A gurgling boiler, and
the pipe nobody looks at

Every condensing boiler produces a small amount of acidic water as it runs, and it has to go somewhere. If the pipe carrying it away is blocked or frozen, the water backs up and the boiler shuts down to protect itself. You may hear a gurgling noise before it stops. Freezing is the winter version of this and a blockage is the version that happens at any time of year.

The noise is the clue

You may hear a gurgling from the boiler, and sometimes that is the only warning you get before it locks out. It is the sound of the boiler trying to push condensate through a pipe that will not take it.

It is worth saying that a gurgle does not always mean the pipe. It is one of the things it points at, and on the jobs we go to it points there often enough to be worth checking first.

Freezing

When it is properly cold outside, condensate pipes freeze, and they freeze in numbers. There have been winters where a big freeze left a lot of people without a working boiler for exactly this reason, and it was common enough that the guidance on how these pipes should be installed was changed because of it.

The pipe is carrying a trickle of water in a thin plastic tube that often runs up an outside wall. In a hard frost it does not take much.

The part that is worth going outside to look at

The current guidance, which comes out of BS 6798 and is set out in the industry installer guide, says a condensate pipe running inside can be 19mm internal diameter, typically 22mm outside, but the moment it goes outside it should step up to at least 30mm internal, typically 32mm outside, and it should make that step before it passes through the wall. It should be insulated with weatherproof material, the external run should be kept to about three metres, and the end should be cut at an angle.

A lot of installations still do not follow that, and it is fairly common for us to find it. Where it is causing a problem for the customer we tell them, and it is something we can put right.

So if your condensate pipe freezes every winter, go outside and look at it. If it is still the thin pipe all the way, or it has no insulation on it, that is very likely why it keeps happening, and thawing it out each time is not going to change anything.

Manufacturers' own instructions vary from boiler to boiler and they take precedence, so the right answer for your appliance is the one in its manual.

Blockages

The other version has nothing to do with the weather. Condensate is slightly acidic and it runs through the same small pipe for years, and debris, sludge from the drain end, or a partial collapse will all do the same thing as ice. That is what my friend had, and it was not winter.

What you can do, and what you should not

If the pipe is frozen and you can reach it safely from the ground, warming it gently along its length will usually clear it, and the boiler will often restart once it does. Do not use anything boiling, do not get on a ladder for it, and do not start dismantling pipework.

If it freezes again next winter, the pipe is the problem and not the weather.

Quick answers