Frozen condensate pipe: how to thaw it

The short answer

A condensing boiler produces water as part of normal running, and drains it through a plastic condensate pipe. If that pipe runs outside, the water in it can freeze in cold weather. The ice blocks the pipe, the water backs up, and the boiler shuts down to protect itself.

To fix it, find where the pipe runs outside, pour warm (not boiling) water along it or hold a hot water bottle against it until the ice clears, then reset the boiler once. If it won’t restart, or the pipe is out of safe reach, call us for a boiler repair.

Signs your condensate pipe is frozen

• The boiler stops working during a frost or a cold snap, often first thing in the morning.
• A gurgling or bubbling sound from the boiler or the pipe.
• A fault code or lockout on the display. The code varies by make, so check your manual.
• The boiler tries to light, then cuts out again after a short while.

If the weather is mild, a frozen pipe is unlikely and something else is wrong. Check the pressure and the gas supply, and if that doesn’t help, get an engineer to look at it.

How to find the condensate pipe

The condensate pipe is a plastic pipe, usually white or grey, coming out of the bottom of the boiler. Follow it from the boiler, through cupboards if needed: it may go into a drain or waste pipe indoors, in which case it’s unlikely to freeze. If it goes through an outside wall, look for where it comes out and runs down to a drain or gully.

The ice usually forms where the pipe is most exposed: at a bend, a joint, or near the open end above the drain. Those are the places to thaw first.

If you can’t see a pipe coming out of the bottom of the boiler at all, or there is no plastic pipe of this kind, your boiler may be an older non-condensing model. In that case a frozen condensate pipe isn’t the cause, and it’s time to call an engineer.

How to thaw it safely

1. 1Boil a kettle and let it cool until the water is warm, not boiling. A jug or watering can makes it easier to pour.
2. 2Pour the warm water slowly along the outside section of the pipe, starting at the open end and working back towards the wall. Or hold a hot water bottle or a warm cloth against the frozen part.
3. 3Keep going, refilling with warm water, until the ice clears and water runs freely from the end of the pipe.
4. 4Reset the boiler once, following the manual. It may take a few minutes to restart.

Take care on the ground below. Water poured outside in a frost can freeze into ice where you’re standing, so pour away from paths and doorsteps, and grit or salt it if needed.

If the boiler won’t restart

If the boiler locks out again after one reset, stop. Repeated resetting won’t clear a blockage and may hide a different fault. Note the code on the display and call us. If you have no heating or hot water and someone at home is elderly, very young or unwell, tell us when you call so we can put them first.

While you wait, electric heaters, and an immersion heater if you have a cylinder, can tide you over. See emergency boiler repair for what else is safe to check.

Be ready before a cold snap

A few minutes of preparation in autumn makes a frozen pipe much easier to deal with, especially early on a dark winter morning.

• Find your condensate pipe now, while it’s light and dry, and check whether it runs outside.
• Keep your boiler manual somewhere you can find it, or save the manufacturer’s online copy on your phone. It tells you how to reset your model.
• Check the boiler pressure is in the normal range before the weather turns.
• Keep a hot water bottle handy, and know where you can safely pour warm water.
• Book your annual boiler service before winter, and ask the engineer to look at the condensate pipe while they’re there.

Some people leave the heating on low through a very cold night so the boiler runs more often and the pipe is less likely to freeze. Whether that suits you depends on your home and how exposed the pipe is.

Stopping it happening again

If your condensate pipe freezes every winter, the way it was fitted is usually the problem. There are a few fixes, and all of them are a job for a heating engineer, because the pipe needs the right fall, size and connection to drain properly:

• Insulation. Lagging the outside section with weatherproof pipe insulation.
• A wider pipe. A larger diameter pipe on the outside run is much harder to block with ice.
• Internal routing. Re-routing the pipe to an indoor waste, such as a kitchen sink or washing machine drain, so it doesn’t go outside.
• A shorter outside run. Keeping any external section as short and direct as possible.

When we fit a new boiler, we route and size the condensate pipe to avoid freezing. If you’re planning a boiler replacement, it’s one of the things we’ll look at on the survey.