Water around the base of a hot water cylinder is worrying, partly because the cylinder is usually shut in a cupboard and partly because it is full of hot water. The good news is that most cylinder leaks come from a small number of places, and they can usually be told apart with a torch and a dry tissue. The important thing is to know which type of cylinder you have before you touch anything, because an unvented cylinder has safety devices that are not for DIY.
This guide covers how to tell vented and unvented cylinders apart, what a discharge from the tundish means, the common leak points, and what is safe to do while you wait for help.
Is your cylinder vented or unvented?
The answer changes what the leak is likely to be and who should deal with it.
| Feature | Vented cylinder | Unvented cylinder |
|---|---|---|
| Where the water comes from | A cold water storage tank, usually in the loft | Directly from the mains |
| Typical appearance | Often copper, sometimes with bare or foam-lagged sides, with an open vent pipe rising from the top | Usually a white or coloured casing, with a cluster of valves, a pressure gauge on some models, and often an expansion vessel nearby |
| Tundish | Not normally fitted to the cylinder | A small funnel with an air gap on the discharge pipe from the safety valves |
| Label | Usually a basic maker's label | A warning label telling the user not to adjust components and to contact the installer if a fault develops |
If you have a vented cylinder, the cold water tank above it is part of the same system. Our guide to leaks from loft tanks and pipes covers that end.
What does it mean if water is coming out of the tundish?
On an unvented cylinder, the safety valves discharge into a tundish so that any release is visible. Building Regulations guidance describes the tundish as a visible indication of a discharge. Water there is not a leak to patch. It is the system telling you something is wrong.
- A drip or trickle of cold or warm water through the tundish, often while the cylinder heats up, is commonly linked to the expansion relief valve. A frequent cause is an expansion vessel that has lost its air charge, so the water has nowhere to go as it expands. A faulty pressure reducing valve or a worn relief valve can cause the same symptom.
- Hot water flowing through the tundish is more serious. It can mean the temperature and pressure relief valve is operating because the water has become too hot. The standard warning label on unvented heaters tells the user, in that situation, to switch the heater off and contact the installer.
In both cases, do not try to adjust, tighten or cap the valves or the discharge pipe. Switch off the heat sources and call someone qualified for unvented systems. If the drip is coming from a pipe outside the house connected to your boiler rather than the cylinder, that is a different valve, and our guide to a dripping boiler pressure relief pipe explains it.
Could the immersion heater boss be leaking?
Many cylinders have an immersion heater screwed into a threaded boss on the top or side. The joint is sealed with a fibre or rubber washer that can harden with age and repeated heating. Signs of a boss leak are:
- water or white scale marks running down the cylinder from the immersion heater;
- rust or green staining around the boss;
- damp at the immersion heater's cable entry.
Because this is where mains electricity meets water, switch the immersion heater off at its switch and at the consumer unit before looking closely, provided the consumer unit is dry. Replacing the washer or the immersion heater means draining down the cylinder, and on an unvented cylinder it is a job for a qualified installer.
Is it a pipe connection or valve?
Most cylinder leaks turn out to be on the pipework around it rather than the cylinder itself. The usual suspects are:
- Compression joints on the cold feed, hot draw-off and coil connections, which can weep after the pipes have moved or been disturbed.
- The drain-off cock near the bottom of the cylinder, which can drip from the spindle or the outlet.
- Zone valves and pumps in the airing cupboard, on the heating side rather than the hot water side.
- Gate valves, which can weep from the spindle when turned after years untouched.
- The coil connections, which carry heating water rather than tap water. A leak here will show as a falling boiler pressure on a sealed heating system.
To find which joint is wet, dry everything with a cloth, then wrap a strip of dry kitchen towel round each joint in turn and check it after a while. The one that darkens first is your leak, or is just below it.
What if the cylinder itself is leaking?
Water weeping from the body of the cylinder, rather than from a fitting, is less common but not repairable in any lasting way. On older copper cylinders, corrosion can cause pinholes, often near the base or around seams. A vented cylinder that has been over-pressurised, for example by being connected wrongly, can bulge or split.
