Boilers in winter
Frozen condensate pipe in a tenement flat
A frozen condensate pipe can stop a condensing boiler, and the maker-approved fix is warm, never boiling, water on the plastic outside pipe, but only where you can reach it at ground level.
By the Pipes Up the Stair site team · how pages are checked · Updated
Modern condensing boilers drain a little acidic water through a thin plastic pipe, and on a hard Edinburgh night the outdoor stretch can freeze solid. The boiler then locks out, usually with a fault code, just when you need it most.
In a ground-floor or garden flat you may be able to clear it yourself. In most tenement flats the pipe leaves the wall high above the street or back green, and then it is a job for a Gas Safe registered engineer.
How do you know the condensate pipe has frozen?
The boiler usually tells you. Vaillant says its boilers show an F28 or F29 code, and Baxi lists E133 or E28; other makes use other codes, so check your manual. Vaillant also mentions gurgling or bubbling from the boiler or the pipe.
How cold does Edinburgh get?
Cold enough for this to happen most winters. At Edinburgh Gogarbank, Met Office averages for 1991 to 2020 record 48.01 air frost days annually; December alone averages 10.63. The Royal Botanic Garden station, nearer Stockbridge and the New Town, averages 42.68.
Can you thaw the pipe yourself?
Only if the plastic pipe is outside at ground level and you can stand safely beside it. Both makers say to use warm water, poured along the frozen part from a jug or watering can, and Vaillant warns that boiling water from a kettle can crack the pipe. Baxi says the same and suggests repeating until the ice clears. Then follow the manual to reset the boiler.
- 1Check the code in your boiler manual
- 2Find the plastic pipe at ground level
- 3Pour warm, not boiling, water along it
- 4Reset the boiler as the manual says
- 5Pipe high up? Call a Gas Safe engineer
What if the pipe is high up the tenement wall?
Leave it alone. Don't lean out of a window, stand on a sill or climb a ladder to reach a condensate pipe on a stone tenement: the drop to the back green is a far bigger danger than a cold flat. Call a Gas Safe registered engineer instead. Tenants should report it to whoever manages the flat, so they can send someone out.
What should you never do?
Never open the boiler casing or touch its flue, gas pipe or burner. Never use a kettle straight off the boil. Never take a heat gun or flame to any pipe. And never confuse this with a frozen water supply pipe, which Scottish Water says to thaw only with a heated cloth; our page on frozen pipes in stone tenements covers that.
Why does it keep freezing?
Usually because of how the pipe was run. Baxi says a Gas Safe registered engineer should carry out any remedial work, such as moving the pipe inside, fitting a wider pipe, lagging it or adding trace heating. Vaillant says to call one if the pipe continues to freeze despite precautions.
Does insulating the pipe help?
Yes, where the pipe can be reached safely. Home Energy Scotland suggests foam pipe insulation on the plastic condensate pipe to keep the water from freezing. On a pipe high on the outside wall, leave the lagging to an engineer as part of the remedial work. If your heating has no frost protection, it also suggests setting the thermostat to 10°C with the programmer on.
Asked on the stair
Is the plastic pipe outside the same as a water pipe?
No. The condensate pipe carries waste water from the boiler to a drain, while your supply pipe brings mains water in. They freeze for the same reason but are thawed differently.
Can a hairdryer go near a frozen condensate pipe?
This site only gives the warm-water method both makers agree on, and only for a pipe at ground level. If that is not possible, call an engineer.
Will the boiler restart on its own once the ice melts?
Not always. Check your manual for the reset steps, and if it locks out again, book a Gas Safe engineer.
A neighbour's pipe runs past your window. Can you clear it?
No. Don't lean out, and don't work on another flat's boiler pipework. Let your neighbour know so they can call their own engineer.
Where this comes from
- Vaillant: Your condensate pipe — codes, warm water, no boiling water, call an engineer
- Baxi: Preventing a frozen condensate pipe — codes, remedial work by an engineer
- Home Energy Scotland: Protect pipes in winter — foam insulation on the condensate pipe
- Met Office: Edinburgh Gogarbank averages — frost days
- Met Office: Royal Botanic Garden averages — frost days