Boiler switching itself off or cycling on and off? The usual causes from low pressure to a frozen condensate pipe, safe checks you can do and when to call an engineer.
By AJ, certified Gas Safe engineer and lead plumber at 247 Emergency Plumbers Edinburgh · Checked
A boiler that keeps turning off is most often low on pressure, overheating because air or sludge is blocking circulation, or shut down by a frozen condensate pipe. Check the pressure, bleed the radiators and check the thermostat; if it still cuts out, or there is a code that keeps returning, call a Gas Safe registered engineer.
Smell gas, or a carbon monoxide alarm is sounding? Do not reset the boiler. Open windows, turn off the gas at the meter if it is safe to do so, leave the building and call the free gas emergency line on 0800 111 999. What to do →
What it could be and what to do
Low water pressure You may be able to fix this
Likely cause: Glow-worm says the system needs enough pressure to circulate water; combi boilers are especially prone to shutting off when it drops.
What to do: Check the gauge and top up to around 1 to 1.5 bar using the filling loop.
Thermostat or sensor You may be able to fix this
Likely cause: A faulty thermostat can make the boiler think the home is already warm.
What to do: Check the thermostat settings and batteries.
Air or sludge in the system You may be able to fix this
Likely cause: Trapped air or sludge stops water circulating, so the boiler overheats and shuts off.
What to do: Bleed the radiators. If it continues, an engineer may recommend a system flush.
Frozen condensate pipe You may be able to fix this
Likely cause: In cold weather a combi boiler can shut down if its condensate pipe freezes.
What to do: Thaw an accessible pipe with warm water, then reset once.
Limescale Engineer needed
Likely cause: In hard water areas, scale can block parts of the boiler and cause overheating. Most water in Scotland is soft, so this is less common in Edinburgh.
What to do: Needs an engineer to inspect.
Safe checks you can do for most codes
These are the only checks we suggest without an engineer. Never remove the boiler casing.
Pressure: look at the gauge. Most boilers need about 1 to 1.5 bar when cold. If it is low, top up using the filling loop as shown in your manual.
Gas supply: check your cooker or hob works, and that a prepayment meter has credit.
Frozen condensate pipe: in freezing weather, warm the white plastic pipe outside with warm (not boiling) water or a hot-water bottle, only where you can reach it from the ground. How to thaw it safely →
One reset: press reset once, as your manual describes. If the code comes back, stop and call an engineer; repeated resets can lock the boiler out.
Controls: check the programmer, thermostat and any smart controls are calling for heat.
Code still showing? Our Gas Safe engineers can fix it.
Boiler repair labour is £90–£350 in standard hours, plus the callout from £75 (out of hours £100–£180). The price is agreed before work starts. Tell us the make, model and code when you call.
Our emergency line is open 24 hours a day, 7 days a week.
Every boiler is different. Check your boiler’s own manual. This page is general guidance, not a diagnosis.
YOUR QUESTIONS, ANSWERED
Helpful answers before you call.
Why does my boiler keep turning off?
Most often low pressure, trapped air or sludge causing overheating, a thermostat problem or a frozen condensate pipe. Check the pressure, bleed the radiators and check the thermostat.
When should I call an engineer?
Glow-worm advises calling quickly if the boiler leaks, smells of burning, bangs or clunks loudly, or shows the same error code repeatedly. If you smell gas, call 0800 111 999.
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