Make it safe first: switch off the immersion heater and boiler, then shut the cylinder’s cold feed or the mains stopcock. A drip from the tundish or discharge pipe usually means a faulty valve or expansion problem, which is fixable. Water weeping from the cylinder shell means the tank is corroding, and replacement is the honest answer.
Hot Water Cylinder Leaking? Causes and What to Do
A hot water cylinder holds well over a hundred litres of water, usually in an airing cupboard surrounded by towels, floorboards and, in a flat, somebody else’s ceiling. So a hot water cylinder leaking, even slowly, matters: the difference between a tightened fitting and a collapsed ceiling is mostly how quickly you act and how accurately you diagnose it.
The encouraging news is that most cylinder leaks are not the cylinder at all. They come from the valves, fittings and pipework attached to it, and those are repairable. This guide walks through making the system safe, working out which type of cylinder you own, finding the actual source of the water, and making the repair-or-replace call without wasting money either way.
What this guide covers
First: make it safe
Before diagnosing anything, take the energy and the water out of the problem. A leaking cylinder full of hot water is not something to poke at while it is live.
Five-minute shutdown
- Switch off the heat sourcesTurn off the immersion heater at its wall switch and set the boiler so it is not heating the cylinder. Heating an emptying tank can damage the element and makes everything more dangerous.
- Shut the cold feedLook for a valve on the cold pipe entering the cylinder (usually low down). No valve, or a seized one? Shut the main stopcock instead. Our guide to finding your stopcock shows where to look, including in tenements.
- Relieve the pressure gentlyRun a hot tap to take pressure off the system while the leak is contained.
- Protect what is belowTowels and a shallow tray under the drip, and move anything stored around the cylinder. Check the floor and the ceiling underneath.
- Photograph everythingTime-stamped photos of the leak and any damage will matter if this becomes an insurance claim.
Vented or unvented: which cylinder you have
Diagnosis differs by cylinder type, so settle this first. A vented cylinder is the traditional arrangement: copper tank, fed by gravity from a cold water tank in the loft, with a vent pipe rising from the top. Common in older Scottish homes. A vented cylinder cannot build dangerous pressure, because the loft tank and vent pipe keep it open to the atmosphere.
An unvented cylinder (Megaflo is the name most people know) is fed directly from the mains at mains pressure. There is no loft tank. Instead it carries a cluster of safety controls: a temperature and pressure relief valve, an expansion vessel or internal air bubble, and a tundish (a small open funnel) on the discharge pipework. One honest warning here: unvented cylinders store pressurised hot water, and the safety controls exist to stop that becoming dangerous. Do not dismantle or adjust them yourself. Work on an unvented cylinder needs an engineer holding the specific unvented qualification, so limit your involvement to isolating it and observing.
Where a hot water cylinder starts leaking
Work down this list before condemning the tank. In our experience the fittings fail far more often than the cylinder shell.
| Leak point | What you see | Fixable? |
|---|---|---|
| Immersion heater boss | Weep or crust around the large circular fitting on the side or top | Yes: new gasket or careful re-seating, done with the cylinder drained |
| Pipe connections and valves | Drips at compression nuts, gate valves or the cold feed connection | Yes: usually a fitting tightened or replaced |
| Temperature and pressure relief valve | Water passing into the tundish or discharge pipe | Yes: valve reseated or replaced, but find out why it lifted |
| Heating coil connections | Boiler pressure falling with no visible drip; see below | Sometimes: depends on the cylinder |
| Cylinder shell (the tank itself) | Damp streaks from under the insulation jacket, rusty tracks, pooling with no wet fitting above | No: corrosion has pierced the metal, replacement time |
The shell verdict deserves emphasis. Once the tank wall itself has corroded through, there is no lasting repair. A pinhole that appears today will be joined by others, because the same corrosion is working on the rest of the metal. Budget for a new cylinder rather than paying someone to postpone it.
A dripping discharge pipe is a message, not a leak
On an unvented cylinder, water appearing in the tundish or dripping from the discharge pipe outside is the safety system doing its job: the cylinder is venting because pressure or temperature climbed too high. The usual culprits are a failed expansion vessel (nowhere for the expanding hot water to go) or a relief valve that no longer seats cleanly.
Treat it as an appointment, not a catastrophe. The cylinder is protecting itself, but it is also telling you a control has failed, and every day it discharges is heated water poured down a drain. An intermittent drip that starts when the water heats and stops when it cools points at expansion; a constant dribble points at the valve itself. Report what you observed to the engineer, because that pattern shortens the visit.
