Why Is My Cylinder Leaking? Causes and Next Steps
A puddle beneath a hot-water cylinder is not something to watch and wait on. If you are asking, “why is my cylinder leaking?”, the cause could be as minor as condensation or as urgent as a fault in the pressure-relief system. Acting early can limit water damage, protect the electrical components and prevent a small repair becoming a cylinder replacement.
First, establish where the water is coming from. Dry the visible pipework and casing with a towel, then check again after the cylinder has heated or after hot water has been used. Water often runs along pipes and collects at the lowest point, so the first damp area you spot is not always the source.
Why is my hot-water cylinder leaking?
Most domestic electric cylinders are either vented or unvented. A vented cylinder is supplied from a cold-water storage tank, usually in the loft. An unvented cylinder is connected directly to the mains and stores water at pressure. The diagnosis and safety requirements differ, but a leak should always be investigated promptly.
A cylinder can leak from its connections, valves, immersion-heater opening, internal tank or discharge pipework. It may also appear to leak when moisture has formed on the outside of the unit. The location, timing and volume of water provide useful clues.
A leaking pipe connection or fitting
This is one of the more common and usually more straightforward causes. Compression fittings, flexible hoses, isolation valves and pipe joints can begin to seep as seals age or after pipework has been disturbed during maintenance. You may see a bead of water at a nut, green staining on copper pipework, or a slow drip that becomes more noticeable while the cylinder is heating.
A competent plumber can often repair a localised connection without replacing the cylinder. Do not keep tightening fittings at random, particularly on an unvented installation. Overtightening can damage a fitting or conceal the real problem.
Water from the immersion heater or thermostat pocket
Electric hot-water cylinders use one or more immersion heaters. Water around the immersion boss, element plate or thermostat pocket may point to a failed gasket, corroded component or loose fitting. This can be more likely on an older cylinder, especially in hard-water areas where limescale accelerates wear.
Treat this seriously. Water and electrical equipment are not a safe combination. If there is water near the immersion heater wiring, do not touch the controls or remove covers. Isolate the electrical supply only if you can do so safely from a dry position, then arrange for a qualified engineer to assess it.
A pressure-relief valve discharging water
Unvented cylinders have safety valves designed to release water if pressure or temperature becomes too high. The discharge pipe often terminates outside, or feeds through a visible tundish near the cylinder. A tundish is a small fitting with an air gap that lets you see whether water is discharging.
An occasional small discharge can happen in some circumstances, but repeated dripping or a steady flow is not normal. It can indicate excessive mains pressure, a failed expansion vessel, a faulty pressure-reducing valve, a temperature and pressure relief valve operating, or an overheating control fault.
Never block, cap or alter a discharge pipe to stop the water. It is a critical safety route. Work on an unvented cylinder and its safety controls must be carried out by an engineer qualified to work on unvented hot-water systems, commonly referred to as G3 qualified.
Condensation on the cylinder casing
Not every wet cylinder is leaking. Condensation can form when a cold cylinder or cold pipework sits in a warm, humid cupboard, particularly after showers, laundry drying or poor ventilation. The moisture is usually spread across the outer casing rather than starting at one fitting or valve.
Modern insulated cylinders should not normally produce heavy condensation. If water keeps returning after you dry the unit, check for damp insulation, a cold-water pipe sweating above the cylinder, or a small hidden leak. Improving ventilation and insulating exposed cold pipework may help, but only once you are sure the cylinder itself is sound.
Corrosion or a failed internal cylinder tank
Water seeping through the cylinder body, staining around a seam, or collecting beneath the insulation without an obvious pipework source can indicate internal corrosion. On some products, a sacrificial anode helps protect the tank. Once corrosion has compromised the cylinder wall, a durable repair is rarely practical.
Internal tank leaks commonly mean replacement is the sensible option. The right replacement depends on your household's hot-water demand, available space, incoming water pressure and existing system type. A compact electric cylinder may suit a kitchen, en-suite or annexe, while a family home often needs a larger capacity and correctly matched safety components.
What to do if your cylinder is leaking
The safest response depends on the amount of water and its source. A few drops at a visible compression fitting need attention, but active water near electrical connections, a flowing discharge or a rapidly growing puddle should be treated as urgent.
Take these steps before arranging a repair:
- Keep clear of wet electrical controls, immersion wiring and any exposed connections. Do not remove cylinder covers to investigate.
- If safe to do so, switch off the cylinder's electrical supply at the consumer unit or local isolator. Do not touch an isolator if the surrounding area is wet.
- For a significant leak, turn off the cold-water supply to the cylinder if you know where the correct valve is and can reach it safely. This may be the cylinder's inlet isolation valve or the property's main stopcock.
- Place towels or a shallow container beneath a small, contained drip to protect flooring, but do not interfere with safety discharge pipework or valves.
When to call an engineer straight away
Arrange urgent professional help if water is coming from electrical parts, the cylinder casing is visibly bulging, the temperature and pressure relief valve is discharging continuously, or you cannot stop a substantial leak. The same applies if there is no hot water alongside unusual noises, burning smells or repeatedly tripping electrics.
For an unvented cylinder, use an appropriately qualified engineer. These systems store water under mains pressure and rely on specific safety controls. A repair that appears simple can affect the expansion provision, discharge arrangement or relief-valve operation if it is not completed correctly.
Landlords should act quickly too. A leaking cylinder can cause damage to a tenant's belongings and may leave the property without reliable hot water. Keeping a record of the fault, the repair and any replacement work is good property management and helps demonstrate that the issue was addressed without delay.
Can you keep using a leaking hot-water cylinder?
It depends entirely on the cause. A confirmed trace of condensation or a very minor external pipe seep may allow limited use until a repair visit, provided there is no risk to electrics and the leak is contained. However, using a cylinder with an unidentified leak is a gamble. Heating cycles can increase pressure and turn a slow drip into a more substantial problem.
Do not continue using the cylinder if a safety valve is operating continuously, water is close to electrics, or the tank itself appears to be leaking. Switching it off and seeking advice is the safer option.
Choosing a replacement if the cylinder has failed
If corrosion or an internal tank failure has been confirmed, replacement is an opportunity to fit a cylinder that better matches the property. Capacity matters, but it is not the only consideration. Think about the number of occupants, baths versus showers, recovery time, available cupboard height, pipe connections and whether your electrical supply can support the immersion-heater rating.
For homes moving away from gas, an electric hot-water cylinder can provide dependable stored hot water with straightforward control. Where energy use is a priority, an air-source heat-pump water heater may be worth considering, provided the installation space and ventilation requirements are suitable. Modern Heat supplies official TESY products across compact and high-capacity domestic hot-water applications, with UK-based support available when specifications need checking before purchase.
A cylinder leak is inconvenient, but the right response is usually clear: identify the source without dismantling anything, make the area safe, and bring in the right professional where pressure or electrics are involved. Fast action protects the home now and gives you the time to choose a repair or replacement that will deliver reliable hot water for years to come.