If your boiler stops working during a cold snap and shows a fault code, a frozen condensate pipe is one of the most common causes. Condensing boilers drain a small amount of slightly acidic water through a plastic pipe, and where that pipe runs outside it can freeze and block. You can usually fix it yourself by pouring warm, not boiling, water over the frozen section, then resetting the boiler as your manual describes. Rerouting or upgrading the pipe so it does not freeze again is a job for a Gas Safe registered engineer.
What the condensate pipe does
Modern gas boilers are condensing boilers. They recover extra heat from the flue gases, and in doing so they produce condensate: water vapour from the flue gases that condenses back into liquid. The condensate is mildly acidic, so it is drained away through a plastic pipe, usually to an internal waste pipe or soil stack, or to an outside drain or soakaway.
If the condensate cannot drain, it backs up inside the boiler. The boiler detects this and shuts down to protect itself, which is why a frozen pipe stops your heating and hot water.
Signs of a frozen condensate pipe
- The boiler has stopped and shows a fault or lockout code during freezing weather.
- Gurgling or bubbling noises from the boiler.
- Ice visible on, or at the end of, the plastic pipe outside.
- The boiler restarts after a reset, then locks out again shortly afterwards.
Check your boiler manual: many manufacturers list the specific code that relates to a condensate blockage. Our guide to boiler GC numbers shows where to find your boiler's make and model if you have mislaid the manual.
Find the pipe
The condensate pipe is normally a plastic pipe, often white or grey, that leaves the bottom of the boiler. Follow it to where it goes through the wall. Outside, it usually runs down the wall to a drain, gully or soakaway. The most likely place to freeze is the lowest point, a bend, or the open end where water drips out.
How to thaw a frozen condensate pipe
- Stay safe. Only attempt this at ground level. Do not climb a ladder in icy conditions. If the pipe is high up or hard to reach, call an engineer.
- Use warm water, not boiling. Fill a watering can or jug with warm water from the tap or kettle mixed with cold. Boiling water can crack the plastic pipe and the fittings.
- Pour it slowly along the pipe, starting at the frozen end and working back towards the wall, until the ice clears and water runs freely. A hot water bottle or warm cloth held against the pipe also works.
- Reset the boiler exactly as your user manual describes. It may take a few minutes to restart.
- Watch it. If the boiler locks out again, the pipe may still be partly blocked or may have refrozen.
Mop up any water that pours onto paths, as it can freeze into an ice hazard. Do not hit the pipe to break up the ice, and do not try to cut or remove the pipe.
Stopping it freezing again
A pipe that freezes every winter needs a permanent fix. The installation standard for gas boilers, BS 6798, manufacturers' installation instructions and the heating industry's own condensate guidance (published by the Heating and Hotwater Industry Council) set out how condensate pipes should be run. Typical improvements an engineer may suggest include:
- An internal route to a soil stack or a kitchen, utility or bathroom waste, so the pipe does not run outside at all. Industry guidance treats this as the preferred method, with an external run only where an internal connection is impractical.
- A larger diameter pipe through the wall. Where the pipe must go outside, the guidance is to increase it to at least 30mm internal diameter (typically 32mm pipe) before it passes through the wall, sloping downwards.
- Waterproof, UV-resistant insulation on the external section, run right to the tip of the pipe.
- A short, direct external run of no more than 3 metres, as vertical as possible and with no horizontal sections where water can sit.
- A condensate pump where the boiler cannot drain to an internal point by gravity, for example in a basement or utility room below the drain level.
Because the condensate pipe is part of the boiler installation, alterations should be made by a Gas Safe registered engineer, following the boiler manufacturer's instructions. Engineers will find condensate pumps, waste pipe and fittings in above ground drainage, and fittings in pipe systems and fittings.
While you wait for a permanent fix
- During an extreme cold spell, industry guidance suggests temporarily turning the boiler's heating flow temperature up, because a hotter boiler produces less condensate. Radiators will run hotter and efficiency drops slightly, so turn it back to normal once the cold spell ends.
- Keep warm water and your boiler manual handy so you can thaw and reset quickly.
When to call a Gas Safe engineer
- The boiler still will not restart after the pipe has thawed and been reset.
- The pipe has cracked, come apart or is leaking.
- The pipe freezes repeatedly and needs rerouting, upsizing or a pump.
- You can smell gas, see scorch marks or have any concern about the boiler itself. In that case, call the gas emergency number straight away.
Find a registered engineer at the Gas Safe Register. If your boiler keeps losing pressure as well, our guide to boiler losing pressure explains the separate checks for that. Engineers sourcing parts can browse boiler spares and domestic boiler accessories.
Frequently asked questions
Can I use boiling water on a frozen condensate pipe?
No. Boiling water can crack the plastic pipe and its joints. Use warm water, or a hot water bottle held against the pipe.
How do I know if my boiler has a condensate pipe?
If your boiler is a condensing boiler, which most gas boilers fitted in recent years are, it will have a plastic condensate pipe leaving the bottom of the boiler.
Why does my condensate pipe keep freezing?
Usually because it runs outside, is too narrow where it leaves the building, is uninsulated, has a long or shallow run, or ends where cold air can reach the water. An engineer can reroute it internally or upgrade it to current guidance.
Is a frozen condensate pipe dangerous?
It is a nuisance rather than a danger: the boiler shuts down safely. But if you smell gas or suspect a fault with the boiler itself, call the gas emergency number.