Cold water storage tank overflowing into loft
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Most likely cause & what to check
Go into the loft and check the cold water storage tank — the water level should be roughly 50–75mm below the overflow pipe outlet. If water is at or above the overflow, the ball valve is not shutting off correctly.
Lift the ball float by hand — if the water stops flowing into the tank, the float is set too high and just needs adjusting lower.
On older brass ball valves: gently bend the float arm downward so the ball sits lower in the water and cuts off the supply sooner.
On modern plastic valves: there is usually a screw adjuster or the float can be repositioned on the arm to achieve the correct cut-off level.
If lifting the float does not stop the flow of water, the valve seat is worn and the whole ball valve needs replacing.
To replace the ball valve: turn off the mains water at the stopcock, drain the tank by running taps until empty, then unscrew the old valve (held by a back nut on the outside of the tank) and fit the new one at the correct height.
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Frequently asked questions
My loft tank doesn't have an overflow pipe — is this normal?
All tanks should have an overflow pipe fitted. If yours doesn't have one, a plumber should fit one to prevent water damage to the ceiling below if the ball valve fails.