Why is there water under my hot water cylinder?
Six things that put water on the airing cupboard floor, how to tell which one you have from where the water is, and which of them is an emergency.
- Difficulty
- beginner
- Time
- 25 min
- Read
- 5 min
- Safety
- warning
Short answer
Find where the water is coming from before anything else: a joint, a valve, the immersion boss, the tundish, condensation, or the cylinder body itself. Joints and valves are repairable. Water from the tundish on an unvented cylinder is a safety device operating and needs an engineer. Water weeping from the cylinder body means the cylinder is finished.
An airing cupboard floor is a good place to spot a leak early and a bad place to ignore one, because the water goes straight into a ceiling. The useful thing is that a cylinder has only a handful of places water can come from, and each of them looks different. Ten minutes with a torch and some dry kitchen roll will usually tell you exactly which one you have.
Safety
What you'll need
- A torch
- Dry kitchen roll or tissue
- A bowl and towels
Step by step
- Make it safe first.Switch off the immersion heater at its double pole switch and turn the boiler's hot water off. Lift anything stored on the floor. If water is running rather than dripping, close the cylinder's cold feed valve — on a vented system that is the gate valve on the pipe from the loft; on an unvented system it is the isolating valve in the control group.
- Dry everything completely and lay tissue down.Wipe every pipe, joint and the cylinder body dry, then lay dry kitchen roll under each suspect point. Come back in an hour. The wet piece tells you where it is coming from far more reliably than guessing from a puddle.
- Check the compression joints and valves.Every olive joint on the pipes into and out of the cylinder, plus any isolating valve or drain cock. A weeping olive shows as a small bright drip forming repeatedly at the same point, often with a lime trail. These are the most common and the easiest to fix.
- Check the immersion heater boss.Where the immersion screws into the cylinder there is a large sealing washer. It hardens over the years and starts to seep, leaving water running down the outside of the cylinder from that point. It is repairable but the cylinder has to be drained.
- Check the tundish, if you have one.On an unvented cylinder, water at the tundish is a relief valve discharging, and that is a fault message, not a leak. Note whether it drips only when the cylinder is heating. Do not plug it — call an engineer with the unvented qualification.
- Rule out condensation.In a cold cupboard, a cold mains pipe can sweat heavily enough to puddle. Condensation appears as a fine film over the whole length of a cold pipe rather than a drip from one point, and it comes and goes with the weather. Lagging the pipe cures it.
- Check the cylinder body itself.Look for green or white corrosion trails on copper, or rust staining on a steel cylinder, particularly around the seams and the base. Water appearing from the body rather than from a fitting means the cylinder has perforated.
- Act on what you found.A weeping olive can be nipped up a quarter turn. An immersion washer needs the cylinder drained. A leaking cylinder body needs replacement and there is no repair. A discharging tundish needs an unvented engineer. Anything you cannot identify with the tissue test, photograph and show to a plumber.
Tips
- Put a shallow tray or a leak-detector mat under the cylinder once you have fixed it. The next one will announce itself before it reaches the ceiling.
- Photograph the pipework before you insulate or box anything in, so a future leak can be traced without pulling it all apart.
- A cylinder over about twenty-five years old that has started weeping anywhere is telling you to plan a replacement rather than chase repairs.
- If the airing cupboard has a wooden floor, check underneath it as well — water tracks along joists a long way before it appears downstairs.
If it doesn't work
Small persistent drip at a compression fitting
Cause: The olive has relaxed or was never fully compressed — Fix: Nip the nut up a quarter turn with two spanners, holding the fitting so you do not twist the pipe. If it still weeps after a second quarter turn, stop — the joint needs remaking with a new olive.
Water tracking down the cylinder from the immersion cap
Cause: The immersion sealing washer has hardened and is seeping — Fix: The cylinder must be drained to change it. Straightforward for a plumber, and worth doing at the same time as replacing an old element.
Water in the tundish, unvented cylinder
Cause: A relief valve is discharging — most often a failed expansion vessel — Fix: Not a leak and not a householder repair. Note the timing relative to heating and call an unvented-qualified engineer.
Whole cold pipe wet, worse in humid weather
Cause: Condensation on cold pipework in a warm cupboard — Fix: Lag the cold pipes. No plumbing repair needed and none will help.
Green or white deposits and water from the cylinder seam
Cause: The cylinder has corroded through — Fix: Isolate the cold feed, switch off all heat sources and arrange replacement. Perforated cylinders always get worse and often quickly.
Water only after the heating has been on
Cause: A leak that only shows when the system is hot and pipework has expanded, or a heating-side joint rather than a hot water one — Fix: Repeat the tissue test straight after a heating period, which is when it will be wet. Expansion leaks usually come from a joint under strain.
Questions people ask
Can a leaking cylinder be repaired?
No. Copper and steel cylinders that have perforated cannot be reliably patched, and any repair will fail. Once the body is leaking, the cylinder is replaced.
Is the water dangerous?
It can be very hot, and it may be running near electrical connections at the immersion. Switch the immersion off and treat the water as scalding until you have felt it carefully.
How do I know if it is the cylinder or the pipework above it?
Water tracks downwards and sideways along pipes, so a puddle at the base proves nothing. The dry-and-tissue test is the only reliable way to find the actual source.