Signs include a wet patch on the side of the cylinder with no fitting above it, green or blue staining on bare copper, or foam insulation that is wet and discoloured in one area. A leaking cylinder body means replacement, and the choice of replacement is worth discussing properly with an installer rather than deciding in a rush.
What if the water is near the cylinder but not from it?
A wet airing cupboard floor does not always mean the cylinder. Before assuming the worst, check for:
- Condensation on cold pipes in a warm cupboard, which can drip steadily, especially on the cold feed.
- Pipes above the cupboard, including a bathroom or loft pipe, dripping down the outside of the cylinder.
- Heating pipes under the floor of the cupboard, where water pools and appears to come from the base of the cylinder.
- A shower or bath on the other side of the wall.
If the floor of the cupboard is wet and none of the visible joints is, the leak may be below the floorboards or in the ceiling below. That is the point where tracing, rather than tightening, is needed.
What should you do right now if your cylinder is leaking?
- Make the electrics safe. Switch off the immersion heater. If water has reached any electrics, turn off the circuit at the consumer unit, but only if the consumer unit is dry.
- Stop the heat. Switch the boiler off, or set it so it is not heating the hot water.
- Stop the supply. On a vented cylinder, close the gate valve on the cold feed from the tank to the cylinder. On an unvented cylinder, close the cold mains inlet valve if you can identify it, or the main stopcock.
- Leave the safety valves alone. Do not open, close, cap or block anything on the discharge pipe or tundish.
- Contain the water. Towels, a tray or a bowl under the drip, and move anything stored in the cupboard.
- Photograph it. Pictures of the leak and any damage help the person repairing it and any insurance claim.
Do not try to drain an unvented cylinder yourself. If a vented cylinder needs draining, the water inside is hot and the drain-off cock can be difficult to reseal, so it is usually best left to the plumber.
Who is allowed to work on an unvented cylinder?
Unvented cylinders store hot water under mains pressure and rely on several safety devices working together. Building Regulations guidance says that installing one is notifiable building work, and that owners should choose installers with the necessary skills, shown for example by registration with a competent person scheme for this work or by a current skills card for unvented hot water systems. In practice, plumbers often refer to this as a G3 qualification, after the part of the regulations that covers hot water safety.
For a householder, the practical rule is straightforward: an unvented cylinder, its valves and its discharge pipe are for a suitably qualified installer. A pipe joint well away from the cylinder can be dealt with by any competent plumber.
When should you call a leak detection specialist?
Most cylinder leaks are visible and need a plumber or an unvented-qualified installer rather than a detection survey. A specialist is useful when:
- the cupboard floor or the ceiling below is wet but no joint you can see is leaking;
- the boiler keeps losing pressure, which may point to the coil or heating pipework under the floor;
- water is appearing in a room below the airing cupboard and the pipe route is hidden;
- you need the source confirmed and recorded for an insurer.
Our emergency leak detection is priced from £300 in working hours and £450 out of hours, and a thermal imaging survey is £150 to £250. Call 020 3912 0200 to describe what you are seeing.
Frequently asked questions
Why is my hot water cylinder leaking from the bottom?
Often it is not the bottom of the cylinder but a joint, valve or drain-off cock above it, with water running down the side. Dry everything and check each joint with a strip of kitchen towel. If the water truly comes from the body of the cylinder, it usually needs replacing.
Is it safe to use hot water if my cylinder is leaking?
Switch off the heat and the supply until the cause is known, particularly with an unvented cylinder or if hot water is flowing through the tundish. A small weep from a joint may allow limited use once isolated, but take advice from the person repairing it.
Why is water dripping into the tundish?
A drip into the tundish on an unvented cylinder usually means the expansion relief valve is opening, often because the expansion vessel has lost its charge. Hot water flowing through it can mean the temperature and pressure relief valve is operating. Switch the heater off and contact a qualified installer.
Can a leaking immersion heater be fixed?
Usually, by replacing the washer at the boss or the immersion heater itself. The cylinder has to be drained, and the electrics isolated, so it is a job for a plumber, and for a qualified installer on an unvented cylinder.
Can a leaking hot water cylinder be repaired?
Leaks from joints, valves and the immersion boss can normally be repaired. A leak from the body of the cylinder, such as a corrosion pinhole, generally means the cylinder needs replacing.
