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Frozen condensate pipe: why your boiler stopped in the cold
If a condensing boiler stops during freezing weather, the plastic pipe that drains it may be blocked by ice. Where that pipe can be reached safely from the ground, gentle warmth on the frozen section can clear it, and the boiler can then be reset using its normal controls.
By AJ, certified Gas Safe engineer and lead plumber at 247 Emergency Plumbers Edinburgh · Published · Updated · 3 min read
How to tell if the condensate pipe is frozen
A condensing boiler produces water as it runs and drains it through a narrow plastic condensate pipe, usually white or grey, which leaves the wall near the boiler and runs to a drain, gully or soakaway. When that water freezes in the pipe, the boiler shuts itself down as a safety measure.
The usual signs are heating and hot water stopping during a cold snap, a gurgling sound from the boiler, and a condensate-related fault code on the display; your boiler manual explains what each code means. Your cold taps still work, because the condensate pipe is separate from the water supply.
Thawing an accessible condensate pipe safely
Only work on a section you can reach while standing on the ground, on a surface that is not icy. Do not use a ladder on frozen ground or lean out of a window.
Run a hand along the outside section of the pipe to find the coldest point. Ice usually forms at the open end, at bends or where the pipe passes through an unheated garage.
Hold a hot-water bottle or a warm cloth against the frozen section, or pour warm water slowly along it. Let a boiled kettle cool for 10 to 15 minutes first; never use boiling water, which can crack the plastic.
Keep water away from outdoor sockets, lights and the boiler itself, and watch for new ice forming on paths and steps where the water lands.
Once the pipe has cleared, reset the boiler using the reset control described in its user manual. If it locks out again after a couple of attempts, stop and arrange an engineer.
When to call an engineer instead
Leave it to a professional if the frozen section is high up, inside the boiler casing or inside the building, if you cannot find the pipe, or if the boiler still will not run after it has thawed. Never remove the boiler casing; gas appliance work must be carried out by a Gas Safe registered engineer.
If you smell gas, a carbon monoxide alarm sounds or you notice soot around the boiler, do not try to reset it. Leave the affected area and call the National Gas Emergency Service on 0800 111 999 from a safe place.
Stopping it happening again
Industry guidance favours keeping the outside run of a condensate pipe as short as possible, giving it a steep fall with few bends so water cannot sit in it, and using the widest external pipe the boiler manufacturer allows, often 32mm or more. Weather-resistant insulation helps, and some properties benefit from routing the pipe internally or fitting trace heating.
Changes to the pipe's route, angle or size should follow the boiler manufacturer's installation instructions, so ask about them at your next boiler service. Until then, check the pipe before a forecast cold snap so you know where it runs and whether you can reach it safely.
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Callout charges and standard-hours service labour guide
Service
Guide charge
What it covers
Standard-hours callout
From £75
Attendance and diagnosis only
Out-of-hours callout
£100–£180
Attendance and diagnosis only
Boiler repair
£90–£350
Labour only
Boiler service
From £60
Service labour only
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Can I pour boiling water on a frozen condensate pipe?
No. Boiling water can crack the plastic pipe. Use warm water, a hot-water bottle or a warm cloth on a section you can reach safely from the ground.
Why do my taps still work when the boiler has stopped?
The condensate pipe only drains the boiler and is separate from your water supply, so cold taps can work normally while the boiler is locked out.
Should I use a ladder to reach a high condensate pipe?
We advise against using a ladder on icy or frozen ground. Leave high or hard-to-reach pipework to an engineer.
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