How to thaw a frozen boiler condensate pipe

Has your boiler has stopped working during a cold snap? Before you go to the expense of calling out a heating engineer, check whether its pipe has frozen.

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

What is a condensate pipe?

Modern boilers will have a a condensate pipe; this is a pipe that runs from your boiler to your household drainage, to take away the waste water from the boiler.

Where is the condensate pipe?

While it's preferable to have this pipe internally, to stop it freezing, this pipe can be located outside, so sub-zero temperatures can cause the water in this pipe to freeze.  

If this pipe gets blocked with ice, or anything else, then your boiler will automatically shut down as a safety measure. It's the boiler's way of keeping you safe, if not warm.

If it's freezing outside, and your boiler has suddenly stopped working, it's worth checking to see if this is what's happened to your boiler; If so, here's what to do.

Keep your energy bills in check this winter: here are our top tips to reduce your heating bills.

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. Use warm water, not boiling water.  It is important that you do not use boiling water. Boil a kettle and leave it to cool for 10-15 minutes so that it's warm rather than boiling. You could also use a hot water bottle or microwavable heat pack.
3. Starting from the top of the pipe, 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.

How to balance your radiators – heat your home evenly and shave money off your bills at the same time

How to stop your condensing pipe from freezing

Prevention is always better than a cure. 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, as it's less likely to freeze.

If your condensate pipe is known to freeze over, you may want to think about moving it. This isn't a DIY job so you won't be able to change yours yourself; you'll need to pay for a heating engineer to complete the work. But 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 instead. 

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 building standards regulations (BS 6798:2014).

Which? Trusted Traders – find a plumber or heating engineer in your area who's been vetted by our rigourous assessment team

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. 

Find out more: Common boiler repairs – how much will a repair cost, and can you fix it yourself?

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 or lower. 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.

Get the best boiler service – expert advice on how to get the most from your boiler's annual service

Reliable boilers and boiler repairs

If your boiler is consistently developing costly faults, read our five signs you need a new boiler guide, as it could be time for a new one. See our reviews of the best gas and oil boiler brands before you buy, or read our expert advice on how to buy the best boiler.