Frozen condensate pipe: how to fix

No heating or hot water? If your boiler‘s stopped working during a snap of frosty weather, there’s a good chance you’ve got a frozen condensate pipe. This common problem is ironically the last thing you need during the winter months! Fortunately, there are a couple of easy solutions to get your boiler up and running again.

What exactly is the condensate pipe?

The condensate pipe is a plastic pipe which allows a condensing boiler to discharge waste water from the condensing process into the sewer.

During the combustion process inside the boiler, water vapour is produced. Instead of expelling this hot vapour out of the flue, condensing boilers draw out so much heat from it, that it cools down and condenses into a liquid. This liquid – moderately acidic water, is then expelled from the boiler via the condensate pipe.

In some households the condensate pipe may discharge into the trap of a sink. Alternatively, it may run independently through the wall and into a drain. It may also connect directly to the soil stack. Residual water in the pipe during the winter may freeze, resulting in a frozen condensate pipe. The boiler’s sensors will detect that the condensate pipe is blocked, and trigger a fault code, preventing the boiler from operating.

How to unfreeze/thaw the condensate pipe

  1. Do not cut the condensate pipe. The condensate pipe is generally considered to be part of the flue of a boiler. Any work on it may only legally be carried out by a Gas Safe-registered engineer.
  2. Identify where the pipe may have frozen. Horizontal lengths of pipe with little to no fall are a likely suspect – they allow the water in the pipe to settle.
  3. Prepare some hot water. You will probably have to use your kettle or the microwave for this. Do not use boiling water.
  4. Using a suitable container such as a watering can, pour hot water over the pipe where you think it has frozen. Pipe elbows and horizontal pieces of pipe are good areas to start with. Alternatively, you can use a hot water bottle. Place it over the pipe and leave it for a few minutes.

If ice in the condensate pipe was indeed the culprit, it should melt away and any water which was behind it should flow away too. After resetting the boiler or clearing any fault codes, the boiler should now operate normally.


You can prevent a frozen condensate pipe in future by insulating the pipe with lagging. The lagging must be waterproof. Otherwise it will soak up water like a sponge and give the pipe a nice ice blanket, which will only make the problem worse.

Other ways to prevent a frozen condensate pipe include rerouting the pipe to give it the appropriate level of fall or rerouting it through the house so that is less likely to freeze. Alternatively, external runs of pipe can be widened from 22 mm to 32 mm. For these modifications, you should consult a Gas Safe-registered engineer.