Boilers and heat pumps ยท In brief
Frozen condensate pipe: what you can safely do
A frozen condensate pipe can often be thawed with warm, not boiling, water, but only on the outside plastic pipe and only from the ground. Stay off ladders, keep the boiler casing shut and call an engineer if unsure.
Most useful for: Condensing boiler households in a cold snap, if the pipe is within reach
Compiled from the sources at the end. Updated .
A condensing boiler makes slightly acidic water that drains away through a thin pipe. When an outside length freezes, the boiler can stop and may show a fault. Codes and restart steps differ between makes, so use the manual for your own boiler.
What you can do from the ground
The steps below follow Baxi's advice for householders. Do them only if the outside pipe is within reach from the ground and the ground is not icy.
- Find the plastic condensate pipe where it runs down the outside wall.
- Pour warm, not boiling, water along the frozen length, or hold a heat pack against it.
- Restart the boiler using the normal user control only, as your manual describes.
- If it will not relight, call a Gas Safe registered engineer.
Boiler maker Baxi shows how to find a boiler's outside condensate pipe in freezing weather and thaw it with warm, not boiling, water. Not made by this site; duration 5:05.
No ladders or steps, no boiling water and no opening the boiler casing. Spilt water can freeze underfoot, so watch the ground. The round flue terminal in the wall is not the condensate pipe, so leave it alone. If a pipe is out of reach or you are unsure which pipe it is, stop and call an engineer; check their card first.
Stopping it happening again
Approved Document H para 1.14 says condensate going to sanitary pipework needs a minimum 22mm pipe through a 75mm trap, preferably to an internal stack, in materials resistant to pH 6.5 and lower, with BS 6798 as the reference. Ask the engineer whether your run could be rerouted or insulated. Where a run cannot fall to a drain, a pump is one option they may look at.
Frozen water pipes are a different problem with different risks, covered on the frozen pipes page. A full boiler change is the natural moment to fix a poor run, so raise it when replacing the boiler.
Sources, read 4 November 2026: Baxi, how to thaw a frozen condensate pipe; Approved Document H, 2015; HSE, the Gas Safe Register.