Cold water tank flooding

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Established Member
5 Jun 2013
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Hi all,

It's a cold water tank in a cupboard in a flat in a block, not a flat in converted house. I think the ball valve stop working, and water continued to fill the tank. There's an overflow pipe (white pipe. top right in photo) but that didn't seem to drain away the excess water. Result is water flooding all over the floor.

Pic 4 show what I think is the overflow pipe going into a wall near the floor at the back of the cupboard.

There's a stopcock leading to the tank but it was stuck, eventually I managed to turn off the water by turning the isolation valve.

I think the ball valve needs replacing/repair and overflow pipe needs to be connected to the bathroom drain. But there's a metal frame/bar going over the top of the tank lid and around the tank and the lid won't open.

The tank was put in by the council when the occupier was a tenant who has since bought the flat, and is now the leaseholder. Is the metal frame there to stop tampering? Is it possible they didn't connect overflow pipe to something, I heard it's supposed to be the bath?

Since buying the flat, a new bathroom and kitchen were put in, maybe the builder didn't reconnect overflow pipe??!!

tank 1.JPGtank 2.JPGtank 3.JPGtank 4.JPG
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The overflow should cope with a failed ballcock, that does look like a council job though and overflow pipe is normally 21mm but I have used larger when the supply is high flow.
Does the overflow have an isolating valve on it (the round circle on it)? It would be odd if it does but it could be in the shut position.
Does the overflow have an isolating valve on it (the round circle on it)? It would be odd if it does but it could be in the shut position.
That’s part of the bylaw 30 kit, it’s not an isolation valve it has metal gauze in it
First thing to check is that the overflow works. The pipe top right.
I don't think that overflow would cope with full flow into the tank. The cold water feed is at mains pressure, the overflow is at the top of the tank, so won't flow away very fast. Both pipes look the same size. It is essentially a warning pipe. Often when a float valve fails, it still reduces the flow when it's in it's shut off position. so in most cases it could be OK.
You possibly need a bigger overflow which can cope with the full flow into the tank. In the meantime, you could turn the input flow down. That looks like a ball valve on the cold water inlet (pipe top left) The valve looks as if it's shut??.
Doing this is not an ideal solution because the input flow may not cope with someone filling a bath or a long shower and the float valve may squeal like a banshee just before it shuts off.

There must be some plumbers on here, so they should know the proper pipe sizing.

A guide shows
""A warning/overflow pipe must be at least 19mm (internal diameter) and capable of accommodating all possible flow rates i.e. the maximum inflow under fault conditions. The effect of any screen on the nominal flow capacity must be taken into account when determining the size of an overflow.""
The overflow looks like 15mm pushfit, I haven't seen a weird elbow like the top one though, could it be like a non return elbow fitted the wrong way round? I would stop the water filling the tank, disconnected the overflow pushfit at the second elbow with a bucket underneath and make sure water is flowing through it first before you do anything else