Is it really the cylinder?
Airing cupboards mislead people. They are full of pipework that has nothing to do with the cylinder: heating flow and return runs, cold feeds passing through, sometimes the loft tank overflow. A puddle at the cylinder base may have travelled from any of them, or dripped from a joint a metre above.
Condensation confuses things further. A cold mains pipe running past a warm cylinder will sweat in humid weather, producing convincing drips with no leak anywhere.
Dry everything thoroughly, lay kitchen towel around the base and along pipe runs, and check back hourly. The first wet spot on the towel points at the true source. If the towel stays dry but a damp patch keeps returning nearby, you may be looking at one of the signs of a hidden water leak rather than a cylinder problem.
One more impostor: if your cylinder is heated by the boiler through an internal coil, a fault on that coil or its connections shows up as the boiler losing pressure rather than as water on the floor. A falling gauge plus a healthy-looking cylinder is worth mentioning to whoever attends.
If the water keeps coming and you cannot trace it to any fitting, stop guessing. Call us on 07700 152 467 and we will locate the source with moisture mapping and thermal imaging before anything gets dismantled.
Repair or replace: making the call
Repair makes sense when
- The leak traces to a fitting, valve, immersion boss or connection
- The tank shell is sound, with no rust tracks under the jacket
- An unvented unit needs a control part (valve, expansion vessel) rather than a tank
Replacement makes sense when
- The shell itself weeps, streaks rust or has pinholed
- Repairs are becoming a subscription: this is the second or third callout
- The cylinder is decades old and parts are obsolete, so each fix costs a large fraction of a new unit
We would rather you spent once, deliberately, than three times in a panic. A failing cylinder rarely picks a convenient moment, and a planned replacement costs less than the same job done as an emergency with a soaked floor added to the bill.
The damage a slow weep does
A burst cylinder is dramatic but rare. The expensive version is the slow one: a weep under the insulation jacket that runs quietly down the back of the tank, into the floor, for months. Chipboard flooring swells and loses its strength, joists stain, and in flats the first person to notice is sometimes the neighbour underneath, staring at a mark on their ceiling. If water is already showing below, our guide on water coming through the ceiling covers the emergency steps.
Escape of water is one of the most common domestic property claims in the UK; the Association of British Insurers puts insurer payouts for it at £1.8 million every day. Cylinders and their pipework contribute a steady share of that. If a hidden weep has already caused damage, your buildings policy is likely to include trace and access cover, which pays for locating the source and making good the access; MoneySuperMarket reports that 94% of policies carry it. Our article on whether home insurance covers water leaks explains what is and is not included, and why insurers respond better to a documented leak than a mystery one.
Frequently asked questions
Is a leaking hot water cylinder an emergency?
It depends on the flow. A weep you can contain with a tray, after isolating the cold feed and heat sources, can wait for a booked visit. Water you cannot contain, a bulging ceiling below, or any leak near electrics is a same-day problem. When in doubt, isolate the cylinder and get advice by phone.
Why is water dripping from a pipe outside my house when the cylinder heats up?
That is the discharge pipe from an unvented cylinder’s relief valve. Dripping during heating usually means the expansion vessel has failed, so the expanding water has nowhere to go and vents instead. It is fixable, but it needs an engineer qualified on unvented systems rather than a DIY attempt.
Can a leaking hot water cylinder be repaired?
Often, yes. Leaks at the immersion heater boss, pipe connections, valves and relief fittings are all repairable at reasonable cost. The exception is the tank shell: once the metal itself has corroded through, repairs do not last, because the corrosion continues around any patch. A weeping shell means replacement.
Should I drain the cylinder if it is leaking?
Not necessarily. Isolating the cold feed and switching off the immersion and boiler stops the situation worsening in most cases, and a cylinder is heavy, awkward work to drain. Leave draining to the engineer unless the shell is failing quickly and water cannot be contained any other way.
Will home insurance pay for a leaking cylinder?
Policies generally cover the damage the escaping water causes, and trace and access cover pays for finding a hidden source. The cylinder itself is usually treated as wear and tear, so the replacement unit is normally your cost. Document the leak and the damage with photos before anything is repaired.
Related reading
- Where Is My Stopcock? How to Shut Off Your Water in an Emergency
- Water Coming Through the Ceiling: Emergency Steps That Limit Damage
- Does Home Insurance Cover Water Leaks? Escape of Water Explained
MCR Leak Detection provides professional leak detection across Scotland, including plumbing and heating systems.
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