The short answer: A hot water cylinder usually leaks because of a corroded tank or pinhole, a perished immersion heater gasket, or a faulty valve. On an unvented cylinder, water dripping from the tundish points to the expansion vessel or relief valve. Switch off the supply and heating, then call a professional.
A hot water cylinder is one of those things you barely think about until the day you open the airing cupboard and find a damp patch, a rusty stain or a puddle on the floor. It is an unsettling moment, partly because the cylinder is often tucked away where water can do real harm before anyone notices. The good news is that most cylinder leaks come down to a handful of common causes, and once you know what you are looking at, the next steps become a lot clearer. This guide walks through why hot water cylinders leak, the damage a slow leak can cause, how a hidden leak is traced, and when it is time to bring in a professional.
Why a hot water cylinder leaks
Cylinders fail in fairly predictable ways. Working out which part is leaking is the key to understanding how serious the problem is and what the repair is likely to involve. Here are the causes we see most often.
Corrosion and pinhole leaks
Most cylinders are made of copper or stainless steel, and over years of holding hot water they can corrode from the inside out. Hard water, which is common across much of Devon, speeds this up by leaving scale and encouraging the slow breakdown of the metal. Eventually a tiny pinhole can form in the wall of the tank, weeping a steady trickle that often looks worse than the size of the hole suggests. A pinhole or any corrosion through the body of the cylinder is generally not repairable. In most cases the cylinder has reached the end of its life and needs replacing rather than patching.
The immersion heater gasket
Where the immersion heater screws into the cylinder there is a rubber sealing washer, often called the immersion gasket or flange gasket. Heat and time make this washer harden and lose its grip, and once it perishes water can seep out around the immersion boss. If you notice water around the immersion heater or running down from that fitting, a tired gasket is a likely culprit. This is one of the more straightforward fixes, since the washer can usually be replaced, though the work involves draining the cylinder and isolating the electrics first.
Valves and fittings
Not every leak comes from the tank itself. The compression fittings, isolation valves and pipe connections around the cylinder can loosen, corrode or fail over time. A drip from a joint at the top or base of the cylinder is often a fitting rather than the vessel, which is reassuring because a connection can usually be remade or replaced without changing the whole cylinder. It is worth tracing the water carefully, because a drip running down the outside of the tank can easily look as though the cylinder body has gone when the real source is a fitting higher up.
The relief valve and discharge pipe on unvented cylinders
Many modern homes have an unvented cylinder, which is fed directly from the mains and runs under pressure. These systems have built-in safety devices, including a pressure relief valve, a temperature and pressure relief valve, and an expansion vessel that absorbs the extra volume as water heats and swells. They also have a tundish, a small open funnel in the discharge pipe that lets you see when water is being released. Water dripping through the tundish is not a leak in the usual sense. It is a sign that one of those safety devices has activated or failed. A common cause is an expansion vessel that has lost its air charge, which leaves the rising pressure with nowhere to go, so it escapes through the relief valve and out via the tundish. High incoming mains pressure can do the same thing. Because these components are there to keep a pressurised system safe, faults on an unvented cylinder should always be handled by a qualified engineer holding the relevant unvented certification.
The damage a cylinder leak can cause
Cylinders are usually sited in an airing cupboard, a loft or a first-floor space, which means a leak often soaks through to the room below before it is spotted. Even a slow weep can keep a structure damp day after day, and the warm water involved tends to make things worse than a cold leak would. Over time you can see swollen or stained ceilings, lifting floorboards, peeling paint, blown plaster and a musty smell that hints at damp taking hold. Left unchecked, persistent moisture can affect joists and timber and create the conditions for mould. Catching a cylinder leak early keeps the repair smaller and limits the knock-on damage to floors, ceilings and decoration.
Signs your cylinder may be leaking
- A damp patch, rust stain or puddle near the base of the cylinder
- Water around the immersion heater or running down the tank
- Dripping from the tundish or discharge pipe on an unvented cylinder
- A ceiling stain or musty smell in the room below the airing cupboard
- Hot water running short or the system losing pressure
What to do first if you find a leak
If you discover a leak, a few calm steps will limit the damage while you arrange help. Turn off the water supply to the cylinder, usually at the isolation valve on the cold feed, or at the stopcock if you are unsure which valve controls it. Switch off the heating to the cylinder, including the immersion heater at the wall and the boiler if it is heating the water. Place a bucket and some towels to catch the drips and protect the floor. Then leave the diagnosis and repair to a professional. There is an important safety point here, because immersion heaters and their wiring sit right next to the water. Do not poke around electrical connections on a wet cylinder, and if anything looks damaged or you have any doubt, keep the power off at the consumer unit until it has been checked.
How a hidden cylinder leak is located
Sometimes the source is obvious, but cylinders sit in tight cupboards surrounded by lagging, pipework and boards, and water has a habit of travelling along a joist or pipe before it appears. When the visible damp does not match an obvious fault, locating the true source without ripping the cupboard apart is exactly where careful leak detection earns its place. Two methods do most of the work, and they complement each other well.
Thermal imaging reads the surface temperature of walls, floors and pipework. Because the water in a cylinder system is hot, a leak tends to leave a telltale warm trail that a thermal camera can pick out against the cooler surroundings, helping to narrow down the area without lifting a single board. Acoustic detection then takes over for the precise location. Water escaping a pressurised pipe makes a distinctive sound, and sensitive listening equipment can pin down where that noise is loudest, which is particularly effective on the copper pipework common in cylinder installations. Used together, these techniques let us find the source with the minimum of disruption, so any opening up that is needed can be small and targeted rather than wholesale. You can read more about how this works on our water leak detection service page.
When to call a professional
A cylinder leak is rarely a job to leave for another day. If you can see active dripping, a spreading damp patch, a stained ceiling below, or water near the immersion heater, it is worth getting it looked at promptly before the surrounding structure suffers. Anything to do with an unvented cylinder, including persistent discharge through the tundish, should go to a suitably qualified engineer because of the pressures involved. And where the damp is clearly there but the source is hidden, professional leak detection will trace it accurately and spare you from guesswork and unnecessary damage. If you are in Devon and unsure what you are dealing with, our team is happy to talk it through and locate the problem for you. You can see the areas we cover across Devon, or browse more guidance in our articles.
Frequently Asked Questions
It depends on where the leak is. A perished immersion gasket or a loose fitting can often be repaired. A pinhole or corrosion through the body of the tank itself usually cannot be patched reliably, and in that case the cylinder has typically reached the end of its life and needs replacing. A professional can confirm which it is.
A dripping tundish means a safety device has activated or failed, rather than the tank leaking. A common cause is an expansion vessel that has lost its air charge, or high mains pressure forcing water through the relief valve. Because unvented systems run under pressure, this should be checked by a qualified engineer.
Water and the immersion heater wiring sit very close together, so caution is sensible. Switch off the immersion heater and, if anything looks damaged or you are unsure, isolate the power at the consumer unit. Avoid touching electrical connections on a wet cylinder and have it inspected before the power goes back on.
We typically combine thermal imaging, which highlights the warm trail left by hot water, with acoustic detection that listens for the sound of water escaping pressurised pipework. Used together they narrow down and then pinpoint the source, so any opening up can be small and targeted rather than wholesale.
Many home insurance policies include cover for the damage caused by an escape of water, and some also contribute towards tracing the leak. Cover varies, so check the wording with your insurer. A clear leak detection report can help support a claim by documenting the source and the affected area.
Worried about a leaking cylinder?
We trace hidden water leaks across Devon with minimal disruption and tell you exactly what we find. Get in touch for a clear, no-pressure quote.