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How to prevent your condensing boiler from freezing and what to do if it does

Keep your boiler working even in the coldest weather with these tips from Which? Trusted trader, BeeXpress Plumbing and Heating.

In this article

Condensing boilers

If your boiler was installed in the last 10 years, it will be a condensing boiler following a change in the law. These boilers are generally considered more environmentally sound and efficient than older styles of boiler.

These boilers typically operate at a lower temperature than non-condensing boilers, which makes them more energy-efficient. However, this can cause problems when temperatures drop below freezing – just when you need them most.

In some cases, the condensate water is unable to run away and fills internal parts of the boiler, which can cause damage to components such as fans and burners. Find out more about condensing boilers in this Which? guide.

What should you do if your condensate boiler pipe freezes?

If your boiler stops working in freezing weather, check your condensate waste pipe. Is the white pipe running from your boiler located outside without any insulation? Does it run almost horizontally? Is it quite long or on a north-facing wall?

If so, you can try to unfreeze that pipe, and hopefully get your boiler up and running again. BeeXpress Plumbing and Heating's Ben Margulies gave us the following tips:

  • Heat water in a kettle – then wait until it has cooled slightly. DO NOT use boiling water as this could crack the pipe.
  • Pour the hot water over the length of exposed pipe to unfreeze any blockage and allow the water to run freely away

Watch BeeXpress’s YouTube video showing you how to unfreeze a condensate pipe.

  • Press the reset button on your boiler and it should fire up as normal.

If unfreezing the pipe doesn’t resolve the issue for you. You will need to call in a heating-engineer. Which? Trusted Traders had endorsed heating specialists in your local area. Use our online search to find heating engineers local to you and read the reviews from previous customers to help find the right business for you.

How to prevent condensing boiler pipes from freezing

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

1. Reduce the amount of outside pipe

If possible, run the condensate waste internally. (In many cases this is impossible.)

The shorter the amount of pipe outside, the better.

2. Siphon trap

Try to get a boiler with a Siphon trap type of condensate (water) release – this is only available on certain models. This releases the water in one amount, reducing the risk of freezing.

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

3. 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. Although this may not prevent freezing in extreme conditions.

4. Insulation

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, however if it’s tricky to access, call in a professional. You can find a plumber, heating-engineer or a handyman in your area with Which? Trusted Traders.

5. Gravity

Make the waste fall as directly and steeply as possible to the exit -so that gravity can help it along. The fewer the number of bends the better. BeeXpress’s Ben Margulies told us: ‘A lot of waste pipes are not installed according to the manufacturer’s instructions – I’ve seen them close to horizontal. That means the water is more likely to collect and gives it more chance to freeze’.

6. Trace

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 the pipe when the temperature outside reaches +5 degrees Celsius. 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. They are designed to protect pipes down to -20 degrees C. This is probably the only guaranteed way to prevent freezing, although it goes against the principle of installing an energy efficient boiler – but if your pipe freezes regularly this could be the solution. A trace can only be installed by a Gas-Safe registered engineer.

7. Reduce exposure

Think about the direction and weather. Is the wall the condensate waste is to run down facing north? Is it particularly exposed to the weather? 

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

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