Cold Water Tank Overflowing

21 Jan 2004
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United Kingdom

My cold water tank in the loft has started to fill and leak water from the overflow. It happens most mornings and started a few weeks ago. The float valve has been replaced twice so that's not the issue.

The loft has 2 tanks, it's the big cold water tank that leaks not the small fe tank. There are 2 motorized valves in the airing cupboard near the pump and cylinder. The boiler is a worcester greenstar Ri.

What happens is that in the morning the HW comes on and after it stops, something causes the float valve to get drowned. Once it's drowned it starts letting more water in until the cold water tank is full. If I use up some water from the tank, then the float valve behaves normally and the situation is resolved until the next morning.

I can't figure out what causes the float to drown. I tied a container under the cold tank vent pipe and there's no water venting into the tank.

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Are you using the correct type of ball valve? Let's see a photo.
Difficult to get a photo due to the awkward space and light but its one of these:

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I'll have to go and look. Am I looking for a white (or red) item like this? Will it be inside the valve mechanism or easily visible?

I'm not a plumber, this is based on helping out at some neighbours whose CW tank was overflowing even though the valve, supplying the ball cock, was off. Their problem was found by a proper plumber.

You say the HW coming on seems to trigger it. It's possible that comparatively high pressure (pumped by boiler ?) HW is mixing with cold and going into the CW tank via the outlet pipe. I'd try turning off the CW tank outlet valve before going to bed and seeing if the problem's there in the morning. The TROUBLE with this is that you'll (almost definitely) only have cold water from the kitchen tap (which includes toilet cistern not refilling).

In my neighbour's case, a faulty shower mixer unit was allowing hot & cold to mix prior to the taps and the combi boiler (water at mains pressure) was sending hot water into the CW tank.
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I cant see its the ballvalve, especially if its been replaced twice. Also, if ballvalve was faulty, it would overflow all the time. As a test, can you draw some water off, (so ballvalve has opened slightly to refill the cistern), then isolate the cold supply to the cistern. Either shut it off or tie the ballvalve up so no more water can get into the cistern from the supply. Then see what happens.

The high pressure seat is usually fitted by default, to change it would involve dismantling the valve to access the internals, any plumber worth his salt is going to do that, especially on a mains fed cistern, unless the supply pressure is always diabolical.

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