Hot water overflow from cold tank

AdrianW2

We've been having some trouble with our water recently. The cold water tank in the loft has been overflowing. When I look in the loft the tank is full and steaming, and the water lukewarm.

The float valve is about 6 months old and appears to be working fine.

So I suspect something in the hot water system is dumping hot water into the cold tank through the vent pipe.

There are lots of similar posts on the web, but I don't really understand the answers so any suggestings of what to check gratefully received.

As an aside, last week we changed one of the taps and I turned the water off to do so and forgot to turn it back on before the heating kicked in. I'm not sure if that's relevant but the cooincidence is suspicious.

gas4you

Is the small heating feed and expansion tank higher than this cold storage tank in your loft?

ormus

do you have two tanks? and which tank are you talking about?
the large tank is the CWS tank and the small tank is the CH expansion tank.
what sort of boiler do you have?

AdrianW2

We've got a two tank fully pumped system with a motorised valve. The header tank is higher than the CW cistern. It's the large cold cistern that's overflowing.

Some experimentation suggests that this happens whenever the boiler starts up, for HW or for CH, but I haven't caught it in the act yet.

fiddiwebb

How old is the system?

Not had a power shower fitted recently?

ormus

sounds like a blockage somewhere, and the boiling hot water is going up the safety vent pipe into the tank.
maybe the motorised valve faulty?

AdrianW2

I suspected the valve earlier this year when the heating got stuck on and refused to turn off, but the man from British Gas declared it fine.

Hard to comment on age, it's a mix of parts 10 to 40 years old.

ormus

on some old systems, ive seen boiler vent pipes put into CWS tanks.
when the boiler overheats, (runs constant), the vent pipe dumps lots of steam/water into the CWS tank.

jaxjax123

I had this last year if I am understanding you properly and we were told it was the Coil in the hot water tank. We had that replaced and everything works great now.

Gloomendoom

I had a similar problem a while ago. I was also told that the heating coil in the hot water tank was leaking. I wasn't convinced, so I drained the tank to check. It made no difference.

The real cause turned out to be simple. The heating expansion tank was too full. When the heating came on the water in the system expanded and the level in the expansion tank rose above the level of the overflow.

The fix required no more than bending the ball valve float down a couple of inches to stop the tank overfilling.

1984ReturnsForReal_2

Have you left your immersion heater on?

If you have a faulty immersion stat would do it.


Also a faulty stat on your cylinder for the boiler control.

Or more than likely a dodgy port valve that doesn't close to central heating only.



Take your pick. Its one of them.