Hot Water Cylinder Leaking: Causes, Safe Checks and Repair vs Replace

Why Hot Water Cylinders Leak: The Six Most Common Causes

Understanding where a leak is most likely to originate helps you describe the problem accurately to an engineer and assess how serious it might be. Here are the causes in rough order of frequency.

• Loose or corroded pipe connections. Compression fittings, flexible hoses, isolation valves and solder joints all degrade over time, especially in hard-water areas. You may notice a bead of water at a nut, green staining on copper pipework, or a slow drip that gets worse while the cylinder is heating. A plumber can often fix a localised connection without touching the cylinder itself.

• Faulty immersion heater gasket. If your cylinder has an immersion heater, the rubber or fibre gasket sealing the element to the tank wall can harden and crack with age, allowing water to weep around the heater boss. This is one of the more repairable faults if the cylinder body itself is sound.

• Pressure relief valve or temperature and pressure relief valve (T&PRV) dripping. Every unvented cylinder has a T&PRV and an expansion (pressure relief) valve as safety devices. A persistent drip from the discharge pipe — usually a copper or plastic pipe running to an outside wall or tundish — normally means the system pressure is too high, the cylinder thermostat is letting water overheat, or the valve seat itself has worn or scaled up. Crucially, replacing the valve without diagnosing why it is opening will not solve the problem.

• Expansion vessel failure. On unvented systems, the expansion vessel absorbs the extra volume of water as it heats up. If the vessel's internal bladder fails or it becomes waterlogged, pressure spikes occur on every heating cycle, causing the T&PRV to weep repeatedly. Engineers always check the expansion vessel first before condemning the safety valve.

• Sediment buildup and internal corrosion. Over many years, mineral deposits accumulate at the base of the cylinder. This sediment acts as insulation, creates hot spots and accelerates corrosion of the tank wall. Once the metal is breached, water begins to escape — often from the bottom of the unit. Discoloured or rusty water from the hot tap is a strong warning sign.

• General age-related tank corrosion. Steel cylinders with a glass or enamel lining eventually suffer liner cracking, particularly in hard-water areas. Constant thermal expansion and contraction — every time the cylinder heats and cools — stresses the lining over decades. Once the tank body itself is leaking, repair is almost never the right answer.

Hot Water Cylinder Leaking from the Bottom: Take It Seriously

A leak from the very base of a hot water cylinder deserves extra attention. The bottom of the unit is where cold water enters, where the greatest heat stress concentrates, and where years of sediment sit in direct contact with the metal. A drip from a pipe fitting at the base might be a simple compression joint repair. But if water is seeping through the tank body itself — through a pinhole, a crack in the lining or a corroded seam — the structural integrity of the vessel is compromised.

On an unvented cylinder, the water inside is at mains pressure and high temperature. A worsening body leak is not something to monitor and revisit in a few weeks. Turn off the cold water supply to the cylinder, switch off the immersion heater or the boiler's hot water circuit, and call a G3-qualified engineer promptly.

Patching a corroded or cracked cylinder body is not a safe or lasting repair. If an engineer confirms the tank itself is the source, replacement is the correct course of action.

Pressure Relief Valve Leaking: What It Means

The pressure relief valve — or combined temperature and pressure relief valve — is a deliberately weak point in the system. It is designed to open and discharge water safely when pressure or temperature climbs beyond safe limits, preventing a far more dangerous uncontrolled failure. So a dripping discharge pipe is not always a sign the valve is broken; it may be doing exactly what it should.

The key questions are: is the discharge intermittent (during heating cycles) or continuous? And is the water scalding hot when it discharges?

Intermittent dripping during heating usually points to an over-pressurised system — a failed expansion vessel is the most common culprit. Continuous or hot discharge suggests the cylinder thermostat or high-limit energy cut-out is not controlling temperature correctly, which is a safety-critical fault.

Homeowners should check that the hot water thermostat is set to around 60°C (the recommended temperature for legionella control and safe operation in most UK homes) and that the cylinder is not running noticeably hotter than usual. Beyond that, diagnosing and rectifying a repeatedly opening PRV requires a G3-qualified engineer — do not simply swap the valve without identifying the root cause.

Mineral buildup on the valve seat can also cause a small but persistent drip even when pressures are normal. In this case, an engineer may be able to cycle or replace the valve, but again, only after confirming the system pressure and temperature controls are functioning correctly.

Is It Condensation or a Real Leak?

Before assuming the worst, it is worth checking whether what you are seeing is condensation rather than an active leak. In a cool airing cupboard, particularly in winter, a cold incoming water pipe can attract moisture from the air, leaving a damp patch on insulation or a puddle on the shelf below.

The simple test: dry all visible pipework and the cylinder casing thoroughly with a towel. Leave it for a couple of hours — ideally through a full heating cycle — then inspect again. Genuine leaks leave a wet trail or a drip you can trace back to a specific fitting or point on the tank. Condensation tends to form evenly on cold surfaces and disappears when the ambient temperature rises.

If the damp returns and you can trace it to a specific fitting, joint or area of the tank, you have a real leak to investigate further.

What You Can Safely Check Yourself

There are a handful of observations and checks that are safe for any homeowner to carry out before calling an engineer. Knowing what you have found will help the professional diagnose the fault faster.

• Locate the source. Dry the pipework and cylinder casing, then observe after a heating cycle. Remember that water runs downhill along pipes, so the first wet patch you see may not be directly above the leak origin.

• Check the discharge pipe. Find where the pressure relief valve discharge pipe terminates — usually at an outside wall, a tundish above a gulley or an internal drain. Is it dripping or running with water? Note whether it is hot or cold, and whether it is continuous or only happens during heating.

