Cold Water Tank Ball Valve Not Shutting Off
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →Turn off the mains water at the stopcock before working on the ball valve. Have towels ready as some water spillage is inevitable.
A constantly running cold water tank or overflowing tank is almost always caused by a faulty ball valve (ballcock). Either the float has sunk (waterlogged), the diaphragm washer is worn, or the float arm is bent so the valve never fully closes. This is a DIY-accessible repair.
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Most likely cause & what to check
Turn off the mains water stopcock (under the kitchen sink, clockwise to close). If there is no stopcock for the loft supply, you can tie the ball float arm up to stop the flow temporarily while you work.
Check the ball float first — if it has water inside it (shake it), the float is waterlogged and will not rise high enough to close the valve. Unscrew the float from the arm and replace it (£3–8).
Check whether the float arm is bent — it should be horizontal when the tank is at the correct water level (25mm below the overflow). If it is angled incorrectly, bend it gently downward to raise the cut-off water level.
If the valve still runs with the float replaced, the diaphragm washer has failed. For a diaphragm (Portsmouth) valve: unscrew the large nut on the valve body, pull out the piston, and replace the rubber washer inside (£2–5 for a repair kit).
For a modern equilibrium or Torbeck valve, the whole valve head can usually be replaced with an inexpensive cartridge — check the brand and order a spare head from a plumbing merchant (£5–15).
If replacing the washer does not resolve the dripping, the valve body seat may be worn. Replacement ball valves (full units) cost £10–25 and take about 30 minutes to swap out — turn off the water, drain the tank partially, undo the tank connector nut, and fit the new valve.
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