Frozen boiler condensate pipe: why it happens and what you can safely do

On the coldest mornings of the year, a common reason for a modern gas boiler to stop working is not the boiler at all. It is a small plastic pipe outside the house that has frozen solid. This guide explains why that happens and what you can safely do on the outside pipe. Anything on or at the boiler is for a Gas Safe registered engineer.

What the condensate pipe is

Modern condensing boilers squeeze extra heat out of their exhaust gases. As those gases cool, water condenses out of them. The boiler collects this slightly acidic water, called condensate, and drains it away through a plastic pipe, usually white or grey.

The pipe normally runs to an internal drain such as a sink waste or soil stack, or through the wall to an outside drain, gully or soakaway. When it runs outside, the water in it can freeze in a cold snap, especially where the pipe is long, exposed to wind or runs with little fall.

When the pipe is blocked by ice, condensate backs up into the boiler. The boiler detects this and shuts down to protect itself. Many boilers show a fault code or flashing light when it happens.

How to tell it might be the condensate pipe

Signs that point to a frozen condensate pipe:

• it is, or has recently been, below freezing outside;
• the boiler has stopped and shows a fault code or warning light;
• you may hear gurgling or bubbling from the boiler when it tries to start;
• there is a plastic pipe from the boiler running outside, often to a drain at ground level.

Your boiler's user manual should list what its fault codes mean. Check that first, because other faults can look similar.

What you can safely do

These steps are only for the outside part of the plastic condensate pipe, where you can reach it from the ground. They follow the kind of advice boiler makers give householders; for example, Vaillant's homeowner page on frozen condensate pipes advises pouring warm water along the affected area of the pipe, and warns against boiling water from a kettle because it "can damage or crack the pipe".

1. Find where it is frozen. Follow the pipe from where it leaves the wall. The blockage is often at the lowest point, at a bend, or at the open end over the drain.
2. Use warm water, not boiling water. Fill a watering can or jug with warm water, comfortable to put your hand in, and pour it slowly along the frozen section. Keep pouring and repeating until the water runs through.
3. Or use gentle warmth. A hot water bottle or a warm cloth held against the pipe can help in an awkward spot.
4. Watch your footing. The water you pour will freeze on paths and steps. Grit or salt the area, and do not climb ladders in icy weather to reach a high pipe.
5. Restart the boiler as its manual says. Once the pipe is clear, follow the user instructions in the boiler's manual for restarting it. Do not open the boiler casing.

Do not use a naked flame, a blow torch or a heat gun on the pipe. Do not hit the pipe to break the ice, and do not pull it away from the wall.

Our general guide to frozen and burst pipes covers water pipes elsewhere in the house.

When to stop and call a Gas Safe registered engineer

Stop and call a Gas Safe registered engineer if:

• the boiler does not restart after the pipe has thawed, or shows another fault;
• you cannot find a frozen section, or the pipe is out of reach;
• the pipe is cracked, split, disconnected or leaking;
• water is leaking from the boiler itself;
• the pipe freezes again and again each winter;
• you smell gas, or the boiler shows any sign of damage. In that case, follow the gas emergency advice on our Gas Safe page and call the National Gas Emergency number straight away.

If the pipe keeps freezing, the engineer can look at whether the way it is run outside, its size, or its insulation should change. That is work on the boiler installation. We do not give instructions for changing the pipe's route or its connections, and you should not alter it yourself. The boiler maker's installation instructions and the engineer decide what is suitable.

Preventing it next time

Things you can do that do not involve the boiler:

• Ask at the next service. Ask the engineer whether your condensate pipe is at risk of freezing and whether anything should be done about it.
• Insulation on the outside run. Boiler makers' homeowner advice, including Vaillant's page above, mentions pipe insulation for the outside part of the pipe. Our pipe lagging guide says not to wrap insulation round a boiler or its condensate pipe without checking first, because the right material and size depend on the installation. Ask the engineer what suits your pipe, and do not insulate or wrap anything at the boiler.
• Keep the drain end clear. Leaves, ice and snow round the drain the pipe runs into can block the outlet.
• Know where it is. Find the pipe before winter, so you are not hunting for it on a freezing morning.

We do not link to any boiler parts or condensate kits.

Can I pour boiling water on a frozen condensate pipe?

No. Use warm water. Boiler makers warn that boiling water can damage or crack the plastic pipe.

Why has my boiler stopped working in cold weather?

A frozen condensate pipe is a common reason, but not the only one. Check the boiler's manual for the fault code. If thawing the outside pipe does not help, call a Gas Safe registered engineer.

Should I insulate my condensate pipe?

Ask your Gas Safe registered engineer. Insulation can help on an exposed outside run, but the right approach depends on how your boiler is installed, and nothing at the boiler should be altered by a householder.