Anti-Vacuum Valve Leaking? What It Does and How It Is Fixed
An anti-vacuum valve, also called a vacuum relief valve, lets air into a cylinder or a pipe if the pressure inside ever drops below the air pressure outside, then seals itself shut again when the water pressure returns. When one leaks, it is nearly always because a bit of grit or scale has lodged on its seal, or the seal has perished with age. It is often worse after the water has been turned off and back on. The fix is to clean or replace the valve. What you must not do is cap it, plug it or take it off, because on some cylinders it is the only thing stopping them being crushed by a vacuum. Put a container under the drip, keep the water off the electrics, and get it looked at.
What an anti-vacuum valve does
Water systems are built to hold pressure pushing outwards. Some parts of them are not built for the opposite: a vacuum pulling inwards. A vacuum can form when the cold supply is shut off, or the mains fails, and water keeps draining out of a pipe or a cylinder from a tap lower down. With nothing letting air in, the pressure inside drops.
In most pipework that just means gurgling taps. In a thin walled cylinder it can mean the vessel is pulled in on itself. On a vented cylinder, the open vent pipe running up to the loft does the job of letting air in, which is why draining one needs a hot tap open at the top, as our guide on how to drain a hot water cylinder explains. On a sealed, unvented cylinder there is no open vent, so some manufacturers fit or specify an anti-vacuum valve, usually on the hot outlet at the top. It is more common on copper unvented cylinders than on stainless steel ones, because copper is softer.
Anti-vacuum valves also turn up on the cold side. They are used where a hose or an outlet below the water level could otherwise siphon dirty water back into the mains, for example on some outside taps, bidets and appliance supplies. The principle is the same: let air in so water cannot be drawn backwards.
How to tell it is the anti-vacuum valve that is leaking
Water on top of a cylinder can come from several places, and they have different causes. The anti-vacuum valve is a small brass valve, usually no bigger than a thumb, often with a plastic cap or a row of small holes, sitting on the pipework at or near the top of the cylinder pointing upwards. Dry everything off with kitchen roll, then watch.
- A drip or a wet crust of scale at the holes or cap of that small valve: the anti-vacuum valve is weeping.
- Water coming out of the open funnel in the discharge pipe, usually lower down near the cylinder: that is the tundish, and it is a different fault, covered in our guide to a tundish dripping.
- Water from a valve with a lever or a twist cap on the cold inlet, or from the valve on the top of the cylinder that has a lever: that is a relief valve, and our page on a pressure relief valve leaking explains what it means.
- Water round the large plastic cap on the side or top of the cylinder: that is the immersion heater boss, not a valve.
- Water from a joint in the pipework, a nut or a soldered fitting: a pipe leak rather than a valve fault, and our page on a hot water cylinder leaking covers where those usually start.
Why they start to leak
The valve works with a small seal that lifts to let air in and is pushed shut by the water pressure. Anything that stops it seating perfectly will let water weep past.
- Debris. When the water has been off for work on the street or in the building and comes back on, grit and flakes of scale are carried through the pipes. One piece on the seal is enough.
- Scale. In hard water, and most of London has hard water, scale builds up on the seat of any valve that sits in hot water. The valve can stick slightly open.
- Age. The rubber seal hardens and loses its shape over the years.
- Pressure problems. A valve that weeps only when the hot water is heating, and then stops, may be seeing more pressure than it should. On an unvented cylinder that can point to a failing pressure reducing valve or an expansion vessel that has lost its charge.
What to do now
A weeping anti-vacuum valve is a slow leak, not an emergency, but it needs dealing with before the water finds its way into the electrics or the ceiling below the airing cupboard.
- Put a container or a folded towel under the drip and keep an eye on it.
- Keep water away from the immersion heater and its switch. If water is dripping onto them, switch the immersion off at its wall switch.
- Do not overtighten the valve or its nut with a spanner. That rarely stops a seal leak and can crack the body.
- Do not fit a blanking cap over it or replace it with a solid fitting.
- Take a photo of the valve and the label on the cylinder, and note whether the drip started after the water had been off.
If the drip is coming from a cylinder and the ceiling below is getting wet, turn off the water at the stopcock. Our guide to water escaping in the house covers the first few minutes.
Why it must never be capped
Capping a leaking anti-vacuum valve stops the drip and removes the protection. On a copper cylinder that relies on it, the next time the mains goes off and someone opens a hot tap downstairs, the cylinder can be pulled in. A deformed cylinder is not repairable. It has to be replaced, which turns a small valve job into a much larger one. That is the same logic that applies to any safety valve on a hot water system: a device that is there to protect the cylinder should be repaired or replaced, not disabled.
How it is fixed
Sometimes the valve can be cleaned. With the supply isolated, an engineer takes it apart, clears the debris or scale from the seal and seat, and refits it. If the seal is hardened or the body is scaled up, the valve is replaced with the correct type for that position on your system.
On a vented cylinder that is a small job. On an unvented cylinder, the hot side is at mains pressure, so the cylinder has to be isolated and the pressure released first, and anything that has made the valve weep, like a failing pressure reducing valve or a flat expansion vessel, should be checked while the engineer is there. That work should be done by a G3 qualified plumber, and it is part of our unvented cylinder repair service. If the cylinder is overdue a service, it makes sense to do both at the same visit, and our guide to what happens in an annual unvented cylinder service shows what is checked.
When to call us
Call us on 020 4652 5111 or send an enquiry. Tell us where the valve is, how fast it is dripping, whether your cylinder is vented or unvented if you know, and whether the water had been off shortly before it started. A photo of the valve and the cylinder label helps us bring the right part. We have 20 engineers across London, and our rate is on the pricing page and agreed before anyone attends.
Questions we get asked
- What does an anti-vacuum valve do on a hot water cylinder?
- It lets air into the cylinder if the pressure inside drops below atmospheric, for example when the mains is off and a hot tap below is opened. That stops a vacuum forming that could deform the cylinder.
- Why is my anti-vacuum valve dripping?
- Usually grit or scale on its seal, often after the water supply has been off and back on, or a seal that has hardened with age. A valve that only drips while the water is heating may be seeing too much pressure.
- Can I just cap off a leaking anti-vacuum valve?
- No. On a cylinder that relies on it, capping it removes the protection against a vacuum and can lead to the cylinder being crushed. Clean or replace the valve instead.
- Is an anti-vacuum valve the same as a pressure relief valve?
- No. A pressure relief valve opens to let water out when pressure is too high. An anti-vacuum valve opens to let air in when pressure is too low. They can look similar and sit near each other, but they do opposite jobs.
- Do all hot water cylinders have an anti-vacuum valve?
- No. Vented cylinders use their open vent pipe instead, and many stainless steel unvented cylinders do not need one. It depends on the cylinder and on the manufacturer's instructions.