Frozen condensate pipe: how to thaw it safely

Gas boilers fitted in England in recent years are condensing boilers. It gets extra heat from its exhaust gases, and in doing so it makes water, called condensate. The condensate drains away through a plastic pipe, usually white or grey, about the size of a garden hose. When that pipe runs outside in a cold snap, the water in it can freeze. Ice blocks the pipe, water backs up into the boiler, and the boiler shuts itself off.

Signs your condensate pipe is frozen

• It has been below freezing, often overnight.
• The boiler has stopped and shows a fault code or a flashing light.
• You may hear gurgling from the boiler or the pipe.
• The outside end of the pipe has ice at it.

London is milder than much of the country, but it still gets frosts and cold spells. Exposed pipes on the backs of houses freeze first.

Why Ilford homes are prone to it

Much of Ilford is long, straight roads of late Victorian and Edwardian terraces. Many were built on the estates Archibald Cameron Corbett laid out in the 1890s, including Grange, Downshall and Mayfield, according to the Corbett Society. Over the years many have had rear kitchen extensions. A boiler moved into a new kitchen at the back often sends its condensate down the outside wall of the extension to a drain or gully.

That outside run is the weak point. It is exposed to wind, it is often in shade at the back of the house, and it can be long. Our Ilford area page has more on local homes.

How to thaw it safely

Only do this if you can reach the pipe safely from the ground. Do not climb ladders in icy weather.

1. Find the pipe. Follow it from the bottom of the boiler to where it goes through the wall, then outside to the drain. The frozen part is usually at the lowest point, a bend, or the open end.
2. Use warm water. Fill a kettle and let it cool until it is warm, not boiling, or use hot tap water. Pour it slowly along the outside section. Worcester Bosch is clear that it should be warm water, not boiling.
3. Or use a hot water bottle. Hold it on the frozen section or wrap it around for a few minutes.
4. Reset the boiler. Once the ice has cleared, reset the boiler as your manual explains.
5. Watch it. If the boiler runs, check it again the next cold morning.

Do not use a blowtorch, heat gun or boiling water. Do not try to remove the pipe or open the boiler casing.

If the boiler does not restart after the pipe is clear, or it locks out again straight away, the problem may be something else. Book a boiler repair.

Before you thaw: other checks

A cold snap can cause other faults at the same time. Before you go outside, check:

Check	What to look for
Pressure gauge	Within the range in your manual
Gas supply	Hob works, if you have one; credit on a prepayment meter
Power	Boiler display lit, fused spur on
Fault code	Matches a condensate or blockage code in the manual

If there is no fault code in the manual for condensate, our guide on no hot water from a combi boiler covers other causes.

Stopping it freezing again

A condensate pipe that freezes once in a bad winter is not unusual. One that freezes every year needs changing. The usual fixes are:

• Reroute it inside to an internal waste pipe, such as the kitchen sink trap, so there is no outside section.
• Use a larger pipe outside. Manufacturer installation guidance often asks for a wider pipe on any external run. A wider pipe takes longer to freeze solid.
• Shorten the outside run and make it fall steeply to the drain so water does not sit in it.
• Insulate it with weatherproof lagging.

These changes should be done by a Gas Safe registered engineer, ideally at a service or when a new boiler goes in. If you are planning a new boiler installation, ask how the condensate will be run.

Landlords and tenants

If you rent, tell your landlord or agent as soon as the boiler stops. Thawing the pipe as above is a safe user task, but rerouting it is not. HSE says landlords are responsible for repairs from general wear and tear on the gas appliances they provide.

Ilford water and your boiler

Ilford is supplied by Essex & Suffolk Water rather than Thames Water. Its reports also describe the water in its zones as hard; for example its Billericay & Brentwood report shows an average of 249 mg/l. Ilford households can check their own postcode on the Essex & Suffolk Water hard water page. Hard water does not cause freezing, but it is another reason to keep up an annual service.