Hot Water Cylinder Leaking? Causes and What to Do

Reviewed by the DCI Leak Detection team · Last updated June 2026

Finding water around or under the hot water cylinder, usually tucked in the airing cupboard, is unsettling. The good news is that most cylinder leaks are slow rather than sudden, and once you have made things safe there is plenty you can do before help arrives. This guide explains where the water is likely coming from, what counts as normal and what does not, the safety steps to take now, and when a hidden leak in the surrounding pipework needs tracing.

What to do right now

The first priority is to stop heating the water and stop the cylinder refilling, then limit the damage. Work through these steps in order.

1. Turn off the heat source. Switch off the immersion heater at its switch or spur on the wall, and turn off the boiler or the heating supply to the cylinder. This stops fresh heat raising the pressure and makes everything safer to be around.
2. Isolate the cold feed. Find the cold water feed into the cylinder and close its isolating or gate valve, usually a small lever or wheel-type valve on the pipe running to the bottom of the tank. Turning it off stops the cylinder refilling as it empties, which slows the leak. WaterSafe recommends knowing where your isolating valves and stop tap are before you ever need them.
3. Catch the water and protect the area. Put a bowl or towels under the drip, lift anything stored in the cupboard clear, and protect the floor and any room below.
4. Leave the electrics alone if water is near them. If water is reaching the immersion heater wiring, a socket or any fitting, do not touch it. Keep the immersion switched off and stay clear of standing water until it is dry or checked.
5. Take photographs. Record where the water is and the date. If you make an insurance claim later, early evidence helps.
6. Call a qualified engineer. Once it is safe and the leak is slowed, the fault still needs diagnosing. On an unvented cylinder this must be a G3-qualified engineer, as we explain below.

Common causes of a cylinder leak

Water around a cylinder can come from the tank itself, from the parts bolted to it, or from the pipework feeding it. Here are the usual culprits and what each one means.

If the water is not obviously from any of these, and the cylinder and its visible fittings look dry, the source may be a buried heating or supply pipe nearby rather than the cylinder at all. Water travels, so a pool in the cupboard does not always sit under the leak.

Normal discharge versus a real leak

This is the point that confuses most people, so it is worth being clear. Not every drop of water near a cylinder is a leak.

On an unvented, pressurised cylinder, water expands as it heats. The system has an expansion vessel to take up most of that increase, and a temperature and pressure relief valve as a backstop. A small amount of water released through the relief valve, down the tundish and out through the discharge pipe, can be perfectly normal now and then, particularly just after the water has heated. That is the safety system doing its job.

A real leak is different. Tell-tale signs include water coming from the cylinder body, the immersion heater seal or a pipe joint rather than the discharge pipe; a drip that is steady rather than occasional; limescale crusting or rust staining where water has been escaping for a while; and a pool that keeps returning under the tank. If the relief valve or discharge pipe is releasing water frequently or continuously, that is not normal either: it points to a fault such as a failed expansion vessel, and it needs investigating rather than ignoring.

Vented and unvented cylinders, and the G3 rule

Which type of cylinder you have changes what is safe to do. There are two broad kinds in UK homes.

A vented cylinder is the traditional setup, fed from a cold water tank in the loft and open to the air through a vent pipe. It is not under mains pressure. The leaking parts are usually the immersion seal, the connections, or the tank itself, and while you should still use a competent plumber, it is a lower-risk system.

An unvented cylinder, such as a Megaflo or similar, is a sealed system fed straight from the mains and held under pressure. Because it is pressurised and stores hot water, it has to have safety devices including an expansion vessel and a temperature and pressure relief valve. Under Building Regulations, Approved Document G3, installing, repairing or replacing an unvented hot water system is notifiable work that must be carried out by a G3-qualified engineer. In plain terms: you can safely switch off the immersion heater, turn off the heat source and close the cold feed, but you must not attempt DIY on the pressurised parts. The relief valve, expansion vessel and pressurised connections are a job for a qualified professional, both for your safety and to keep the system compliant.

Safe checks versus when to call a pro

There are a few things you can safely do yourself, and a clear line beyond which it is a job for a professional.

Safe to do yourself:

• Switch off the immersion heater at the wall and turn off the boiler heat to the cylinder.
• Close the cold water feed at the cylinder's isolating valve to stop it refilling.
• Catch and mop up water, and move stored items clear.
• Look at where the water is appearing, the discharge pipe, a fitting, the immersion seal or the tank body, and note it down for the engineer.

Call a professional when:

• you have an unvented cylinder and anything beyond the steps above is needed, as it must be a G3-qualified engineer;
• the leak is from the cylinder body itself, which usually means a replacement;
• the relief valve or discharge pipe is releasing water frequently or constantly;
• the cylinder also feeds, or sits within, your central heating, and you suspect the heating side is involved;
• water is pooling but you cannot find the source, which points to hidden pipework.

That last point is where leak detection earns its keep. If the cylinder and its fittings look dry but water keeps appearing, the leak is often in buried heating or supply pipes running under the floor or behind the cupboard. Chasing it by opening up floors on a hunch causes needless damage. Instead, a specialist uses thermal imaging to spot the warm trail of a heating or hot water leak, acoustic equipment to listen for a pressurised pipe escaping, tracer gas to pinpoint a small leak, and moisture meters to map how far the damp has spread. The leak is traced back to its real source, so any opening up is aimed at one small spot.

If your heating pressure is dropping at the same time, the two can be linked, and our guide on why a boiler keeps losing pressure walks through the causes. For leaks on the heating side, our central heating leak detection service finds the source without ripping pipes out, and where the problem turns out to be general plumbing, our plumbing leak detection service covers it. If you have wet underfloor heating in the mix, see our guide on underfloor heating leaks.

Catching a cylinder leak early limits the damage to the airing cupboard, the floor and the room below, and it keeps the eventual repair smaller. The sooner the cause is found, the less water has the chance to spread.

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