What is a condensate pipe?
Modern condensing boilers produce a small amount of acidic water (condensate) as they run. That waste water is carried away through a plastic pipe – the condensate pipe – usually to a drain outside.
If that pipe runs externally and the weather drops below freezing, the water in the pipe can freeze, block the pipe and trigger a fault code that shuts the boiler down for safety.
Typical signs your condensate pipe is frozen
- Boiler suddenly stops working on a cold day or night.
- Fault code on the boiler display related to “condensate” or “flame failure”.
- You may hear gurgling or sloshing noises before it shuts down.
- Outside, the plastic pipe feels rock‑solid or you can see ice at the end.
Short‑term thawing: safe things a homeowner can do
If you can see the plastic condensate pipe outside and it’s accessible from ground level, there are a few safe steps most people can take:
- Pour warm (not boiling) water slowly over the outside section of the pipe, especially any low points and the outlet. A jug or watering can works well.
- Gently warm the pipe with a warm towel or cloth that’s been soaked in hot water and wrung out, then wrapped around the pipe.
- Once thawed, reset the boiler following the manufacturer’s instructions and see if it runs normally.
Do not use open flames, blowtorches, or hit the pipe hard – it’s usually plastic and can crack, leaving you with leaks when it thaws.
Long‑term fixes your installer should be considering
If your condensate pipe freezes once in a freak cold snap, that’s annoying. If it freezes every time the temperature drops, the design is probably wrong. Common improvements include:
- Increasing the pipe size: External sections should usually be upsized to at least 32mm so they’re less likely to freeze.
- Reducing external runs: Keeping as much of the condensate run inside the property as possible and minimising exposed lengths outside.
- Improving fall (slope): Making sure the pipe has a continuous fall with no low spots where water can sit and freeze.
- Adding insulation or trace heating: Purpose‑made insulation sleeves or, in some cases, an electric trace‑heat cable to keep things moving in extreme weather.
When to call a professional
Anything that involves taking the boiler case off or altering the flue or combustion side of the appliance should only be done by a properly qualified engineer (e.g. Gas Safe in the UK). You’re fine to warm an exposed plastic pipe, but don’t start dismantling things you’re not sure about.
How The Plumbing Advisor can help
On a video call I can look at photos or a quick video of your boiler location, condensate run and the outside pipe, then:
- Confirm if a frozen condensate pipe is the likely culprit.
- Talk you through safe short‑term steps while you wait for the weather or an engineer.
- Suggest the sort of permanent improvements you should be asking your installer about – so it’s fixed properly, not just patched.