How to identify – and fix – a frozen condensate pipe

When the temperature outside drops below freezing, your boiler might malfunction or produce an uneven heat supply that could be caused by a frozen condensate pipe, a problem that’s simple enough to fix with plain old hot water.

But first, what is a condensate pipe?

A condensate pipe releases residual water outside the house. This water is acidic in nature and is produced when the water in the boiler comes into contact with flue gases in a process called condensing. The release of this waste-water increases your boiler’s efficiency to give you an uninterrupted heat supply.

Why does a condensate pipe Freeze?

Because the condensate pipe is outside the house, the water flowing through it can freeze and block the pipe when the temperature drops. When this happens, valves inside the condensate pipe are automatically turned off to prevent damage to the boiler. But when water can’t find a way out, it accumulates in the boiler, causing it to shut down or make unusual gurgling noises.

Before you can unblock the pipe, you need to find it.

The condensate pipe is easy to recognise. Unlike other boiler pipes, a condensate pipe is made of plastic, usually white or black in colour, and leads outside the house.

How will you know if it’s frozen?

In most boilers, a frozen pipe means the boiler won’t start and will flash a warning or an error code for the frozen pipe. These vary depending on your boiler brand.

These are the error codes of a few popular brands:

Baxi E133 or E28
Ideal Boilers L2, LF, or F2
Glow-Worm F28 or F29
Potterton E133 or E28. Or, the error code may alternate between E1 and E33
Worcester Bosch EA229 or D5
Vaillant F28 or F29
Viessmann F4

Without a code showing, your boiler might just make an unusual gurgling sound to indicate the problem.

If your boiler was serviced recently and performing well before the temperature dropped to freezing, this is itself could indicate a condensate pipe problem.

Here’s how to defrost your pipe.

The problem is easy enough to fix without special tools or materials.

1: Locate the blockage.

Feel the pipe with your hand to find the freezing spot - usually the most exposed part. The blocked part will be colder than the rest of the pipe.

2: Pour hot water over the frozen spot.

Boil water in your kettle and let it cool for a few minutes. (Boiling water will cause the pipe to melt.) Pour the water over the frozen spot. Or you could pour water all over the pipe until it unfreezes.

3: Restart your boiler.

When the pipe is unfrozen your boiler should work ok. If it still shows the error code or doesn’t start, pour more hot water to make sure the ice has completely melted.

Can you stop your pipe from freezing?

You could leave the heating on overnight. While this can keep the condensate pipe warm and help it stay unblocked, it won’t help your energy budget.

You could turn up your heating system temperature. This will increase the condensate pipe’s temperature, but will also raise your heating bills.

Install a Trace heater. This is an inexpensive electrical element installed beneath the pipe which warms it up when the temperature drops extremely low. It’s the best way to stop your pipe freezing.