Frozen Condensate Pipe: How to Thaw It and Get Your Boiler Back On

If your boiler has stopped during a cold snap, check the condensate pipe before anything else. It's the plastic pipe that takes waste water from the boiler to a drain, and where it runs outside it can freeze. When that happens the water can't get away, so the boiler shuts itself down and you lose your heating and hot water.

You can usually sort this yourself if the frozen part is within easy reach from the ground. It involves the outside pipe only, with no need to touch the boiler's casing or anything inside it.

How to tell if the pipe is frozen

The usual signs are a boiler that has stopped working in freezing weather, a fault code on the display, and sometimes a bubbling or gurgling noise from the boiler or the pipe.

Fault codes vary by make. Vaillant and Glow-worm use F28 or F29. Baxi says most of its boilers show E133 or E28. On models such as the Exclusive 2, Ideal says a blocked condensate pipe makes the boiler gurgle and lock out on L2, which our Ideal L2 guide covers. Check your manual for any other make, because the same code can mean something else. Vaillant says F28 can also point to an ignition or gas supply problem, so if the weather isn't freezing, the pipe is unlikely to be the cause.

It's also worth checking that your other gas appliances are working, and that a prepayment meter still has credit.

How to thaw a frozen condensate pipe

Ice usually forms in the most exposed part of the pipe, which is often the open end near the drain or a bend where water collects. Before you start, make sure you can reach it from the ground. If it runs high up a wall, as it can on a tenement or an upper flat, don't climb up to it. Call an engineer instead.

1. Find the frozen section outside. It may feel solid or look frosted over.
2. Warm it with a hot water bottle, a microwaveable heat pack or a cloth soaked in warm water, or pour warm water along it from a watering can or jug. Keep going until the ice clears or the pipe drains.
3. Reset the boiler using the steps in your manual, then restart it.

Never use boiling water straight from the kettle, as Vaillant warns it can crack the pipe. Take care on the ground too. Baxi points out that the water you pour can refreeze and leave it slippery.

If the boiler still won't start

Try the warming again if the pipe may still be blocked. If the pipe is clear and the boiler still won't run after a reset, call a Gas Safe registered engineer. Baxi gives the same advice. Don't take the casing off or try to repair anything inside the boiler.

If you smell gas, don't wait to speak to an engineer. Call the National Gas Emergency Service on 0800 111 999.

How to stop it freezing again

Insulation is the first thing to try. Vaillant and Glow-worm both recommend fitting pipe lagging to the outside section, and Baxi specifies lagging that is waterproof and weatherproof. Make sure the pipe has thawed and water is running freely before you fit it.

Vaillant also advises keeping the boiler running on a timer through freezing weather, especially when you're away, because a pipe is more likely to freeze when the boiler isn't being used.

If it keeps freezing, the way the pipe is run may be the problem. A Worcester Bosch technical bulletin says running the pipe inside the building virtually eliminates the chance of freezing, and Worcester Bosch's current guidance limits outside runs on its gas and oil boilers to 3 metres, with insulation required on most. Rerouting or upsizing the pipe is a job for an engineer, so ask at your next boiler service or call us about it.

Need a hand?

We repair boilers across Edinburgh and the Lothians. If you can't reach the pipe, or the boiler still won't start, call us on 0131 644 0045 with the make, model and any fault code, and we'll talk through cost and when we can come out. Our boiler repairs page has the other checks to make before you call, and if the gauge is reading low, see our guide to a boiler losing pressure.