What should I do if my condensate pipe freezes?

If your boiler has stopped working on a frosty morning, a frozen condensate pipe is one of the most likely reasons. When ice blocks this small plastic pipe, waste water backs up and the boiler shuts itself down to stay safe. In most cases you can thaw it yourself in a few minutes with warm water.

This guide explains what the condensate pipe does, how to tell if it has frozen, how to thaw it safely and how to stop it happening again. Everything here is done from outside the boiler. You never need to take the boiler casing off, and you should never try to.

What the condensate pipe does

Almost every gas boiler fitted in the UK since 2005 is a condensing boiler. It recovers extra heat from its exhaust gases, and in doing so it turns water vapour into liquid, called condensate. This water is slightly acidic, so it is piped away to a drain rather than left to collect.

The condensate pipe is usually white or grey plastic, about the width of a thumb or a little wider. It comes out of the bottom of the boiler and runs either to an inside waste pipe, such as the kitchen sink waste, or through the wall to an outside drain. It is the outside stretch that freezes. Condensate trickles out slowly, so in a cold spell it can freeze, often at a bend or the open end over the drain.

Signs your condensate pipe has frozen

It usually happens first thing in the morning after a hard frost. Look out for:

• the boiler stopping, or refusing to light, when it is freezing outside

• a gurgling or bubbling noise from the boiler or the pipe

• a fault code or flashing light on the boiler display (Worcester Bosch boilers, for example, can show EA or D5)

• an outside section of plastic pipe that feels solid and icy to the touch

Fault codes vary from one make to another, so check your boiler's user manual for what yours means. If the boiler stopped in mild weather, or your condensate pipe runs entirely inside the house, a frozen pipe is unlikely to be the cause.

How to find the frozen section

Follow the plastic pipe from the bottom of the boiler to where it leaves the house, then outside to the drain. Run your hand along the outside section. The coldest part, or where you can feel ice, is usually where the blockage is, often at a bend or the very end.

Only check the parts you can reach safely from the ground. If the pipe runs high up the wall, don't climb a ladder in icy conditions.

How to thaw a frozen condensate pipe safely

There are two simple ways to thaw it.

1. Warm water. Fill a watering can or jug with warm water, either from the hot tap or from a kettle that has been left to cool for 10 to 15 minutes. Pour it slowly along the frozen section, starting near the end of the pipe and working back towards the wall. You may need a few refills.

2. A hot water bottle or heat pack. Hold a warm hot water bottle or microwavable heat pack against the frozen section. This is slower but avoids soaking anything nearby.

A few things to avoid:

• Never use boiling water. It can crack or split the plastic pipe, leaving you with a leak as well as a cold house.

• Never use a heat gun, blowtorch or any naked flame on the pipe.

• Keep water away from outside sockets, lights and cables.

• Watch your footing. The water you pour will freeze on the ground, so put down salt or grit afterwards.

Restarting your boiler after the pipe thaws

Once the ice has cleared, water should start to run freely out of the end of the pipe. Now reset the boiler. Most boilers have a reset button or a reset option on the display, and your user manual will show you which. The boiler should then fire up as normal.

It's also worth glancing at the pressure gauge. If the needle is below about 1 bar, our guide on how to top up boiler pressure safely explains what to do.

When to stop and call a Gas Safe engineer

Thawing the outside pipe is a safe job for a homeowner. Anything inside the boiler is not. Call a Gas Safe registered engineer if:

• the boiler still won't restart after the pipe has been thawed and reset

• it locks out again within a day or two, even though the weather hasn't changed

• the pipe has cracked, split or come apart at a joint

• you can't find or reach the frozen section

• water is leaking from the boiler itself

A boiler that keeps locking out may have a blocked condensate trap, a part inside the boiler that collects the water before it leaves, or a different fault altogether. Our boiler repair service covers these. At Home Heat the same engineer comes every time, so if we've worked on your boiler before, we already know how its condensate pipe is run and what was done last time.

How to stop it freezing again

If your condensate pipe freezes most winters, the way it was fitted is usually the real problem. An engineer can make it far less likely by:

• re-routing the pipe to an inside waste pipe, so little or none of it is outside

• keeping any outside run as short as possible, with a good fall and few bends

• fitting a wider pipe outside (Worcester Bosch suggests 32mm)

• lagging the outside section with waterproof, weather-resistant insulation

You can help yourself by fitting foam pipe lagging to an exposed run, and by leaving the heating on low overnight in a cold snap rather than off. Worcester Bosch also suggests turning up the boiler's heating temperature during freezing weather.

An annual boiler service is also a good time to have the condensate trap cleaned and the pipe checked before winter sets in. Our winter heating maintenance guide has more cold-weather checks.

If your boiler won't restart after a freeze, or its condensate pipe keeps freezing every winter, Home Heat can help. Book a boiler repair and we'll find the cause, put it right and, where it makes sense, re-route or upgrade the pipe before the next cold spell.