Why does my isolating valve still let water through when it is closed?

Debris and scale on the ball seat, the exercise that sometimes clears it, and why replacing a small valve is usually the right answer.

Difficulty: intermediate
Time: 45 min
Read: 3 min
Safety: caution

Short answer

The ball or its seat has debris or scale on it, so it cannot seal. Working the valve fully open and closed a few times sometimes clears it. If it still passes, close the stopcock and replace the valve — they are inexpensive, and a valve you cannot trust is worse than no valve at all.

Small isolating valves are cheap ball valves with a rubber or PTFE seat. Grit from the mains, a flake of solder, or a build-up of limescale sits on that seat and holds it fractionally open. It is common, and it is why you always confirm at the tap.

Safety

If a valve will not hold, do not start dismantling the fitting downstream of it. Close the main stopcock first and prove the water is off at the outlet. Cutting into a pipe you believe is isolated when it is not floods a room in seconds.

What you'll need

A screwdriver that fits the slot
A replacement valve of the correct type and size
Two spanners for a compression valve
Cloths and a container

Step by step

Prove the leak first.Close the valve and open the outlet. A dribble that stops after a minute is just the pipework draining. A steady flow that continues is a passing valve.
Exercise the valve.Turn it fully open and fully closed several times, briskly. That sometimes rolls the debris off the seat and restores the seal.
Run water through it while open.Opening the outlet with the valve open flushes loose debris through rather than leaving it sitting on the seat.
Do not part-close it and hope.A valve that nearly closes is not isolated. Treat it as failed for the purposes of any work downstream.
Close the main stopcock to work on it.Find the internal stopcock, close it, and open the lowest cold tap in the house to drain the pressure down.
Identify the valve type.Compression valves have a nut and olive at each end; push-fit ones have a collet. Buy a like-for-like replacement of the same pipe size.
Replace a compression valve.Undo both nuts with two spanners, ease the valve off, and fit the new one with new olives. Note the flow direction if the valve has an arrow — many are bidirectional but not all.
Replace a push-fit valve.Push the collet in with a release tool or your fingers and pull the pipe out. Check the pipe end is clean and undamaged, and use an insert if it is plastic pipe.
Consider fitting a lever valve.A small lever-handled valve costs a little more, is easier to operate, and gives an unambiguous open or closed indication.
Test thoroughly.Restore the stopcock, check the new joint by hand, then close the new valve and prove it holds at the outlet.

Tips

Fit a lever valve rather than a slotted one where access is awkward — you can operate it without a screwdriver.
Buy two while you are there. If one has failed, others of the same age on the same water are likely to follow.
Always use new olives on a compression joint. The old one has taken the shape of the old valve.

Common mistakes

Starting work downstream of a valve that only mostly shuts — It floods the moment the fitting comes off. Confirming at the outlet before undoing anything takes ten seconds and prevents a very expensive mistake.
Overtightening a compression nut to stop a weep — The olive is already deformed to shape. Extra force distorts it further and usually makes the leak worse, or splits the fitting.

If it doesn't work

Steady flow with the valve closed

Cause: Debris or scale on the ball seat — Fix: Exercise it several times; replace it if it still passes.

Passes only on the hot side

Cause: Scale build-up, which is faster in hot water — Fix: Replace the valve; consider descaling the system if it is widespread.

Valve turns but nothing changes

Cause: Spindle sheared from the ball — Fix: Replace it — there is nothing to repair.

New valve weeps at the nut

Cause: Old olive reused or nut cross-threaded — Fix: Undo, fit a new olive, start the nut by hand and tighten a turn past hand-tight.

Questions people ask

Can an isolating valve be repaired?

Not practically. They are sealed low-cost ball valves with no serviceable parts. Replacement is quick and costs very little.

Should every fitting have one?

Ideally yes — every tap, toilet and appliance. It turns every future repair into a small local job instead of draining the house down.