Frozen condensate pipe: how to get your boiler going again

What the condensate pipe does

Every condensing boiler produces condensed water as it recovers heat from the flue gases. That water drains away through a small plastic pipe — the condensate pipe — which usually runs to a waste pipe or an outside drain. Because the water inside is only a trickle, a hard frost can freeze it solid, and the boiler shuts itself down to stop the water backing up.

How to tell it's the condensate pipe

The clues line up like this: it's freezing outside, the boiler was working fine before the cold snap, and now it's locked out with a fault code (often relating to ignition or the flue) or making gurgling sounds before it stops.

Look outside for the plastic pipe leaving the wall near the boiler — usually white or grey, around 20–30mm across, running to a drain. If it's exposed and the weather is below freezing, you've almost certainly found the problem.

Thawing it safely

This is a safe job for most homeowners, done from outside:

• Find the frozen section — usually the exposed outdoor run or the end near the drain
• Pour warm (not boiling) water along the pipe, or hold a hot water bottle or warm cloth against it
• Work along the pipe until you hear or feel the ice shift and water drains away
• Reset the boiler following the instructions in your manual
• If the boiler fires and stays running, you're done

What not to do

Never use boiling water straight from the kettle — it can crack a frozen plastic pipe. Don't try to take the pipe apart while it's frozen, and don't be tempted to bypass or block the condensate pipe: the boiler needs it to drain. If the pipe is somewhere you can't reach safely, such as high up a wall, leave it and call an engineer.

Stopping it happening again

A condensate pipe that freezes once will freeze again in the next cold snap unless something changes. The lasting fixes are:

• Insulating the outdoor section with proper pipe lagging
• Increasing the pipe size for the outdoor run, which freezes more slowly
• Re-routing the pipe so less of it is exposed, or so it drains to an internal waste pipe
• In exposed spots, a trace-heating cable fitted to the pipe

When it's not the condensate pipe

If thawing the pipe and resetting the boiler doesn't get it going, or the pipe isn't frozen at all, the fault is something else — and anything inside the boiler case is Gas Safe engineer territory. Note the fault code before you ring; it helps narrow the problem down quickly.

Need it sorted? Book a boiler repair

If the boiler still won't run after thawing the pipe, or you'd like the pipe re-routed or insulated properly before next winter, see our boiler repairs page. If your heating is off in freezing weather and you need help urgently, our emergency boiler repair page explains what to do — or call 07429 632804 and Chris will tell you honestly whether and when he can attend.