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How to prevent your boiler from freezing


Avoid a big freeze with these tips from recommended boiler engineer Alan Ferguson.

A boiler being serviced

Condensing boilers

All new central-heating boiler installations in Britain must be condensing boilers (with a few exceptions).

What's the problem?

Alan has found that, during cold winters, condensing boilers can freeze up and fail when we need them most.

This is due to the way condensing boilers use water. In some cases, the condensate water is unable to run away and fills internal parts of the boiler. This damages components such as fans and burners.

How to prevent condensing boiler pipes from freezing

1. Siphon trap

Try to get a boiler with a Siphon trap type of condensate (water) release. 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.

2. 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.

3. A large pipe

Make the condensate waste pipe as large as possible. It should be at least 32mm.

4. Insulation

Consider insulating the pipe to protect it, especially if it's high up and therefore difficult to unfreeze using hot water.

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.

6. Trace

What work can you do yourself?

You can insulate a pipe yourself.

However, fitting a trace or a new condensate waste pipe to a boiler must be done by a Gas Safe-registered engineer.

Read about the Gas Safe Register

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 Trace to a typical 5 metre long waste pipe takes the equivalent power as 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.

However, Alan makes the point that installing an energy efficient boiler only to then have to warm a pipe outside to stop it from freezing seems contradictory.

Alan says: "It doesn't seem very environmentally-friendly to me. However, sometimes, needs must."

7. Shorten the pipe

Make the condensate waste pipe as short as possible.

8. 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.


About our expert trader

Boiler engineer Alan Fergusson

Our expert: Alan Ferguson

Boiler engineer Alan Ferguson's business, Alan's Gas Services, was the regional winner for the Which? Local Business of the Year (North West) in 2011.

Reviewers have praised Alan's Gas Services for its excellent service and reasonable price.

View reviews for Alan's Gas Services in Altrincham