
• Your thermostat. If it's set close to room temperature, the boiler may simply be cycling normally as the room hovers around the target. Turn it up a few degrees and see whether the behaviour changes.
• One reset. Per your manual — once. If it locks out again, stop there.

Not sure which fault matches what you're seeing? Our Boiler Problem Finder walks through the symptoms.

When it's more than a nuisance

If the boiler is locking out on overheat — your display or manual will identify the code — stop using it and book a diagnosis. Overheat protection exists to stop a fault becoming damage, and repeatedly resetting through it is the one version of this problem we'd tell you firmly not to live with.

What the engineer tests

Diagnosis is £99 including the first hour. The engineer reads the boiler's fault history, then tests the suspects in a sensible order: system pressure and the expansion vessel, the condensate route, pump operation and circulation, the thermistor's readings against actual temperatures, and — where the symptoms point that way — combustion and flue performance with a flue gas analyser. The cause gets confirmed before anything is replaced, and any part is quoted first. Most repairs carry a 12-month parts warranty, and 88% are finished on the first visit. A worked example: a Walsall rental locking out on overheat, where the pump had burnt out on sludge and was replaced from the van the same visit.