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10 November 2020

How to thaw a frozen boiler condensate pipe

Watch our video to find out how to safely thaw your boiler's condensate pipe.
Boiler pipe 474094
Matthew Knight

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

Your boiler's condensate pipe will run directly into a drain, in most cases externally to your property. 

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

In cold weather, one of the most common reported faults that will stop a boiler working is a frozen condensate pipe. 

The boiler shutting down is its way of keeping you safe, if not warm. 

Quick guide to defrosting 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. 
  3. Starting from the top and working your way down, pour the water over the pipe until the ice within has melted. 
  4. Reset your boiler and it should work as normal.

How to stop your condensing pipe freezing again

Prevention is always better than cure. Follow our tips to stop your pipes from freezing in the first place.

A large 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. However, this may not prevent freezing in extreme conditions. A heating engineer will be able to fit a fatter pipe, should that be necessary. 


Consider insulating the pipe to protect it, especially if it's high up and therefore difficult to unfreeze using hot water. You can insulate a pipe yourself, but if it’s tricky to access you should call in a professional. You can find a plumber, heating engineer or handyman in your area with Which? Trusted Traders.

Reduce the amount of outside pipe

If possible, the condensate pipe should run internally as far as is 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. 


The waste should fall as directly and steeply as possible so that gravity can help it along. The fewer bends in the pipe, the better. A lot of waste pipes are not installed according to the manufacturer’s instructions, with some running almost horizontally. If your pipe isn't at a steep angle, the water spends longer in the pipe and is more likely to freeze. Again, if you need to change the angle of the condensate pipe, you will need to pay for a heating engineer.

Siphon trap

If you're buying a new boiler, then consider one with a 'siphon trap' type of condensate (water) release – it's only available on certain models. This releases the water in one amount, reducing the risk of freezing.

Most boilers release the water as a continual drip, which makes freezing more likely.


If you live in an area where temperatures regularly fall below freezing for several days at a time, ask your installer about 'trace'. This is an electrical element that is bonded underneath the pipe and warms it when the temperature outside goes down to +5°C. It needs lagging to insulate it on the pipe.

Adding a trace to a typical 5-metre-long waste pipe will use power equivalent to a 50W light bulb. It's designed to protect pipes down to -20°C, and is probably the only guaranteed way to prevent freezing, although it goes against the principle of installing an energy-efficient boiler. If your pipe freezes regularly, a trace could be the solution. Only a Gas-Safe registered engineer can install one.

Reduce exposure

Think about the pipe's placement and the weather. Is the condensate waste pipe positioned on a wall that will get any sun? Is it particularly exposed to the weather?

If the wall gets sun and the pipe is black, it can often absorb enough heat to prevent freezing. If not, consider fitting a trace and insulating the pipe.

Try calling a Which? Trusted Trader who can help you prevent this problem from happening again. 

Installing a thicker pipe and attaching insulation to it should help the pipe remain ice-free, even in the coldest of winters. 

Reliable boilers and boiler repairs

If your boiler is consistently developing costly faults, then 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.

Every year we survey more than 10,000 boiler owners and almost 200 Which? Trusted traders. We use this data to tell you everything from which brands' systems will last the longest, to the ones that engineers think have the best build quality and are most likely to recommend. 

If you just want to know which are the best brands, head straight to our best boiler brands.

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