Boiler not working? How to fix a frozen boiler condensate pipe

Condensing boilers might promise you the best energy savings but the way that they’re often installed in the UK often leads to problems when the temperature drops, freezing up the condensate pipe and stopping the boiler from working. If you’ve woken up to find that you don’t have any heating and your boiler’s flashing up a strange error message or is making a weird gurgling noise, chances are that a frozen boiler pipe is to blame.

The problem is down to the way that condensing boilers work. When burning fuel, gasses escape up the flue. A condensing boiler recaptures some of this heat by condensing the waste gasses, with the by-product of slightly acidic water that runs out of the condensate pipe. If this pipe runs, uncovered, outside, then the water inside can freeze and block the piper and prevent your boiler from working.

Although boilers are supposed to show an error message that helps you track this down, sometimes it’s not that simple. For example. my Glow-worm boiler has shown error messages F1, F9 and F13, even though the problem is the same.

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The fix

Before you start, turn your boiler off at its dedicated switch. If your boiler is on the ground floor, go outside and look for a plastic pipe that comes out from the wall that your boiler is mounted on. If you can see the end of it, take a look and see if it’s frozen throughout; if you can’t see the end, then it’s worth trying the method to fix it anyway. If you can’t see a pipe coming through the wall, you may have an older type of boiler and will need to call a heating engineer.

Likewise, if your boiler is mounted higher up, it’s dangerous to get to, so you’ll need to call a registered heating engineer.

Now, you need to thaw out the pipe, clearing the ice blockage. You can strap a hot water bottle to the pipe, although this takes time. The better option is to use warm water to melt the ice. Don’t use boiling water, as this could be dangerous; start your a kettle, but cut it off when the water is warm, but not boiling.

Now, go outside and pour the warm water up and down the length of the condensate pipe. Use a full kettle-load of water to make sure that you’ve got everything out; at my house, a popsicle-like block of ice slid out of the pipe. Repeat this step as necessary.

Now, go back in and turn your boiler on. It should start automatically, running through a startup cycle. If you still get an error message (I did on my Glow-worm), then you may have to reset the boiler by checking the manual (these are available online if you Google the manufacturer and model name). My Glow-worm has a reset button that you can push with a pin, underneath the display.

With the boiler fired up, you can now try your heating or, if you have a combi-boiler, turn on the hot water. The boiler should fire into life, and you should get hot water.

However, I had to go through an extra step, as my boiler had also lost a bit of pressure and wouldn’t start. Boilers should be pressurised to 1bar. To top up, you need to open the filling loop, turning off when the boiler’s display indicates 1bar (a little over won’t cause any harm, say 1.2bar). On my boiler, the filling loop consists of two taps located underneath the boiler; both must be opened to fill and both closed when done. Your boiler’s manual will tell you the correct procedure if you’re not sure.

If your boiler is at the correct pressure and not working, do not fill it further, as this could damage it. If you’re not confident doing this step, then it’s best to call a heating engineer. Given that you had a problem with your boiler, it’s a good time to book a service and get it looked at properly, anyway.

How to prevent the condensate pipe freezing

There are several steps you can take to prevent your condensate pipe from freezing. First, find out if a plumber can run the waste into a drain, so that there’s nothing exposed outside. As the condensate pipe’s water is slightly acidic, it should only be run into plastic piping as it can corrode metal pipework.

If this isn’t possible, then lagging the pipe can help prevent problems, although this tends not be very effective in the coldest temperatures. Getting a plumber to fit a larger pipe (25mm+ in diameter) is a good call, as larger pipes make it harder for the ice to clog.

There’s another option that can help. My guide on setting a condensing boiler’s temperature talks about setting the best central heating temperature to make sure that your boiler is operating at its most efficient. The short version is that due to the way that condensing boilers work the return water temperature has to be 55C or lower; if it’s not, then the gasses can’t condense and turn to water. Turning your central heating up to 65C or higher should push your boiler so that it’s no longer condensing. This will reduce fuel efficiency but could help avoid frozen pipe issues; once the worst of the cold weather has passed, you can dial the temperature back down to make your boiler more efficient.

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