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Home & garden.

14 September 2021

How to thaw a frozen boiler condensate pipe

Watch our video to find out how to safely thaw your boiler's condensate pipe.
Sam Morris
condensate pipe with ice on it

If your boiler has stopped working in a cold snap, check whether its condensate pipe has frozen before you call out a heating engineer.

When cold weather sets in, one of the most commonly reported faults is that a boiler's condensate pipe has frozen.

This fault will cause your boiler to shut down – but it's the boiler's way of keeping you safe, if not warm. 

If this pipe gets blocked with ice, or anything else, then your boiler will automatically shut down as a safety measure.

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How to defrost your condensate pipe

  1. Locate your condensate pipe. This will be a white pipe that comes out of the wall behind your boiler and runs directly into an outside drain. 
  2. Boil a kettle and leave it to cool for 10-15 minutes so that it's warm rather than boiling. It is important that you do not use boiling water. You could also use a hot water bottle or microwavable heat pack.
  3. Starting from the top and working your way down, apply the water (or wrap the hot water bottle/heat pack) over the pipe until the ice within has melted. 
  4. Reset your boiler and it should work as normal.

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How to stop your condensing pipe from freezing

Prevention is always better than a cure, and with the UK weather becoming more 'extreme' due to the effects of global climate change, we may see prolonged periods of sub-zero temperatures.

Follow our tips to stop your pipes from freezing in the first place.

Reduce the amount of outside pipe

If possible, the condensate pipe should run internally as far as possible. The less pipe that sits outside, the better. 

You won't be able to change yours yourself; you'll need to pay for a heating engineer to complete the work. But if your condensate pipe is known to freeze over, it will be time and money well spent getting a solution put in place.

When you next get your boiler serviced, ask the engineer if any improvements can be made. They will be able to tell you if your condensate pipe can be run internally. 

Add insulation

If the pipe cannot be connected internally, the next thing to consider is insulating any sections of the condensate pipe that run externally. 

This is especially important if it's high up and therefore difficult to unfreeze using hot water. 

You can insulate a pipe yourself, but unless you absolutely know what you're doing you should call in a professional for an assessment.

This will ensure the correct material is used for the insulation (it should have a waterproof and UV-resistant coating) and be installed to the latest regulations according to BS 6798:2014.

Which? Trusted Traders – find a plumber, heating engineer or handyman in your area


Install a larger pipe

Make the condensate waste pipe as large as possible. Check your manufacturer’s instructions – some will recommend that it should be at least 32mm or up to 40mm. 

The internal diameter shouldn't be smaller than 30mm.

Get your heating engineer to inspect the condensate pipe and install a wider pipe, should that be necessary. 

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More tips to prevent a frozen condensate pipe

If you've already implemented the tips above but your external condensate pipe still freezes over, here are some more changes that could help. 

  • Use gravity – pipes should fall as directly and steeply as possible. The fewer bends the better. If your pipe isn't at a steep angle, the water spends longer in the pipe and is more likely to freeze. If you need to change the angle of the condensate pipe, you will need a heating engineer to do so. 
  • Have a siphon trap – if you're getting a new boiler, consider one with a 'siphon trap' type of condensate water release. It's only available on certain models. It releases the water in one amount, which reduces the risk of freezing.
  • Install a 'trace' system – if you live in an area where temperatures drop below freezing for several days at a time, ask your installer about trace systems. It involves an electrical element that's bonded to the pipe and warms it when the temperature outside drops to +5°C. It's designed to protect pipes down to -20°C, and is probably the only guaranteed way to prevent freezing. Only a Gas Safe-registered engineer can install it.
  • Reduce exposure – is your condensate pipe exposed to the weather? Is it on a wall that gets sun? If your condensate pipe is black, and on a wall that gets sun, it can often absorb enough heat to prevent freezing. If not, consider getting a heating engineer to insulate the pipe or install a trace system.

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Reliable boilers and boiler repairs

If your boiler is consistently developing costly faults, it could be time for a new one. 

At Which? the boilers we recommend on our Best Buy Boilers page are only from brands that have an excellent reliability rating, are loved by their customers and are well respected by Which? Trusted Trader heating engineers.

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