How do I thaw a frozen boiler condensate pipe?

If your boiler has stopped on a freezing morning and you can hear it gurgling, or it is showing a fault code, the most likely cause is a frozen condensate pipe. In most cases you can sort it yourself by warming the frozen part of the pipe outside with warm, not boiling, water and then resetting the boiler.

This guide covers what the pipe does, how to tell it has frozen, how to thaw it safely and how to stop it happening again. Only work on the pipe where you can reach it from the ground, and never remove the boiler's casing or touch anything inside it.

What the condensate pipe does

Almost all modern gas boilers are condensing boilers. They take extra heat from the hot waste gases before they leave the flue, and as those gases cool, some of the water vapour in them turns back into water. This water is called condensate. It is mildly acidic, so it has to drain away.

It leaves the boiler through a plastic pipe, usually white or grey, which in many homes runs out through the wall to a drain outside. When it stays below freezing, the water in the outside section can turn to ice and block it. With nowhere to drain, the condensate backs up and the boiler shuts itself down. That is a built-in safety feature, not a breakdown.

Signs your condensate pipe has frozen

A frozen condensate pipe is the likely cause if several of these are true:

• it has been below freezing overnight or is still freezing outside
• the boiler has stopped and will not stay on, even though the gas and electricity are on
• you can hear gurgling from the boiler or the pipework near it
• the boiler is showing a fault code or flashing light, such as E133 or E28 on many Baxi boilers, or EA or D5 on Worcester Bosch boilers (your manual lists the code for your model)
• there is frost or ice on the outside pipe, often at its open end

If it is mild outside, something else has made the boiler stop, and it needs an engineer rather than repeated resets.

Find the frozen part of the pipe

Follow the pipe outside from where it comes through the wall to where it ends. It usually freezes at the most exposed point: the open end, a bend, or a dip where water collects.

If the pipe runs high up an outside wall, or you would need a ladder to reach it, stop here. Climbing a ladder on icy ground is not worth the risk.

How to thaw it safely

If you can reach the pipe from the ground, thawing it takes a few minutes:

1. Boil a kettle, then let it cool for 10 to 15 minutes so the water is warm rather than boiling.
2. Pour it slowly over the frozen part of the pipe, starting at the top and working down. You may need to refill the kettle a few times.
3. Or hold a hot water bottle, a microwavable heat pack or a cloth soaked in warm water against the frozen section.

Once the pipe is clear, reset the boiler. The steps vary by make, so check your manual or our guide on how to reset a boiler.

Never use boiling water. It can damage the plastic pipe and its joints, and it could scald you. Water poured on the ground can also freeze into a slippery patch, so mop it up or put down some salt afterwards.

When to call a Gas Safe engineer

Call an engineer on the Gas Safe Register, the official list of engineers allowed to work on gas appliances, if:

• the boiler will not restart after the pipe has thawed, or shows a different fault code
• you cannot reach the frozen part of the pipe from the ground
• the pipe freezes every time there is a cold spell

Do not try to clear the pipe from inside the boiler. Inside the casing is a condensate trap, a small U-bend that holds water to stop flue gases escaping into your home, and only a qualified engineer should touch it.

When we are called out to a boiler that has stopped in the cold, our Gas Safe registered engineers find the cause first, tell you what the work will cost before it goes ahead, and check the boiler is running safely before we leave.

How to stop it freezing again

If your condensate pipe freezes more than once, the way it was fitted is usually the problem. Guidance from the Heating and Hotwater Industry Council (HHIC) and the boiler makers points to a few fixes:

• Run the pipe inside where possible. Connecting it to an indoor waste pipe, such as the kitchen sink waste, keeps it out of the cold altogether.
• Use a wider pipe outside. Manufacturers often recommend at least 32mm for any outside run, as a wider pipe takes longer to freeze solid.
• Keep the outside run short and steep. A short pipe with a good fall and few bends drains quickly, so less water sits in it.
• Insulate it properly. Any outside section needs at least 13mm of waterproof insulation that stands up to sunlight, not ordinary indoor foam.
• Consider a condensate pump or trace heating. Where the pipe cannot run inside, a small pump or a heating cable that warms the pipe in very cold weather can help.

These changes should be made by a Gas Safe engineer, following the boiler maker's instructions and BS 6798, the British Standard for fitting gas boilers. That also helps keep your boiler's warranty valid.

During a very cold spell, HHIC also suggests turning up the heating temperature dial on the boiler for a few days. A hotter boiler makes less condensate, so there is less water to freeze. Turn it back down once the cold has passed, as it uses slightly more gas.

Other pipes are at risk in a freeze too. Our guide on what to do if a water pipe bursts covers the rest of the house.

If your boiler has stopped in the cold and the pipe is out of reach, or it will not restart once thawed, RDS Plumbing Ltd can help. Book a visit through our boiler repair service and one of our Gas Safe registered engineers will find the cause, get your heating back on and, if the pipe keeps freezing, explain how it could be rerouted or protected.