Key points
- Condensing boilers drain waste water through a plastic condensate pipe, which can freeze where it runs outside.
- Pour warm, not boiling, water along the frozen section, then reset the boiler.
- Insulating the outside pipe and making sure it’s wide enough are the best long-term fixes.
Why condensate pipes freeze
Every modern gas boiler is a condensing boiler. It recovers extra heat from its exhaust gases, which produces a small amount of slightly acidic water. That water drains away through a plastic condensate pipe, usually white or grey. Where the pipe runs outside, the water can freeze in a cold snap, and once it’s blocked the boiler shuts down to protect itself.
Signs your condensate pipe is frozen
- The boiler has stopped and shows a fault code. On Worcester Bosch boilers, for example, this is often “EA”.
- A gurgling noise from the boiler.
- It’s been below freezing, especially for more than a day or overnight.
- You can see ice at the open end of the pipe outside.
How to thaw it safely
- Find the pipe. Follow the plastic pipe from the bottom of the boiler to where it leaves the house and runs to a drain. The blockage is usually at the end, or at a bend or dip in the outdoor section.
- Pour warm water along it. Use a watering can or jug of warm water. Never use boiling water, which can crack the plastic. Water from a kettle that has cooled for 10–15 minutes is fine. A hot water bottle or warm cloth held against the pipe also works.
- Reset the boiler. Once the ice has cleared, reset the boiler following its manual. It should restart normally.
Stay safe: only do this if the pipe is at ground level and you can reach it easily. Never climb a ladder in icy conditions. If the pipe runs high up the wall or you can’t find the frozen section, call an engineer. Watch out too for the warm water freezing on paths and steps.
Stopping it happening again
Worcester Bosch recommends insulating the external pipework and fitting a cover over the pipe end. Its installer guidance adds that outdoor condensate runs should be kept to a maximum of 3 metres and insulated, and that it may be advisable to increase the pipe to 32mm, since a wider pipe is less likely to freeze solid.
If your pipe is thin, uninsulated or has a long outdoor run, ask us to look at it during your annual service. It’s far better sorted before winter than during a freeze.
When it isn’t the condensate pipe
If you’ve thawed the pipe and reset the boiler a couple of times and it still won’t run, the problem may be elsewhere: low pressure (see our guide to boiler pressure), frozen pipework elsewhere, or a fault inside the boiler. That’s the point to call us. Same-day emergency call-outs are available. See our Prices page for rates.