How to Deal with a Frozen Condensate Pipe

One of the most common reasons for boiler lockouts in cold weather is a frozen condensate pipe. The good news: it's usually fixable in minutes without an engineer.

What is a condensate pipe?

Modern condensing boilers produce acidic waste water (condensate) as a byproduct of their high efficiency. This exits through a plastic pipe — usually 22mm or 32mm diameter, white or grey — typically through an outside wall to a drain. In temperatures below approximately -2°C, this pipe can freeze solid, causing the boiler to lock out with a fault code.

How to identify a frozen condensate pipe

Signs: the boiler shows a fault code in cold weather, and everything else checks out (correct pressure, thermostat working, other gas appliances fine). The condensate pipe is always the first thing to investigate in freezing conditions.

How to thaw it

1. Locate the condensate pipe exiting your property — note where it goes from the boiler to outside.
2. Fill a jug or watering can with warm (not boiling) water — boiling water can crack the plastic pipe.
3. Pour the warm water slowly along the length of the frozen pipe, starting from the top and working down.
4. Alternatively, wrap a warm (not hot) hot water bottle around the frozen section for 10–15 minutes.
5. Once thawed, reset the boiler and wait 3 minutes for restart.

Preventing it next winter

Wrap the external condensate pipe with foam pipe lagging — £3–5 from any DIY store. Takes 10 minutes to fit and prevents this issue in all but extreme cold. Ask us to check and lag the pipe at your next annual service.

Still not working?

If the boiler doesn't restart after thawing, there may be a secondary fault. Call us for same-day attendance.