Frozen condensate pipe: how to thaw it safely

When a cold snap arrives, a boiler that suddenly stops working is often down to one small plastic pipe outside the house. Ideal Heating calls a frozen condensate pipe a very common cause of boilers stopping in cold weather. The good news is that thawing it is one of the few boiler problems you can usually sort out yourself, safely, from outside.

Safety: Only ever thaw the outside plastic pipe. Never open the boiler casing, pour water on the boiler or touch any gas parts. If you smell gas, or think there could be carbon monoxide, stop, get everyone out into fresh air and call the National Gas Emergency Service on 0800 111 999.

What is a condensate pipe?

Modern condensing boilers make a small amount of waste water while they run. A plastic pipe, usually white, carries this water away from the bottom of the boiler. It often goes through the wall and runs down the outside of the house to a drain.

The water drips slowly, so in freezing weather it can turn to ice inside the pipe. Once the pipe is blocked, the boiler shuts itself down. Baxi explains this is a failsafe that stops water backing up into the boiler and damaging parts.

How do I know if my condensate pipe is frozen?

Look for these signs, especially after a night or two below freezing:

• The boiler has stopped and you have no heating or hot water.
• You can hear gurgling or bubbling from the boiler or the pipe.
• The boiler display shows a fault code or a flashing light.
• You can see ice or frost on the outside section of the pipe, or nothing is dripping from its end.

Fault codes vary by make. Vaillant says its boilers show F28 or F29, and Baxi says most of its boilers show E133 or E28. Worcester Bosch lists codes such as EA or D5, or a slow flashing blue light, depending on the model. Check your manual to see what your boiler's code means.

How to thaw a frozen condensate pipe

You only need warm water, a jug or watering can, and somewhere safe to stand. Only do this if you can reach the pipe from the ground.

1. Follow the pipe from the bottom of the boiler to where it comes out of the wall and runs outside.
2. Find where it's frozen. Vaillant and Baxi say this is usually the most exposed part: the open end, a bend, or a dip where water collects.
3. Pour warm water slowly over the frozen section. Which? suggests boiling a kettle and letting it cool for 10 to 15 minutes first. Start at the top and work down until the ice clears and water drips out.
4. If you'd rather not pour water, hold a hot water bottle or a microwavable heat pack against the pipe instead.
5. Once the pipe is clear, reset the boiler as your manual describes. Give it a few minutes to restart.

If the pipe has foam lagging on it, Ideal Heating suggests taking it off while you thaw the pipe, then putting it back afterwards to help stop it freezing again.

Why shouldn't you use boiling water?

Which?, Baxi, Vaillant and Ideal Heating all say not to use boiling water. The pipe is plastic, and Vaillant warns that water straight from the kettle can damage or crack it. A cracked pipe can be costly to repair and leave you without heating for longer.

Take care with the water you pour, too. It can freeze on the path or patio and make it slippery, which both Baxi and Ideal Heating warn about. Wear shoes with grip and clear up any puddles if you can.

What if the boiler still won't work after thawing the pipe?

Reset the boiler once, as the manual says. If it locks out again or shows the same code, stop there. Pressing reset over and over won't fix the cause. The pipe may still be partly blocked further up, or the boiler may have a different fault.

At that point, call a Gas Safe registered engineer. Only they can work on the boiler itself. When they arrive, ask to see their Gas Safe ID card and check the photo matches, the card is in date and the back lists the work they're qualified to do, such as domestic boilers.

If you rent, tell your landlord instead of booking an engineer yourself. If you own your home, check whether home emergency cover or a boiler warranty includes the call-out.

How can I stop my condensate pipe freezing again?

If your pipe freezes every winter, there are some simple and some bigger fixes. Manufacturers and Which? suggest:

• Insulating the outside section with waterproof, weatherproof pipe lagging. Which? says it should also be UV resistant.
• Running the pipe inside the house for as much of its length as possible, so less of it is exposed.
• Fitting a wider pipe outside. Which? says this is typically 32mm to 40mm, depending on the boiler maker's instructions.
• Making the outside run steep and direct, with as few bends as possible, so water drains away quickly.
• Keeping the outside run short. Worcester Bosch says external runs for many of its boilers should be no longer than 3 metres.
• Leaving the heating on a timer while you're away in cold weather, so the boiler doesn't sit idle, as Vaillant suggests.

Some of these are jobs for a professional. Which? says rerouting the pipe isn't a DIY job, and insulating it is best left to an engineer unless you're confident. A good time to ask is during the annual boiler service, when an engineer can check the pipe's size, fall, route and insulation.

Are there other ways to protect the pipe?

Which? mentions a couple of other options. Some newer boilers have a siphon trap that releases condensate in one go rather than a slow drip, which makes freezing less likely. There are also trace heating systems, an electric heating element fixed to the pipe that switches on when it gets to around 5°C. Which? says this must be fitted by a Gas Safe registered engineer.

These are worth discussing if your pipe has a long or exposed outside run that can't easily be changed. Keep away from any wet electrics around the boiler, and leave all wiring to a professional.