Waterfall from overflow in loft - Help!

[Kathy]

Apologies for lengthy explanation - Two weeks ago I noticed a small pool of water outside the back door. Hmmm. Later in the week I was outside and out of the blue water started coming out of the overflow pipe in the loft. It lasted about 3 minutes. Called the plumber who couldn't come for a few days. In the meantime, it happened again, just for a few minutes. I have gravity system with header tanks in the loft. The plumber replaced the ballcock yesterday. This morning half of the whole back porch was wet! The water must have been coming out for a long time overnight to create such a big splash and for the first time I can see the bricks are damp. I also see what looks like bits of mortar that might have washed off the bricks. It has really got me stumped. It is so completely random. I have lived here for 25 years and this is a first. I should add that I had to replace a 30 year old boiler with a new one in December (same plumber). It is a condensing boiler. Could there be a connection? Should I call a civil engineer or surveyor to study this problem? Do I have a ghost?! Really appreciate your help. Thank you.

 

[paulc]

About the only thing that I can think of that would cause a header tank to overflow intermittently is the DHW tank getting too hot and the water level set too high in the header tank. Does the water over flow when the boiler is running, or is it random ?

I'd give the ballcock a little tweak to reduce the water level, and check the thermostat on the tank (50-55°C is plenty hot enough).

 

[Stevers]

What @paulc says!

Whilst it's not unusual for affected cold water overflows to run until someone flushes a loo, the volume of water overflowing is never more than what the leaky ballcock is passing.

 

[Flyfisher]

I once lived in an area with hard water and had problems with an intermittently dribbling overflow pipe. It turned out that scale had built up around the ballcock valve mechanism and was preventing it from fully shutting off the water. The problem was intermittent, or rather the overflowing was intermittent, because it would take many hours for the tank water to rise to the overflow level, which it would do while we were at work, but when we were at home our usual water consumption habits kept the level below the overflow. It turned out this was why I noticed the problem more often during the weekdays when the house was unoccupied.

I initially solved the problem by giving the ballcock valve a gentle rub to break off the worst of the scale and the problem went away for a month or two. After a few attempts at this I finally capitulated and did what I should have done in the first place and replaced the ball valve itself. I never had the problem again, though 25 years on from when I did the fix, I wonder if the current owners of the house have experienced the same problem?

Anyway, this may or may not be the problem in this particular case but it might help someone out there.

 

[CliffordPope]

Get up in the loft and look in the top of the tank.

 

[CliffordPope]

I meant to complete that advice; look at the water level compared with the overflow. Get someone to flush a loo and watch as the tank refills. Does it cut off firmly about 1/2" before it reaches the overflow, or does it dribble continuously?

 

[Kathy]

Thanks for your thoughts. The ballcock is brand new. That was the first idea. The tank levels are fine. The plumber came back yesterday. He said to turn the heating on. There was a little dribble. He thought the pressure setting in the new condensing boiler might be too high so he has lowered it. Waiting to see if that solves the problem. I will ask about the temperature in the HWT if it doesn't. Makes sense it could be involved.

 

[a twig]

I’m potentially missing something here but why would there be a pressure setting for a boiler in a gravity system?

Can you share the details (make and model) of the boiler?

 

[paulc]

I’m potentially missing something here but why would there be a pressure setting for a boiler in a gravity system?

I had made the assumption that the central heating was now pressurised and the original gravity DHW system had been retained. Keeps costs down, but a missed opportunity to install an unvented tank (or switch to a combi).
Can't see any way of lowering the operating temperature of a sealed heating system short of draining a little water out or lowering the pressure in the expansion vessel. Neither would would affect the water levels in a header tank.

 

[Kathy]

Thanks all. The plumber lowered the pressure on the new combi boiler and no waterfall so far, hopefully that has solved the problem.