Hot Water Cylinder Leaking? Causes and What to Do

Finding water pooling under your hot water cylinder, or dripping down the outside of the tank, is one of those moments that makes your stomach drop. The airing cupboard smells damp, the carpet on the landing is wet, and you are not sure whether you are looking at a small nuisance or the start of a flooded ceiling. The good news is that most cylinder leaks fall into a handful of predictable categories, and several of them are far less serious than they first appear. The trick is knowing how to tell them apart, and knowing which ones you can safely leave until morning versus which ones need dealing with right now.

This guide walks through where hot water cylinders actually leak, how to distinguish a genuine tank failure from a valve doing its job, the immediate steps to take to protect your home, and the situations where you should stop and call a professional rather than touch anything yourself. It applies to both older vented copper cylinders and modern pressurised unvented systems, and it flags clearly where the two behave very differently.

First, understand what kind of cylinder you have

Before you can diagnose a leak, it helps to know which type of cylinder is sitting in your cupboard, because they fail in different ways and carry very different risks.

Vented cylinders (the traditional setup)

A vented cylinder is the classic copper tank, usually wrapped in green or grey foam insulation, fed from a cold water storage tank in the loft. The water inside is at low pressure, driven only by the height of that loft tank. If a vented cylinder springs a leak, water tends to weep or trickle rather than spray. These systems have been the standard in British homes for decades, and many London properties, especially older terraces and conversions, still run them.

Unvented cylinders (pressurised systems)

An unvented cylinder, often sold under brand-neutral terms like a pressurised or mains-fed hot water cylinder, is typically a white or stainless steel tank connected directly to the cold mains. There is no loft tank. Because the water is held at mains pressure, an unvented cylinder stores a significant amount of energy, and it is fitted with several safety devices precisely to manage that. These systems are governed by Building Regulations, and by law they must be installed and serviced by a qualified engineer holding the relevant unvented hot water certification, usually referred to as a G3 qualification. This is not a formality. A neglected or wrongly repaired unvented cylinder is genuinely dangerous, which is why the DIY question comes up again and again on forums like r/DIYUK and DIYnot, and why the consistent, sensible answer there is the same: leave unvented cylinder faults to someone qualified.

Where hot water cylinders leak, and why

Water appearing near a cylinder does not always mean the tank itself has failed. In fact, in a large share of call-outs the tank is perfectly sound and the water is coming from a fitting, a valve, or a component bolted to the side of it. Working through the likely points in order will save you a lot of worry and, often, a lot of money.

1. Immersion heater gasket

Most cylinders have at least one immersion heater, an electric element screwed into a boss on the side or top of the tank, sealed with a rubber fibre gasket. Over years of heating and cooling, that gasket hardens and shrinks, and it eventually weeps. You will see water tracking down from the immersion boss, often with a little limescale crusting around it. This is one of the more benign leaks: the tank is fine, and the fix is usually a new gasket or a replacement immersion element. It still needs isolating properly because there is mains electricity involved.

2. Connections and fittings

Every cylinder has a cluster of pipe connections: cold feed in, hot draw-off out, and on many systems a coil for the central heating to warm the water. Compression fittings can loosen with thermal movement, and older soldered joints can develop pinholes. A leak here shows up at a specific joint rather than from the body of the tank. Sometimes it is as simple as a nut that has backed off slightly; sometimes a fitting has corroded and needs remaking. Either way, the copper of the cylinder itself is intact.

3. TPR valve or expansion valve discharge (often not a leak at all)

This is the big one for unvented systems, and the most commonly misread. Unvented cylinders are fitted with a temperature and pressure relief valve (often called a TPR or T&P valve) and an expansion relief valve. Their entire job is to release water safely if the pressure or temperature inside the cylinder rises too high. That released water is piped away through a discharge pipe, usually to an outside wall via a tundish. So if you see water coming from a pipe outside, or dripping into a tundish, the valve may simply be doing exactly what it was designed to do. It is a symptom, not the fault itself. Repeated discharge points to something else being wrong, a failed expansion vessel, a faulty valve, or excessive mains pressure, but the valve releasing water is a safety feature, not a hole in your tank.

