Frozen condensate pipe: how to thaw it safely

A frozen condensate pipe is one of the most common boiler faults in winter. It is not dangerous. The boiler shuts itself down on purpose, to stop water backing up inside it. Once the pipe thaws, the boiler should work again.

If you can't reach the pipe, or the boiler won't restart after thawing, you'll need a boiler repair from a Gas Safe engineer. This guide is part of our common boiler problems series.

What is a condensate pipe?

A condensate pipe is the small plastic pipe that carries waste water from a condensing boiler to a drain.It's usually white or grey, about as wide as a thumb, and it comes out of the bottom of the boiler. On many homes, part of it runs down an outside wall.

Modern gas boilers are "condensing" boilers. They take extra heat out of the hot gases they make. As those gases cool, they turn into water, a bit like steam on a cold window. This water is called condensate.

Water sits in the pipe and drips slowly, so the outside part can freeze in a cold snap. The ice blocks the pipe. The water then has nowhere to go, so the boiler stops.

How do I know if my condensate pipe is frozen?

The main sign is a boiler that stops working on a freezing day, often after a cold night. You may also see a fault code, hear gurgling, or find ice at the end of the pipe.

• The weather is below freezing, often after a cold night.
• The boiler has stopped, so you have no heating or hot water.
• A fault code or flashing light shows on the boiler. The code depends on the make.
• A gurgling sound comes from the boiler or the pipe.
• There's ice at the end of the pipe, or the pipe feels hard and very cold.

If the weather is mild and your boiler has stopped, the pipe is less likely to be the cause. See our list of common boiler problems for other checks.

How do I thaw a frozen condensate pipe?

Pour warm water, about as hot as a bath, slowly over the frozen part until water drips out of the end. Then reset the boiler. Never use boiling water, as it can crack the pipe and scald you.

Only do this if you can reach the pipe from the ground. Don't climb a ladder on ice.

1. Find the frozen part. Follow the pipe from the boiler to where it goes outside. It usually freezes at a bend, a dip or the open end. Feel for the coldest spot.
2. Warm it up. Fill a jug or watering can with warm water, about as hot as a bath. Pour it slowly over the frozen part. You can also hold a hot water bottle or a heat pack against it.
3. Keep going. It may take a few goes. Keep warming the pipe until water drips out of the end again.
4. Reset the boiler. Use the reset button or setting. Your manual shows how. Give it a few minutes to start up.
5. Check the pressure. Look at the gauge on the boiler. If it's below 1 bar, see our guide to a boiler losing pressure.

Never use a blowtorch or any flame either. And watch your step. Water you pour outside can freeze on the path. Put down some salt or grit afterwards, or mop it up.

When to call a Gas Safe engineer

Call a Gas Safe engineer if you can't reach the frozen part safely, or the boiler won't restart once the pipe has thawed. A pipe that keeps freezing, even with lagging on, also needs looking at.

• you can't reach the frozen part safely from the ground
• the boiler won't restart after the pipe has thawed
• it keeps freezing, even with lagging on
• the pipe is cracked or leaking
• you're not sure the condensate pipe is the problem

We fix boilers of all makes. The engineer first runs a thorough check to find the fault and the parts needed. See our boiler repair page for what we cover.

How do I stop my condensate pipe freezing?

Wrap any outside part of the pipe in foam lagging, and leave the open end clear. In a cold snap, running the heating for longer on a lower setting also helps. Bigger changes, like a wider pipe, are jobs for an engineer.

• Lag the outside part. Lagging is foam tube that wraps round a pipe to keep the cold out. You can buy it at most DIY shops.
• Run the heating for longer on a lower setting. This keeps warm water moving through the pipe. Water that keeps moving is less likely to freeze.
• Ask an engineer about bigger changes. Guidance for installers says outside condensate pipes should be wider than inside ones, as a wider pipe is slower to freeze. An engineer can also shorten the outside run or move the pipe indoors. Some fit a small heated cable to it.

A yearly boiler service is a good time to ask about this. The engineer checks the condensate trap, a small part that collects the water inside the boiler. A blocked trap can cause the same fault as a frozen pipe.

What if my water pipes are frozen instead?

Frozen water pipes are a different problem from a frozen condensate pipe. If the pipes to your taps have frozen, turn off the stopcock first. The stopcock is the tap that shuts off the water to your home.

See our emergency plumber page for what to do about frozen or burst water pipes.

Other boiler guides

• Boiler losing pressure: what the gauge should read and how to top it up.
• Heating works but no hot water: checks to try, and what the diverter valve does.
• Cold radiators: what cold at the top or bottom means.
• Leaking boiler: what to do first, and where the water is coming from.
• All common boiler problems.

If none of these help, book a boiler repair and we'll find the cause.