Hot Water Cylinder Leaking: What to Do and What It Means

Updated July 2026 · Emergency Plumber Chelsea

Make it safe first. Turn off the immersion heater at its switch, usually a red switch on the wall near the cylinder, and turn the hot water off at the boiler or programmer. If the leak is anywhere near electrics, including the immersion wiring on top of or on the side of the cylinder, switch off at the consumer unit and keep your hands dry. Then stop more water arriving. Put towels and a tray down, and take photos of the cylinder, the leak and any damage below for your insurer.

How you shut off the water depends on the type of cylinder. A vented cylinder, the traditional copper cylinder in an airing cupboard fed by a cold water tank in the loft, has a gate valve on the cold feed pipe coming down from that tank, usually near the top or bottom of the cylinder. Close it clockwise. An unvented cylinder, the pressurised type fed directly from the mains, usually white or grey with a pressure reducing valve, an expansion vessel and a tundish, is shut off at the isolation valve on its cold mains inlet or at the main stopcock. If you are not sure which you have, our guide to vented and unvented cylinders explains how to tell them apart.

Now find the leak. Dry the cylinder and pipework with a towel and look closely with a torch. There are four common places. Pipe connections at the top, side and bottom of the cylinder, where compression fittings or soldered joints can weep. The immersion heater boss, the large round fitting where the element screws in, which has a fibre or rubber washer that can fail. The valves and safety devices on an unvented system. And the cylinder body itself, most often low down or along a seam.

A leaking joint is usually the least serious. A weeping compression nut can sometimes be stopped by a gentle tightening, but joints on a cylinder are awkward to hold steady and it is easy to twist the pipe or the cylinder boss. If it does not stop with a small nip, leave it for a plumber. A leak around the immersion heater boss usually means the washer has perished and the element needs taking out and refitting with a new one, which means draining down at least part of the cylinder. If the immersion heater itself has also stopped heating, see our immersion heater guide, because it may be sensible to replace the element at the same time.

On an unvented cylinder, water coming out of the tundish, the small funnel with a visible gap in the pipework near the cylinder, is a warning, not a leak in the normal sense. It means either the expansion relief valve or the temperature and pressure relief valve is releasing water. A steady drip usually points to a lost air charge in the expansion vessel, a faulty pressure reducing valve letting mains pressure build up, or a relief valve that has worn and no longer seats. Hot water or steam pouring from the tundish is more serious: it can mean the cylinder has overheated. Turn off every heat source, the boiler and the immersion, keep people away from the discharge pipe, and call an engineer straight away. Never cap, block or tie up the tundish or discharge pipe to stop the drip. Those valves are what stop a pressurised cylinder from failing dangerously. Our guide to what a tundish is explains each cause and what an engineer checks.

Work on an unvented cylinder falls under Part G3 of the Building Regulations, and it should be carried out by an engineer holding the unvented hot water qualification. Resetting a tripped high-limit thermostat, recharging an expansion vessel or replacing relief valves are all quick jobs for someone qualified, but they are not DIY jobs. Keep the installation paperwork if you have it, as it shows the model and when the cylinder was last serviced.

A leak from the body of a vented copper cylinder usually means the copper has corroded through. You will often see green or blue crusting, white limescale deposits or a pinhole weeping near the base or around a seam. London's hard water, older cylinders and poor quality water in the loft tank all speed this up. Once the body is leaking it cannot be repaired properly; soldering over a pinhole in thin, corroded copper rarely lasts. It needs replacing, and the choice is usually between a like-for-like vented cylinder or an upgrade to an unvented system if your mains pressure and flow are good enough. Our guides on what size hot water cylinder you need and cold water storage tank problems help with that decision.

If the cylinder has to be drained before the engineer arrives, only do it with the heat sources off and the water cooled. On a vented cylinder, with the cold feed closed, open the drain cock near the bottom with a hose run to a drain or outside, and open a hot tap to let air in. On an unvented cylinder, leave the draining to the engineer, since the system is pressurised and has to be depressurised in the right order. Our step-by-step guide on how to drain a hot water cylinder covers the vented drain-down in full. In many Chelsea flats the airing cupboard sits above someone else's ceiling, so if water has already gone through the floor, tell your neighbour and the managing agent and keep a record of what happened for an escape of water insurance claim.

Some signs mean it is worth having a cylinder checked before it leaks badly: a damp airing cupboard floor, rust-coloured or cloudy hot water, rumbling or banging from the cylinder as it heats, the tundish dripping on and off, or hot water running out much faster than it used to. Unvented cylinders should be serviced regularly to check the expansion vessel charge and the safety valves, often alongside the annual boiler service.

If your hot water cylinder is leaking, dripping into the tundish or you have no hot water after isolating it, call Emergency Plumber Chelsea on 020 3002 6667. We diagnose cylinder leaks, repair leaking joints, valves and immersion bosses, and quote clearly for replacement where a cylinder has failed, in houses and flats across Chelsea, Knightsbridge, South Kensington, Sloane Square and Fulham, and explain the options before any work starts. If water has got into floors or ceilings, our leak detection service can check how far it has spread. There is no call-out fee on jobs over 3 hours.

Quick tips

• Turn off the immersion and the boiler's hot water first, then the cold feed into the cylinder.
• Vented cylinders isolate at the gate valve on the cold feed from the loft tank.
• Never block or cap the tundish or discharge pipe on an unvented cylinder.
• Hot water or steam from the tundish: turn off all heat sources and call an engineer now.
• A leak from the body of a copper cylinder means replacement, not repair.

FAQs