4. Corrosion in older vented cylinders

Copper cylinders do not last forever. Over many years, and especially in hard-water areas, the tank wall can corrode from the inside or suffer pinhole failures. Once the body of a cylinder is leaking through the metal itself, there is no meaningful repair. Patching a corroded pressurised vessel is unsafe and pointless; the metal has thinned and further failures are only a matter of time. A tank that is weeping from its body, rather than from a fitting, has generally reached the end of its life and needs replacing.

5. Unvented cylinder component faults

Unvented systems carry extra parts that can fail: the expansion vessel (which absorbs the increase in volume as water heats), the pressure reducing valve on the incoming mains, and the internal thermostats and thermal cut-outs. When an expansion vessel loses its air charge, the system has nowhere to put the expanding water, so pressure climbs and the relief valve discharges. This is a classic case where water appears and the instinct is to blame the tank, when the real culprit is a serviceable component. Diagnosing and correcting it is qualified-engineer territory.

Leak point, likely cause and what to do

Where you see water	Likely cause	What to do
Around the immersion heater boss, with limescale crust	Perished immersion heater gasket or element	Isolate the immersion power and water; book a plumber to replace the gasket or element
At a specific pipe joint or nut	Loose or corroded compression or soldered fitting	Contain the drip; a plumber can remake or tighten the joint
From an outside discharge pipe or tundish	TPR or expansion valve releasing water (a safety function)	Not usually urgent, but repeated discharge needs investigating; call an engineer for unvented systems
Weeping from the body of an old copper tank	Corrosion or pinhole failure of the cylinder wall	Cylinder is at end of life; arrange replacement, it cannot be safely patched
Constant discharge plus poor hot water on an unvented system	Failed expansion vessel, faulty relief valve or thermostat	Stop, isolate, and call a G3-qualified engineer, do not attempt yourself
Steam, very hot discharge, or hissing on an unvented system	Possible thermostat and cut-out failure, overheating	Emergency: cut power and cold supply, keep clear, call a professional immediately

How to tell a genuine tank leak from valve discharge

This distinction saves people a fortune, so it is worth slowing down on. A genuine tank or fitting leak is continuous and localised. The water appears at the same spot, it tracks down the cylinder or gathers under a particular joint, and it keeps going regardless of what the system is doing. If you dry the area completely and the same patch is wet again within the hour, and it is coming from the metal or a connection, you are dealing with a real leak.

Valve discharge behaves differently. It is intermittent and it is piped away deliberately. On an unvented cylinder the relief valves are plumbed to a discharge pipe that runs to the outside of the building or into a tundish, so the water you notice is at the end of that pipe, not pooling under the tank. Discharge also tends to coincide with the cylinder heating up, because that is when pressure and temperature peak. If you find water outside near a small pipe low on the wall, or a periodic drip into a tundish that lines up with the heating cycle, that is very likely the valve doing its job rather than a failure of the vessel. It still deserves attention, because valves should not discharge repeatedly under normal conditions, but it is a different problem with a different fix.

A quick, safe check: trace the water back to its highest point. If the highest wet point is a fitting, a boss, or the tank wall, it is a leak. If the highest point is a valve outlet feeding a discharge pipe, it is discharge. Never loosen or cap a relief valve to stop it discharging. That valve is releasing water because pressure or temperature is too high, and blocking it removes the very safety device protecting the cylinder.

Immediate steps to take

If you have an active leak, working through these in order will limit the damage while you get help. The exact isolation points vary between systems, so if anything is unclear, isolate at the most upstream point you are confident about and call a professional.

