Plumbing mystery overflow from loft

[Kathy]

Plumber stumped so hoping someone here might have an explanation and solution -

Dec 2025 - Old boiler lasted 30 years, hardly ever serviced (bless). New combi boiler installed Dec 2025 (hot water tank in airing cupboard).
Shortly after, noticed puddle of water at back door (on a dry day). Hmm. called plumber, changed the ballcock, didn't solve, then returned and said he adjusted pressure (?)and that seemed to work.
Dec 2026 It's started again, and more frequently, lasts 2-3 minutes. Photo is on a windy day. If you enlarge you can see the water spray.
It must have something to do with the new boiler as we never had such a problem with the old one or with this one in summer.

It actually happened while the plumber was here servicing the new boiler which was good (in a way). Plumbing firm is local and did a superb job installing the boiler (see thumbnail photo).

Appreciate any insights. Thanks.
It must have something to do with the new boiler as we never had such a problem with the old one or with this one in summer.

It actually happened while the plumber was here servicing the new boiler which was good (in a way).

Appreciate any insights. Thanks.

 

[Cubist]

Can I just verify a few things with you please before considering the potential cause(s)?

• Is your house connected to a mains water supply?
  • I am assuming it is as you describe the new boiler as a 'combi'.
• Do you know whether all your hot water services around the house are supplied from the boiler alone?
• You say that you have a hot water tank (cylinder) in an airing cupboard.
  • Is this a new tank installed at the same time as the boiler or is it the old system tank/cylinder?
  • Is this tank/cylinder topped up with water from the boiler, a direct cold water feed or from a header tank located in the loft?
  • Is this tank/cylinder equipped with its own immersion heater?

Judging by the location of the overflow pipe it can only be associated with either a water storage or header tank in the loft space OR directly from can expansion pipe and probably from the hot water cylinder.

 

[Kathy]

• yes mains water and gas fired combi boiler (condensing = combi I think?)
• all hot water services from the boiler
• cylinder is the original, fed from cold water fee in header tank in loft. There is a switch for an immersion heater but never used in 25 years.

Plumber examined the cold water storage tank, very minor drip into it but did not think would have any bearing on the problem.

 

[a twig]

Have you traced the overflow pipe back on the inside to see what it is connected to?

 

[a twig]

Friend of mine had an old galvanised water tank in the roof long disused but too heavy to remove so left in place. The overflow was still connected and occasionally let water out - turned out rain was coming in through a hole in the roof above it and filling the old metal tank it until it overflowed

 

[Kathy]

There are two containers in the loft - one is cold water with the ballcock and the other is on a stand higher up for the hot water (per the plumber).
There is a pipe from from the cold water storage container to the outside, assume it's the overflow (as seen in the photo)
Not quite the same explanation for my problem as your friend's, a twig!

 

[stuart45]

Friend of mine had an old galvanised water tank in the roof long disused but too heavy to remove so left in place. The overflow was still connected and occasionally let water out - turned out rain was coming in through a hole in the roof above it and filling the old metal tank it until it overflowed

That must have been a big leak. Lucky it went into the tank.
PS, you're going to tell me it's just a wind up.

 

[Cubist]

Both tanks in the loft AND the hot water cylinder in the airing cupboard are completely redundant - they can and should be drained and the cold water feed to them should be severed and capped.

ALL your Cold water services throughout the house will be on mains water pressure as will all your Hot water via the combi boiler. The only reason that overflow pipe would have been carrying water would have been if the ball valve for one of the tanks was temporarily stuck open.

 

[phillc]

Before we get too hasty, how many thermostats - separately heatable areas - do you have in your house? How many of those zones are radiators and how many of them are underfloor heating?

I ask because, looking at that boiler I think I see two 22mm pipes feeding the heating system at the top. I can't see clearly what is at the bottom but I'm not compelled to believe there's a cold water in, hot water out and gas. You definitely have two motorised valves for the heating circuit.

This could all indicate a system (not combi) boiler. Still condensing, so all fine. But, instead of heating up water on demand like a 'combi' it continues to heat a tank of hot water for you. It does this as the second 'radiator zone' - basically, inside the tank is a radiator like coil of pipe that heats up the water around it.

If this is the case, your water tanks might not be quite so redundant as Cubist suggests.

Water spurting out periodically like that suggests to me a problem with an expansion vessel. If you have a system boiler, and if it's a sealed system, you'll have at least two.

Out of interest, is the water that is coming out cold, hot, or boiling-steam-hot?

*I am not a plumber*

 

[fernicarry]

You have a header tank in the loft but apparently another hot water tank higher than it? If that's a hot water storage tank then it must be pressurised and it would have its own blow off valve which might be the source of the dribble. But then you also have a vented hot water tank in the airing cupboard? I'm not sure I understand the setup.

Intrigued by the two pipes at the very top of the boiler picture. They don't appear to go anywhere. Air bleed or something?

 

[phillc]

You have a header tank in the loft but apparently another hot water tank higher than it? If that's a hot water storage tank then it must be pressurised and it would have its own blow off valve which might be the source of the dribble. But then you also have a vented hot water tank in the airing cupboard? I'm not sure I understand the setup.

Intrigued by the two pipes at the very top of the boiler picture. They don't appear to go anywhere. Air bleed or something?

Yeah, I don't understand it either - I am wondering whether the OP has some sort of residual hot water tank setup.

And, yes, I saw those two pipes at the top, pointing towards us and going nowhere. One of them seems to have a ball valve style tap and the other a classic drain valve. So, I assume it's some sort of rudimentary filling, draining and flushing setup?