Boiler stopped in cold weather? How to thaw a frozen condensate pipe

It is a freezing morning, the house is cold and the boiler has stopped. Before you assume the worst, there is a common and often simple cause on condensing boilers: a frozen condensate pipe. This guide answers the questions people ask while standing outside in the frost, in the order they usually ask them.

What is the condensate pipe?

Modern condensing boilers recover extra heat from their exhaust gases. As they do, water condenses inside the boiler. That water is slightly acidic, so it is carried away through a plastic pipe, usually white or grey, to a drain, a soakaway or an internal waste pipe.

Where part of that pipe runs outside, it is exposed to the weather. In a hard frost, the small amount of water sitting in it can freeze and block it. The boiler senses the water cannot drain and shuts down to protect itself.

How do I know that is the problem?

Signs that point to a frozen condensate pipe include:

• The boiler cuts out or locks out during freezing weather, sometimes after starting normally.
• You hear gurgling or bubbling from the boiler or the pipe.
• The boiler shows a fault code or warning light. Which code depends on the make, so check your manual. Our boiler fault codes guide explains how to read the display.
• You have no heating and possibly no hot water. If only the hot water is affected, our post on a boiler not heating water covers other causes.

If you smell gas at any point, stop. Leave the property, do not use switches, and call the National Gas Emergency line on 0800 111 999 first.

Where do I find the pipe outside?

Start at the boiler and look for a plastic pipe leaving the bottom of it. Follow it to where it passes through the wall. Outside, it usually runs down the wall to a drain or gully, or disappears into the ground. The parts most likely to freeze are bends, the open end, any section that runs along the wall rather than straight down, and anywhere water might sit.

The frozen section often feels much colder than the rest, and you may see ice at the outlet. Do not confuse it with the copper pressure relief pipe, which is metal, or with the flue, which is the larger outlet the exhaust comes from.

How do I thaw it safely?

1. Switch the boiler off at its controls while you work.
2. Fill a jug or watering can with warm water, such as hot tap water. Do not use boiling water; it can crack the plastic pipe and scald you.
3. Pour it slowly over the frozen section, starting near the end of the pipe and working back towards the wall. Repeat as needed. It can take several jugs.
4. Watch your footing. The water you pour can freeze on paths and patios. Salt or grit the area afterwards if needed.
5. Do not open, cut or take apart the pipework. If the pipe will not clear, leave it for an engineer.

You may hear the boiler or pipe gurgle as the ice clears and water drains. That is a good sign.

Then what? Resetting the boiler

Once the pipe is clear, the boiler usually needs a reset. Follow the procedure in your manual and avoid pressing the reset button repeatedly. Our guide on resetting a boiler safely explains why. It is also worth checking the pressure gauge. If it has dropped below the marked range, our step-by-step guide to repressurising a boiler shows how to top it up.

Why does it keep freezing, and what can be done?

Some condensate pipes freeze every cold snap because of how they were installed: a long external run, a narrow pipe, a shallow fall that lets water sit, or an exposed open end. An engineer can look at your installation and talk through options that may reduce repeat freezing, such as:

• insulating the external run with suitable weatherproof lagging;
• changing the route so more of the pipe runs inside the building, where suitable;
• fitting a larger diameter pipe on the external section, where suitable for the boiler.

These are options to discuss rather than guarantees; the right answer depends on the boiler, the wall and where the drain is. A heating engineer visit is a good time to raise it, and it can be looked at alongside other boiler repair work. Frozen water supply pipes are a separate problem, covered in our guide to frozen pipes.

When should I call an engineer?

Call if the boiler will not restart after thawing and resetting, if there is water inside the boiler or leaking from it, or if the pipe stays blocked and the cause is not ice. Our guide to a boiler leaking water helps if you find water around the casing. A boiler that has stopped is handled through boiler breakdown.

Ring our Sevenoaks team on 01732 444555, Monday to Friday 7am to 10pm and Saturday to Sunday 9am to 8pm, or use our contact page. We are Gas Safe registered, and our minimum charge is £90 + VAT, including the first hour on site.