Cold water tank overflowing — how to fix the ball valve
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Most likely cause & what to check
The overflow pipe from a loft tank exits through the eaves and should normally be dry. If water is dripping or trickling from it, the ball valve (float valve) in the tank is not shutting off properly.
Go to the loft and look in the tank. The float arm should be pulling the ball valve closed when the water reaches the correct level. If the tank is full but still filling, the valve is not seating properly.
Common causes: the float is waterlogged and sitting too low (causing the valve to remain open), the rubber washer inside the valve is worn, or the valve seat is corroded.
Test the float — lift the arm by hand. If the flow stops when you hold it up, the float is set too low or is waterlogged. If it doesn't stop, the valve body is faulty.
To reset the float arm height: bend the arm slightly downward so the float rises at a lower water level, shutting the valve before the tank overfills.
To replace the washer: turn off the mains at the stopcock, flush the toilet or run a tap to lower the tank level, then unscrew the valve body. The washer is inside the end cap — replace it and reassemble.
If the valve is very old or badly corroded, replace the whole ball valve. Modern equilibrium ball valves are much more reliable than older diaphragm or piston types and cost about £10–£15.
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