• Look at pipe fittings and joints. Check compression nuts, flexible connectors and solder joints for green or white staining, mineral deposits or visible moisture. Do not attempt to tighten compression fittings unless you are confident doing so — overtightening can split an olive and make the leak worse.

• Check the immersion heater. With the immersion heater switched off at the fuse spur, look at the area around the heater boss on the side of the cylinder. A damp ring or staining here points to a gasket fault.

• Check the thermostat setting. Make sure the cylinder thermostat is set to around 60°C and has not been accidentally turned up. An overheating cylinder will trigger the PRV to discharge water.

• Note the age of the cylinder. If you do not know when it was installed, look for a commissioning sticker on the cylinder or paperwork in a Benchmark logbook. Cylinders over 15 years old — particularly if they are steel-lined rather than stainless — are at higher risk of corrosion-related failure.

Do not attempt to open the boiler casing, adjust system controls inside an unvented cylinder's control pack, or work on any gas components. Unvented cylinders must be serviced and repaired by a G3-qualified engineer under Building Regulations Part G3 (England and Wales) or the equivalent Scottish Technical Handbook provision.

What a G3-Qualified Engineer Will Do

When you call an engineer to a hot water cylinder leaking from the bottom, from a valve or from the tank body, they will typically carry out the following:

• Confirm the leak source by pressure-testing and visual inspection.
• Check expansion vessel pre-charge pressure (should be set to match the incoming mains pressure, typically 1–3 bar) and test for waterlogging.
• Test the T&PRV and expansion valve for correct operation and seating.
• Check the high-limit thermostat and energy cut-out are functioning.
• Inspect the cylinder body, connections and immersion heater boss.
• Advise on repair or replacement based on the cylinder's age, condition and the nature of the fault.

For any work on an unvented system, the engineer must hold a current G3 qualification (such as BPEC HWSS or City and Guilds 6189). New installations or like-for-like replacements must be notified to Building Control, typically via a Competent Persons Scheme such as WaterSafe or BESCA, and a Benchmark commissioning logbook must be completed. Ask for this documentation — it is required for warranty purposes and may be needed when you sell your home.

Repair or Replace? How to Decide

This is the question most homeowners want answered. Here is a practical framework.

Repair is likely the right choice if: - The cylinder is under 10 years old and in otherwise good condition. - The leak is clearly from a pipe fitting, compression joint or immersion heater gasket rather than the tank body. - The pressure relief valve is dripping but the expansion vessel fault has been confirmed and is repairable. - There are no signs of rust in the hot water, no banging or rumbling noises, and no history of repeated repairs.

Replacement is likely the right choice if: - The cylinder is 15 years old or more (stainless steel models can reach 20+ years with good maintenance, but copper and steel models typically do not). - Water is leaking from the tank body itself due to corrosion or a cracked lining. - Hot water from the taps is discoloured or has a metallic taste, suggesting internal rust. - You can hear rumbling, banging or popping from the cylinder — classic signs of heavy sediment buildup and metal fatigue. - The same fault has been repaired more than once in the past few years. - You are upgrading from a vented gravity-fed system to an unvented mains-pressure system for better flow rates.

A useful rule of thumb used by many UK heating engineers: if the cylinder is under a decade old and the fault is minor, fix it. If it is older, showing multiple symptoms or leaking from the tank itself, replacement is almost always the more economical decision over a five-year horizon.

Modern unvented cylinders, particularly stainless steel models, are significantly more corrosion-resistant than older copper or mild-steel units. If you are replacing a corroded cylinder, it is worth asking your engineer about upgrading to stainless steel.

Annual Servicing: The Best Way to Avoid Emergency Leaks

An unvented hot water cylinder should be serviced every 12 months by a G3-qualified engineer. A typical annual service covers testing of all safety valves, checking and recharging the expansion vessel, inspecting the anode rod (on stainless steel cylinders), verifying thermostat accuracy and checking for any early signs of corrosion or connection wear.

Regular servicing dramatically reduces the risk of unexpected failures and keeps the cylinder operating at peak efficiency — important for energy bills, particularly if your hot water is heated by a heat pump or solar thermal system.

Vented cylinders are less regulated but still benefit from periodic inspection of the cold water feed, immersion heater elements and any associated pumps or motorised valves.

Step by step

1. Switch off the heat source

   Turn off the immersion heater at the fuse spur and switch off the hot water circuit at your boiler or programmer. This stops the cylinder heating further and prevents pressure from rising while you investigate.

2. Turn off the cold water supply to the cylinder

   Find the isolation valve on the cold water feed pipe entering the cylinder (usually a slotted screw you turn 90° to close, or a lever valve). Closing this prevents more water entering the tank if it is actively leaking.

3. Dry the area and identify the source

   Use a dry towel to wipe all visible pipework and the cylinder casing. Wait for one to two hours, then inspect carefully. Look for drips at compression fittings, staining around the immersion heater boss, and check the discharge pipe outlet outside or at the tundish.

4. Check the thermostat setting

   Look at the cylinder thermostat dial. It should read around 60°C. If it is set significantly higher, this may be causing the pressure relief valve to open repeatedly. Adjust it if needed, but do not open the boiler casing or touch any gas controls.

5. Note what you have found and call an engineer

   Record where the water appears to be coming from, whether the discharge pipe is active, the age of the cylinder if known, and any unusual sounds (banging, rumbling). Share this with the engineer when you call — it will speed up diagnosis and may save you money on call-out time.

Typical costs

Typical UK ranges as a guide only — prices vary by region (expect the top end, or 20–30% more, in London and the South East) and by how accessible your system is. Always get a written quote.

Frequently asked questions