Isolation Valve Leaking: Why It Happens and What to Do

What an isolation valve is and where you will find them

Isolation valves, also called isolating valves or service valves, are the small inline valves fitted to pipes feeding a single outlet. They let a plumber, or you, turn off one tap, toilet or appliance without shutting the whole house down. The common ones are brass or chrome, about the size of a thumb, with a slot on top that you turn a quarter turn with a flat screwdriver. Some have a small plastic or metal lever instead.

You will find them under kitchen and bathroom sinks, on the pipe into the toilet cistern, behind washing machines and dishwashers, on the pipes to a combi boiler, and on the outlets of the cold water tank in the loft. When the slot or lever lines up with the pipe, the valve is open. When it sits across the pipe, it is closed.

Where exactly is it leaking from?

Dry the valve and the pipe either side with kitchen roll, then watch it for a few minutes with the water on. Wrap a dry strip of tissue around each end and one around the top. The one that gets wet first tells you where the leak is.

• From the slotted spindle on top: the O-ring on the spindle is worn, hardened or was disturbed when the valve was turned. The valve needs replacing.
• From the nut at one end: the compression joint has loosened or the olive is not sealing on the pipe.
• From the body itself, with a crack or a green crust in the metal: the valve has corroded or split, usually from frost or hard water. Replace it and treat it as urgent.
• Water running down from above: check the tap, cistern or appliance connection above the valve before blaming the valve.

Why it started leaking after you turned it

This is the most common story. A valve sits open for ten or fifteen years with scale building up around the spindle. Someone turns it off to change a tap or a toilet part, and when it is turned back on, it starts to drip from the slot. The scale was helping to hold the old seal in place, and moving the spindle has broken that seal.

Turning it back and forth a few times occasionally lets the O-ring reseat, and it is worth trying gently. But a valve that has started weeping from the spindle rarely stops for good, and in London hard water it tends to get worse as more scale forms around the damaged seal. Our guide to hard water and hot water cylinders in London explains what that scale does across the rest of the system.

Do not force a valve that will not turn with a normal screwdriver. The slot is soft brass and the spindle can snap, leaving the valve stuck open and leaking. Turn the water off at the stopcock instead.

What you can safely try

For a leak at a compression nut, hold the valve body still with one spanner and nip the nut up with another by no more than a quarter turn. Holding the body stops you twisting the pipe or loading the joint at the other end. If the drip stops, leave it and check it again the next day. If it does not, stop tightening. An over-tightened olive distorts and leaks worse.

For a spindle leak, some valve types have a small grub screw or retaining nut around the spindle that can be tightened a little, but most modern ones do not. There is no reliable fix short of replacing the valve. In the meantime, put a container underneath and keep an eye on it, especially if it is behind a washing machine or in a cupboard where a slow leak will go unnoticed for weeks.

If the valve is on a toilet feed or under a sink, you can still use the outlet normally while you arrange a repair, as long as the drip is slow and contained.

When the valve will not turn off properly

A valve that turns but does not stop the water fully has scale or debris lodged in the ball or gate inside it. That is not a leak in itself, but it means the valve cannot do its job, and you will find out at the worst time. It needs replacing.

If you are trying to isolate a leaking tap, toilet or appliance and the isolation valve will not close, turn the water off at the stopcock. On a tank fed system, the hot water and some cold taps may come from the loft tank rather than the mains, so the valve on the tank outlet is the one to close for those. Our where is my stopcock guide covers the first, and our guide on how to turn off hot water supply from a cylinder covers the second.

Replacing a leaking isolation valve

The job is to turn the water off, drain down the pipe, undo the compression nuts, cut out the old olives or the pipe section they are on, fit the new valve and test it under pressure. Done properly, it takes well under an hour for a single valve in an accessible place. Awkward ones, behind a fitted appliance, low in a boxed-in space or on old pipework that has been painted over, take longer.

We usually suggest replacing any other old valves that are part of the same job at the same time, since the water is already off. If the valve is on a combi boiler pipe or the outlet of a cold water tank, draining down takes longer and the system needs refilling and bleeding afterwards.

If the valve is fine but the pipework either side of it is corroded or the olives will not come off cleanly, a short section of pipe is usually replaced with it. Our guide to a leaking pipe under the sink covers the joints around it, and stopcock leaking covers the main valve if that is the one giving trouble.

Is a leaking isolation valve an emergency?

A slow drip from the spindle usually is not, provided it is contained and you can check on it. It is urgent if the body is cracked, if water is dripping onto electrics or into a ceiling, or if the valve feeds something you cannot turn off another way. A leaking valve behind a washing machine or under a kitchen unit is also worth doing sooner, because the damage builds up where you cannot see it. If you are unsure whether it is spreading, our guide to water escaping in the house explains what to do first.

Getting it sorted

Call us on 020 4652 5111 or send an enquiry with a photo of the valve and where it sits. Replacing an isolation valve, or a few of them in one visit, is a routine job for our engineers, and we check the joints and connectors around it while we are there. We have 20 engineers across London, and our rate is set out on the pricing page and agreed before we attend.