Switch off the immersion heater and the boiler heat to the cylinder, then close the cold feed valve so it stops refilling. A weep at the immersion boss or a pipe fitting is usually a small repair. Water from the cylinder body means it has failed. On an unvented cylinder, a constantly dripping tundish is a safety fault for a G3 engineer.
You open the airing cupboard for a towel and the floor is damp, or there is a rusty tide mark around the base of the cylinder, or a small funnel on a pipe beside it is dribbling. It is unsettling, because the cylinder is usually above a bedroom or hallway and holds well over a bath’s worth of hot water. The reassuring part is that cylinders fail in a handful of predictable ways, and where the water appears tells you most of what you need to know. This guide covers how to make it safe, how to tell your cylinder type, what counts as a drip and what counts as a failure, and when the water is not from the cylinder at all.
Make it safe before you look for the cause
- Switch off the immersion heater. There is usually a switch or fused spur on the wall beside the cylinder. If water is on or near the immersion wiring, leave the wiring alone and turn the circuit off at the consumer unit instead.
- Turn off the boiler’s heat to the cylinder. Set hot water to off on the programmer, or switch the boiler off if you are not sure.
- Close the cold feed. Look for a valve on the pipe running into the bottom of the cylinder, a small lever or a wheel. Closing it stops the cylinder refilling as it empties. If you cannot find it, use the main stopcock.
- Catch the water and clear the cupboard. Towels, a bowl, and everything stored on the floor out of the way. Photograph the water and the date before you mop.
Do not open a drain valve or any fitting on the cylinder yourself, and do not try to bleed or reset anything on a pressurised cylinder. The steps above are safe on every type. Anything beyond them depends on which type you have.
Vented or unvented: know which cylinder you have
A vented cylinder is the traditional arrangement. It is fed from a cold water tank in the loft, an open vent pipe rises from the top of it back up to that tank, and it is not under mains pressure. Most older Cornish houses with a loft tank have one. If yours is leaking, the culprits are the immersion seal, the connections or the cylinder itself, and a competent plumber can deal with all of them.
An unvented cylinder is fed straight from the mains and runs under pressure. You will see an expansion vessel (a round steel tank, often red or white) nearby, a pressure reducing valve on the incoming pipe, and a tundish, which is a small open funnel fitted into a pipe that runs from the safety valves to outside. Because stored hot water under pressure needs its safety devices working, Building Regulations require installation and repair work on an unvented cylinder to be carried out by an engineer with a G3 qualification. You can switch it off and isolate the feed. Everything else is theirs.
The tundish: a drip that is not a leak, until it is
The tundish exists so you can see when the safety valves have released water. A few drops there now and again, usually just after the water has heated, is the system working. Water expands as it warms, and if the expansion vessel cannot absorb all of it the relief valve lets a little out.
A tundish that drips steadily, runs every time the water heats, or has a limescale crust building under it is telling you something has failed. The most common cause is an expansion vessel that has lost its air charge, so the pressure has nowhere to go but the relief valve. A tired relief valve or a pressure reducing valve passing too much pressure does the same. None of these are the cylinder leaking, and none of them are a job to ignore or, worse, to solve by capping the discharge pipe. Book the G3 engineer.
Where cylinders actually leak
Dry the whole cylinder with a cloth, then watch where the water reappears. A drip running down the outside can look like a body leak when it started at a fitting above.
| What you notice | Most likely cause | What to do |
|---|---|---|
| Damp ring or drip below the immersion heater boss | Perished immersion gasket or a loose immersion | Plumber drains the cylinder and replaces the gasket |
| Drip from a nut on the hot outlet, cold feed or heating coil connections | Compression joint weeping | Gentle nip on the nut, or the joint remade |
| Drip from the drain valve at the base | Washer failed in the drain cock | Replace the drain valve |
| Weep from the thermostat pocket or a boss you cannot name | Seal failed at that fitting | Plumber to reseal or replace |
| Water from the cylinder wall or a seam, rust staining, a bulge | Corrosion pinhole or split | Replace the cylinder |
| Steady drip from the tundish on an unvented cylinder | Expansion vessel or relief valve fault | G3 engineer |
The immersion gasket is the one we see most. The rubber washer where the immersion heater screws into the tank hardens over years of heat and eventually lets water creep out around the boss. It is a cheap part, though changing it means draining the cylinder. Fittings come next. A cylinder body that is weeping through the metal cannot be patched reliably, and once a copper cylinder has corroded through in one place it tends to go elsewhere soon after, so replacement is the honest answer.
