Frozen Condensate Pipe? How to Thaw It Safely and Stop It Happening Again

It is the first properly cold morning of the winter, the house is freezing and the boiler is showing a fault code. You reset it, it tries to fire, gurgles and locks out again. Before you assume the worst, go outside and find the condensate pipe. In a cold snap, a frozen condensate pipe is one of the most common reasons a perfectly healthy boiler stops working.

It is also one of the few boiler faults you can usually fix yourself, safely, with nothing more than warm water. Here is how to spot it, how to thaw it, and how to stop it happening again next winter.

What is a condensate pipe?

Almost every gas boiler fitted in the last twenty years is a condensing boiler. Since 2005, new and replacement gas boilers in England have had to be condensing models under the Building Regulations, because they recover heat from the exhaust gases that older boilers simply threw away.

Recovering that heat produces water, known as condensate. It is mildly acidic, so it cannot just run down the wall. Instead it drains away through a plastic pipe, usually white or grey and about the width of a thumb, that runs from the bottom of the boiler to a drain, a gully or a soakaway.

When that pipe runs outside, it is exposed. On a cold night the water sitting in it can freeze, usually at the open end, at a bend, or at a dip where water collects. The condensate then has nowhere to go and backs up into the boiler.

Signs your condensate pipe has frozen

• The weather is below freezing, or has been overnight. This is the biggest clue.
• A gurgling or bubbling noise from the boiler as it tries to run.
• A fault code on the display. The code varies between manufacturers. Baxi, for example, lists E133 or E28 on most of its boilers. Your manual will name the code for yours.
• The boiler fires briefly then locks out, and does the same after every reset.
• Ice visible on the pipe or at its end where it meets the drain.

If you have heating and hot water problems in mild weather, a frozen condensate pipe is unlikely. Our guide to a boiler losing pressure covers the other common cause of a sudden lockout.

How to thaw a frozen condensate pipe

1. Find the pipe. Follow the plastic pipe from the bottom of the boiler to where it goes through the wall, then trace it outside to the drain.
2. Find the frozen section. It is most likely to be the coldest, most exposed part: the end of the pipe, a bend, or a sagging section.
3. Warm it up. Hold a hot water bottle or a microwaveable heat pack against the pipe, or wrap it in a cloth soaked in hot water. You can also pour warm water slowly along the pipe from a kettle that has been left to cool for a while, or a watering can.
4. Keep going until water flows freely from the end of the pipe. It can take several goes on a very cold morning.

Resetting your boiler

Once the frozen condensate pipe is clear, the boiler will not usually restart on its own. You need to reset it. Depending on the model, that means pressing and holding a reset button, turning a selector knob to reset, or switching the boiler off and on. The exact method is in your boiler manual or on the manufacturer’s website.

Give it a few minutes after the reset. When it fires up and stays running, you have fixed it. If it locks out again straight away, the pipe may still be partly blocked, so check the end again. Should it still refuse to run after that, the cause may be something else and it needs a Gas Safe engineer.

How to stop it freezing again

If your condensate pipe has frozen once, it will freeze again in the next cold spell unless something changes. The options, from simplest to most permanent:

• Lag the outside section with waterproof, weatherproof pipe insulation. Standard foam lagging soaks up rain and freezes, so it needs to be the weatherproof type.
• Keep the heating running at a lower setting through very cold nights, rather than letting the boiler sit off for hours. Warm condensate moving through the pipe is far less likely to freeze.
• Upsize the external pipe. Heating industry guidance for external condensate runs recommends a larger bore pipe, typically 32mm, because a wider pipe takes much longer to freeze solid.
• Shorten or tidy the outside run. Removing sags and dips, and making sure the pipe falls steadily to the drain, means water does not sit and freeze.
• Reroute it indoors. Where possible, connecting the condensate to an internal waste pipe, such as a sink or washing machine waste, removes the problem altogether.

Your engineer can look at the condensate route at your annual boiler service and suggest the right fix before winter. It is a far cheaper job than a call out on the coldest morning of the year.

When to call an engineer

Call a Gas Safe registered engineer if:

• You cannot safely reach the frozen section.
• The pipe is cracked, split or has come apart at a joint.
• The boiler still locks out after the pipe is clear and it has been reset.
• It keeps freezing every cold night and you want it fixed properly.
• You can smell gas at any point. In that case, call the National Gas Emergency Service on 0800 111 999 first.

Do not take the boiler casing off to look for the problem. Anything inside the boiler is a boiler repair job for a qualified engineer. And if the freeze has caught your water pipes too, and you have a burst or a leak, turn the water off at the stopcock and call for emergency plumbing help.

Frequently asked questions