In short
Find the plastic pipe that runs from the boiler to a drain, usually outside. Pour warm, not boiling, water along it, or hold a hot water bottle on it, starting at the open end and bends. When it is clear, reset the boiler once. If it keeps freezing, ask an engineer to reroute, upsize or insulate it.
What is a condensate pipe?
Modern condensing boilers recover extra heat from the flue gases, which produces mildly acidic water called condensate. A plastic pipe, usually white or grey and around 20 to 32 mm across, carries it away to a drain. If part of it runs outside and freezes, the water backs up and the boiler shuts down to protect itself.
How do I know it is frozen?
- The boiler stopped during freezing weather, often overnight or first thing in the morning.
- It shows a fault code or flashing light, and may gurgle as it tries to light.
- The condensate pipe outside has ice at the open end or at a bend.
How to thaw it, step by step
- Switch the boiler off or leave it locked out while you work.
- Find the pipe where it leaves the boiler and follow it outside to the drain. It usually freezes at the open end, a bend or a low point.
- Pour warm water from a kettle that has been left to cool for a while, or a watering can of warm water, slowly along the pipe. Do not use boiling water: it can crack the pipe and scald you. Watch for ice underfoot.
- Or hold a hot water bottle or a warm cloth on the frozen section.
- When water runs freely from the end, reset the boiler following your user guide. It may take a few minutes to restart.
Never cut the pipe, and never open the boiler case.
How do I stop it freezing again?
Industry guidance from HHIC says the condensate should go to an internal drain wherever possible. Where it has to run outside, it says the pipe should be at least 30 mm internal diameter (typically 32 mm outside), slope down through the wall, be as short as possible to a maximum of 3 m outside, take the most direct and vertical route with no horizontal sections, and be insulated. Worcester Bosch's leaflet gives similar advice. If your pipe is thin, long or runs flat along a wall, it is worth having it upgraded before next winter.
What CombiCert would check
If the pipe keeps freezing, we look at its diameter, length, fall, insulation and whether it can be run to an internal drain. HHIC guidance favours an internal termination where possible, with outdoor runs kept short, steep and well insulated. We can thaw an emergency freeze and then quote a proper fix.
When to call a Gas Safe engineer
Call us if the boiler will not restart after the pipe is clear, if it freezes repeatedly, or if you cannot reach the pipe safely. Rerouting, upsizing or insulating the condensate is a job for an engineer, because it has to follow the manufacturer's instructions. If the boiler shows a different code, look it up in our fault code search, or book a boiler repair.
Need a hand? CombiCert in Telford
CombiCert Ltd is a Gas Safe registered (621146) gas and heating company based at The Old School House, Park Lane, Telford TF3 4TG, working across Telford and Shropshire for homes and businesses. We are open Monday to Saturday, 08:00 to 19:45 (closed Sunday).
Boiler repairs and services from £90 + VAT. Commercial work by quote.
See Boiler repairs in Telford, or call or WhatsApp 07393 220694, or email hello@combicert.co.uk.
We cover Telford and nearby, including Wellington, Madeley, Dawley, Oakengates, Shifnal, Newport, Ironbridge and Shrewsbury.
Sources
Frequently asked questions
Can I pour boiling water on a frozen condensate pipe?
No. Boiling water can crack the plastic pipe and fittings, and can scald you. Use warm water from a kettle left to cool for a while, or a hot water bottle held on the frozen section. Work from the open end and any bends, and watch out for the water freezing on the ground.
Why does my condensate pipe keep freezing?
Usually because it runs outside, is too narrow, too long or too flat, or is not insulated. Industry guidance from HHIC says it should go to an internal drain where possible, and where it runs outside it should be at least 32 mm, short, steep and insulated. An engineer can reroute or upgrade it.
My condensate pipe is thawed but the boiler will not restart. What now?
Reset the boiler once, following your user guide, and give it a few minutes. If it still will not run, note any fault code and check the pressure. The pipe may still be blocked further along, or there may be a different fault. Book a Gas Safe engineer rather than resetting repeatedly.
Does every boiler have a condensate pipe?
Every modern condensing boiler has one, and almost all gas boilers fitted in the UK in recent years are condensing. Older non-condensing boilers do not. If yours has a plastic pipe running from the bottom of the boiler to a drain, it is a condensing boiler and the pipe can freeze in cold weather.
This answer is general information, not a substitute for an on-site assessment by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.