1. Turn off the water supply to the cylinder. On a vented system, this usually means the gate valve on the cold feed from the loft tank, or the stopcock. On an unvented system, close the cold mains supply valve to the cylinder. Stopping the incoming water stops the tank refilling.
2. Turn off the heat source. Switch off the immersion heater at its dedicated switch or fuse spur, and if the cylinder is heated by a boiler, turn off the boiler or the hot water demand. A leak plus an active heat source is the combination you want to avoid, especially on pressurised systems.
3. Cut the power safely if the leak is near electrics. If water is running anywhere near the immersion element or its wiring, switch it off at the consumer unit rather than reaching for a wet switch. Do not touch electrical connections with wet hands.
4. Run off some hot water. Opening the hot taps draws down the level in the cylinder and relieves pressure, which can slow a leak while you organise a repair. On a vented system this also lowers the water sitting above a leaking fitting.
5. Contain and protect. Place a bucket or tray under the drip, lay towels to soak up spread, and move anything valuable out of the airing cupboard. If water is finding its way toward light fittings or into the ceiling below, treat it as more urgent.
6. Note what you have seen. Where the water is coming from, whether it is constant or intermittent, and which type of cylinder you have. This helps whoever attends bring the right parts and reach a diagnosis faster.

Why a corroded cylinder needs replacing, not patching

When the leak is coming through the wall of the tank itself, the honest answer is that the cylinder has failed and needs replacing. It is tempting to hope for a quick seal or a bit of solder, but a cylinder that has corroded to the point of leaking has thinned metal throughout, not just at the one visible spot. Repair a pinhole and another will appear nearby before long. On any pressurised or stored-hot-water vessel, a bodged patch is also a safety concern, because the repair is unlikely to hold under thermal stress. A new cylinder, correctly sized and installed, is the sensible outcome, and on an older system it often comes with better insulation and lower running costs as a bonus. The forums are consistent on this too: the general consensus among experienced posters on the DIY communities is that a leaking copper tank is a replacement job, not a repair.

Unvented cylinders: a safety line you should not cross

It is worth repeating because it matters. An unvented, mains-pressure hot water cylinder stores a large amount of energy, and its safety depends on relief valves, an expansion vessel and thermal cut-outs all working together. Working on one without the right knowledge and the G3 qualification is not just against Building Regulations, it is genuinely hazardous. The relief valve discharging, an expansion vessel that has lost its charge, a thermostat that has failed, these are all things a qualified engineer diagnoses and corrects safely. If your cylinder is unvented and it is leaking, discharging repeatedly, or making unusual noises, isolate it and call someone qualified. Do not loosen valves, do not cap discharge pipes, and do not try to top up or drain the system yourself.

When a leaking cylinder is an emergency

Not every cylinder leak is a middle-of-the-night crisis. A slow weep into a bucket, contained and away from electrics, can usually wait until a plumber can attend at a reasonable hour. Treat it as an emergency, though, if any of the following apply: water is coming through fast enough that you cannot contain it; it is running into light fittings, sockets or down through a ceiling; you have an unvented system that is discharging continuously, hissing, producing steam or very hot water; or you simply cannot find the isolation valve to stop the flow. In those cases the priority is to cut the water and power as best you can and get a professional out quickly, because the risk of structural water damage or, on a pressurised system, overheating, outweighs everything else.

If the leak has also left you without hot water, our guide on no hot water causes and emergency fixes covers what to check and how to keep things safe in the meantime. And if water has already spread beyond the cupboard, our water leak repair in London page explains how we trace and stop leaks with as little disruption as possible.

How we handle cylinder leaks in London

When you call us about a leaking cylinder, we keep it straightforward and honest. We give you a realistic arrival window rather than a vague promise, and we agree the price with you before we travel, so there are no surprises on the doorstep. Where we quote figures, we work from typical UK trade cost-guide ranges rather than pulling numbers out of the air, and once an engineer has seen the actual fault we confirm the cost before any work goes ahead. For unvented systems we send someone with the right qualification to work on them safely and legally. If you need someone out quickly, our emergency plumber in London service is set up for exactly this kind of call, isolating the leak, protecting your home, and putting things right properly rather than patching over a tank that has reached the end of its life.

A leaking hot water cylinder is rarely as catastrophic as that first wet patch suggests, but it does reward calm, methodical action: work out where the water is really coming from, isolate the water and heat, contain the mess, and be honest with yourself about whether you are looking at a simple gasket or a tank that is on its way out. Do that, and you turn a stressful surprise into a manageable job.