When the puddle is not the cylinder at all
Water travels. It runs along pipes, follows the fall of a floor and pools at the lowest point, which in an airing cupboard is often under the cylinder. If you have dried everything, watched, and the cylinder and its fittings stay dry while the floor keeps getting wet, the source is somewhere else: a heating pipe under the boards, the cold feed inside the wall, or a pipe from the bathroom next door.
A useful clue is the boiler gauge. If the heating pressure has been dropping at the same time, the leak may be on the heating side, and our guide to why a boiler keeps losing pressure explains the other things to rule out. If the loft tank has been overflowing as well, read our article on a cold water tank in the loft overflowing or leaking, because a failed coil in the cylinder can push heated water back up into the tank.
This is the point where opening up the cupboard floor on a hunch starts costing money. Our water leak detection survey uses a thermal camera to pick up the warm trail hot water leaves under a floor or behind a wall, then acoustic listening to pin down the exact point on the pipe, so any opening up is small and in the right place.
The room below, and your insurance
Because the water is warm, a cylinder leak does more harm per litre than a cold one. It dries plaster unevenly, lifts floorboards, and encourages mould in a cupboard that is already warm. A ceiling stain in the room below the airing cupboard, a musty smell on the landing, or a hot water supply that keeps running short are all reasons to act this week rather than next month.
Most UK buildings policies include trace and access cover, which pays for finding a hidden leak and making good whatever had to be opened to reach it, usually up to a limit in the region of £5,000 to £10,000. It does not pay to replace the failed cylinder or gasket. Our guide to what trace and access insurance covers explains the difference, and a written survey report with photographs and moisture readings is what a loss adjuster wants to see.
A note for Cornish homes and holiday lets
Cornwall’s soft water means limescale rarely shortens a cylinder’s life here, but age does. Many cob and granite cottages between Truro and Penzance still have a vented copper cylinder that was fitted decades ago, squeezed into a cupboard with the pipework buried in a solid floor. Holiday lets and newer builds around Falmouth and Newquay tend to have unvented cylinders, which run harder in a busy changeover week than in a family home, and their manufacturers recommend an annual service of the safety valves and expansion vessel. If your let sits empty over winter, isolate the cold feed and switch the immersion off before you lock up. And if you are unsure whether the water is the cylinder or something behind it, we provide leak detection across Cornwall and can usually survey within the week.
Frequently asked questions
Is a leaking hot water cylinder an emergency?
Treat a steady or growing leak as urgent, because the cylinder sits above other rooms and holds a lot of hot water. Switch off the immersion heater and the boiler heat, close the cold feed valve, and catch the water. A few occasional drips from the tundish on an unvented cylinder can be normal, but anything from the body or fittings needs an engineer.
Why is water dripping from the tundish on my unvented cylinder?
An occasional drip as the water heats is the safety valve releasing expansion, which is normal. A tundish that drips constantly or every heating cycle usually means the expansion vessel has lost its air charge or the relief valve has failed. Because the cylinder is pressurised, that must be checked by a G3 qualified engineer.
Can a leaking cylinder be repaired or does it need replacing?
It depends where the water is coming from. A perished immersion gasket, a weeping compression fitting or a failed drain valve can all be repaired. A leak through the wall or a seam of the cylinder itself cannot be patched reliably, and in that case the cylinder has reached the end of its life and should be replaced.
The cylinder looks dry but the cupboard floor keeps getting wet. What now?
Water tracks along pipes and floors before it pools, so the puddle is often not under the leak. If the cylinder and its fittings stay dry after you have wiped them, the source is likely a heating or supply pipe under the floor or in the wall. Non-invasive leak detection can find it without opening the cupboard up on guesswork.
Will home insurance cover a leaking hot water cylinder?
Most buildings policies cover sudden escape of water damage, and many include trace and access cover for finding a hidden leak and making good the access. The failed cylinder, gasket or valve itself is usually treated as wear and tear and is not covered. Check your own wording, keep photographs and ask for a written report.
Wet cupboard, dry cylinder?
If you have wiped everything down and the water keeps coming back, we will find where it is really escaping before anything is opened up, and give you the report your insurer needs for a trace and access claim.
