Hot Water Cylinder Leaking? What to Do First and Where the Water Comes From

A leaking hot water cylinder is leaking from one of three places: the discharge pipe, a joint or fitting, or the cylinder itself. Water at the tundish or discharge pipe on an unvented cylinder means a safety device is working, and it does not by itself mean the cylinder has failed. Water weeping from a joint is usually repairable. Water coming out of the shell is the serious case, because a body leak cannot be fixed in place. Switch off, isolate and look before you decide which one you have.

What to Do First

Do these in order. They apply to vented and unvented cylinders alike.

1. Stay away from electrics that are wet. Immersion heaters, their wiring and the consumer unit are the hazard. If you can reach the immersion switch or the consumer unit without standing in water, switch the immersion off there. If water is near wiring or sockets and you cannot reach the switch dry, do not go in. Ask an electrician to isolate it.
2. Stop heating the water. Set the hot water to off on the programmer or boiler controls so nothing is adding heat.
3. Shut off the cold supply to the cylinder. On an unvented cylinder this is the isolation valve on the inlet group or the main stopcock. On a vented cylinder it is the valve on the cold feed from the loft tank, or the stopcock if you cannot find one.
4. Leave the discharge pipe open. Never plug or cap the tundish or its discharge pipe. It is there to stop the cylinder over-pressurising.
5. Contain the water. Move belongings, put towels and a bucket down and check the ceiling below for staining or sagging.
6. Do not drain a hot cylinder yourself. The water inside can scald.

If you smell gas near a gas boiler that heats the cylinder, turn the supply off at the meter if it is safe to do so and call the National Gas Emergency Service on 0800 111 999.

When It Is Urgent

Treat it as urgent, and get a qualified engineer, if any of these apply:

• Water is running from the body of the cylinder, not dripping from a pipe.
• The discharge pipe is running continuously, or very hot water or steam is coming from it. That can mean the cylinder is overheating, so make sure every heat source is off.
• Water has reached electrics, or is coming through a ceiling.
• You cannot close any valve that stops the flow.

If you cannot stop the water, shut the main stopcock and call a qualified plumber. A slow, intermittent drip from a joint or the tundish is not an emergency, but it needs looking at soon.

Vented or Unvented: Which Do You Have?

A vented cylinder is fed from a cold water tank in the loft and has a vent pipe running up over it. An unvented cylinder is fed directly from the mains, has no loft tank and carries a group of safety valves, an expansion vessel and a discharge pipe ending in a tundish, a small visible funnel. Where the water appears depends on which you have, so identify it first. Our guide to unvented cylinder repair or replacement covers the unvented kit in more depth.

Where the Water Comes From

The tundish and discharge pipe (unvented)

A few drips as the cylinder heats up are common and usually mean the expansion vessel has lost its air charge, so the extra volume goes down the discharge pipe instead. A steady flow points to a pressure or temperature relief valve that is passing, often from debris on the seat, or an inlet pressure that is too high. The valves are safety devices and should not be adjusted or tampered with. Work on an unvented cylinder is notifiable under Part G of the Building Regulations and needs an engineer with the right unvented qualification.

The immersion heater flange

The immersion heater screws or bolts into a boss on the cylinder, sealed with a gasket. Seepage around the flange is usually a tired gasket or a corroded boss. A gasket can be replaced after the cylinder is isolated and partly drained. A corroded boss means the cylinder itself has a problem.

Joints, valves and connections

Pipe connections on the top and sides, isolation valves and the cylinder thermostat pocket can all weep. So can the motorised valve that sends heat to the coil; our article on zone valve problems describes what to look for, and zone valve installation covers replacing one. Dry each joint, wrap tissue around it and wait. The first tissue to darken marks the source.

The heating coil

An indirect cylinder has a coil inside, through which the boiler's heating water passes. If the coil splits, heating water and stored water can mix. You may see no water on the floor at all. Signs include the heating system losing pressure with no visible leak, or discoloured hot water. Our piece on why a heating system will not hold pressure lists the other causes to rule out first.

The cylinder body

Water weeping from a weld, a seam or the shell itself means the vessel has failed. It cannot be repaired in place and the cylinder must be replaced. Size and type are worth reviewing at the same time; see our hot water cylinder size guide.

Check It Is Really the Cylinder

Airing cupboards hold other sources of water: cold pipes sweating with condensation, a leaking radiator valve, a stopcock or an overflow from the loft tank above. Dry everything and run the tissue test before you blame the cylinder. The method matches the one in our leaking radiator guide.

Repair or Replace?

A weeping gasket, a worn valve or a failed expansion vessel are repairs, and the cylinder carries on. A leak from the shell, a corroded boss or a failed coil usually means replacement. If the cylinder is old and has had several faults, replacing it is often the sounder choice. Our page on unvented cylinder installation in London explains what a replacement involves, including discharge pipework and Building Regulations. If the rest of the hot water and heating setup is tired, a central heating system upgrade may be worth considering at the same time. Whatever you choose, check controls: what heating controls does a house need lists what the cylinder should be wired to.

When to Call Us

If water is coming from a cylinder in London, isolate it as above and get in touch. We will look at where the water starts, tell you whether it is a repair or a replacement, and quote before any work begins. If the problem is a low flow after a repair, see low hot water pressure on an unvented cylinder.