Ball valve constantly running — cistern or loft tank
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Most likely cause & what to check
A ball valve (or ballcock) that never fully shuts off is wasting water and often causing the constant sound of running water. In a toilet cistern this shows as water trickling through the overflow pipe (usually a pipe going outside, or water constantly running into the pan). In a loft tank, water drips from the overflow at eaves level.
Isolate the water supply: for a toilet cistern, turn the isolation valve on the supply pipe (a slot-head screw in the pipe — turn 90° to close). For a loft tank, there is usually an isolation valve on the rising main in the loft.
Flush the toilet or open a tap to empty the cistern, then examine the ball valve. Modern cisterns use a Torbeck or diaphragm valve (a compact unit with a float on the side) rather than the traditional long-arm Portsmouth valve.
If it is a diaphragm type: unscrew the front cap, remove the plunger, and inspect the small rubber diaphragm. This is usually the culprit — a split or hardened diaphragm lets water continuously past. Replacement diaphragms cost £3–£5 and push directly back into the valve body.
If it is an old Portsmouth-type valve (brass body, long float arm): remove the back nut, pull out the piston, and replace the rubber washer at the tip. If the valve body is heavily corroded, a full replacement is better value (£8–£20).
After reassembling, restore the supply and check the shut-off level — the water should stop filling approximately 25mm below the overflow outlet. Adjust the float arm (or float height on modern valves) to achieve this level.
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Frequently asked questions
How much water does a constantly running ball valve waste?
A continuously running overflow wastes 100–300 litres per day depending on flow rate — potentially adding £150–£400 per year to a metered water bill. Fixing a faulty ball valve costs very little and is one of the most cost-effective repairs in a home.
Can I stop a running ball valve temporarily without turning the water off?
You can try gently bending the float arm slightly downward to lower the water level and stop the overflow — this sometimes works as a temporary measure on older brass ball valves. However, this is only a stopgap. A proper replacement valve is inexpensive and straightforward to fit.
What is the difference between a ball valve and a float valve?
They do the same job — both control the water level in a cistern by shutting off the inlet when the water reaches the correct level. "Ball valve" refers to the traditional design with a copper or plastic ball on an arm. "Float valve" is the modern equivalent, which uses a cylindrical float. Modern float valves (Torbeck style) are more reliable and less prone to overflow problems.