Frozen condensate pipe: how to get your boiler going again

Modern gas boilers are condensing boilers. They squeeze extra heat out of their exhaust gases, and in doing so make water, called condensate. It is slightly acidic and drains away through a plastic pipe. If that pipe runs outside and the weather turns cold enough, the water inside it can freeze.

Why the boiler stops

When the pipe freezes, condensate cannot drain. It backs up into the boiler. A sensor detects the blockage and the boiler shuts down to protect itself. Many boilers show an ignition lockout or a condensate fault code. Some make a gurgling noise first.

Maker	Codes often linked with a frozen condensate pipe
Vaillant	F28 (failed ignition)
Ideal	L2 (ignition lockout)
Worcester Bosch	EA 227 (no flame detected)

Codes vary by model, so check your manual. Our posts on Vaillant fault codes and Ideal F1 and L2 codes explain more.

Is it the condensate pipe?

It is likely if:

• It is below freezing, or has been overnight
• The boiler was working yesterday
• The condensate pipe runs outside, even for a short length
• The boiler shows an ignition lockout code or is gurgling

How to find the pipe

The condensate pipe is usually white or grey plastic, about the width of a thumb or a little wider. It comes out of the bottom of the boiler and goes either to a drain inside, such as under the kitchen sink, or through an outside wall to a drain or gully.

Follow it from the boiler. If it goes outside, look for the coldest, most exposed part: the end near the gully, bends, and anywhere it runs along a wall. Ice usually forms at the lowest point or the open end.

How to thaw it safely

1. Fill a kettle or jug and let the water cool until it is warm, not boiling.
2. Pour it slowly along the outside pipe, starting at the end and working back towards the wall.
3. Or hold a hot water bottle or a warm cloth on the frozen part.
4. Keep going until you hear water draining.
5. Reset the boiler using the reset button. It may take a few minutes to start.

Be careful of ice on the ground where you pour water. Do not climb ladders in icy conditions to reach a high pipe. Book an engineer instead.

London terraces and cold snaps

London cold snaps are usually short, but they bring a burst of these calls. In Lewisham, Catford and Brockley, many terraced and semi detached houses have the condensate pipe running down an outside side or back wall into a gully. Those exposed lengths freeze first. Our Lewisham area page covers the housing there.

Boilers in garages and lofts are also at risk, because the space around them is cold too.

Stopping it happening again

Fix	What it involves
Lagging	Insulating the outside length of pipe with weatherproof lagging
Wider pipe	Larger diameter pipe outside freezes less easily
Shorter outside run	Rerouting so less pipe is outside
Internal drain	Rerouting to an inside drain, such as under a sink
Trace heating	A small heating cable along the pipe, used in some cases

Industry guidance recommends a larger diameter pipe and insulation for external runs, and boiler makers publish their own guidance too. An engineer can look at the route at your next annual boiler service and suggest what suits your home. Any changes are priced after a look.

When to call an engineer

Book a boiler repair visit if:

• You cannot reach the pipe safely
• You have thawed it but the boiler still will not start
• The fault comes back after the weather warms up
• There is water leaking from inside the boiler
• The pipe is cracked or loose

Do not take the boiler case off to clear the condensate trap yourself. That is gas work and must be done by a Gas Safe registered engineer.

Other cold weather checks

• Pressure. Check the gauge. Cold weather sometimes shows up a slow leak.
• Thermostat. Make sure it is calling for heat.
• Gas supply. Check other gas appliances work and a prepayment meter has credit.

If you smell gas at any point, call the National Gas Emergency Service on 0800 111 999.

Before the next cold snap

If your boiler stopped last winter with a frozen pipe, look at the outside run in autumn. Note how long it is, whether it is lagged, and whether it ends over a gully or drips onto the ground. Take a photo and mention it when you book your service.

Summary

A frozen condensate pipe is a nuisance, not a disaster. Warm water, a reset and patience fix most cases. If it happens every winter, get the pipe looked at.

Gas work in the UK must be done by a Gas Safe registered engineer. Check the ID card and the Gas Safe Register.