Water leaking from the bottom of a hot water cylinder usually comes from a valve, a seal or a joint above it, not from the tank splitting. The water runs down the outside of the cylinder and gathers on the floor, which makes the base look like the source. Which cylinder you have decides where to look: a vented cylinder fed by a loft tank, or an unvented cylinder fed straight from the mains. Work on relief valves, seals, the immersion heater or the expansion vessel is for a qualified engineer.
Is water leaking from the bottom of a hot water cylinder dangerous?
Treat water under a hot water cylinder as a fault that needs a qualified engineer. Most wet patches are a slow drip rather than an emergency. Hot or steaming water is a different matter, because stored water can sit close to scalding temperature.
HSE sent a safety alert about scalding risk from domestic hot water systems to local authorities in England in May 2008. It followed two similar fatal incidents in four years, where near-boiling water came through bedroom ceilings. The risk was highest where a fixed immersion heater worked with a plastic cold water tank in the loft, most often in houses built between 1945 and 1975. HSE names unusually loud noises from the cylinder as a warning sign of thermostat failure and overheating.
Loud noises, hot water on the floor, or water appearing through a ceiling below the cylinder all mean the same thing. Stop guessing and get a qualified engineer to the cylinder.
Vented or unvented cylinder: how the type changes where the water comes from
The type of hot water cylinder you have decides where a wet patch most likely starts. A vented cylinder is fed by gravity from a loft tank, an unvented cylinder directly from the mains.
| Point of difference | Vented cylinder | Unvented cylinder |
|---|---|---|
| Water supply | Gravity feed from a cold water tank in the loft, with an open vent pipe to atmosphere | Fed direct from the mains, so cylinder and pipework sit at mains pressure |
| Safety devices | The open vent pipe takes up expansion and keeps the stored water at atmospheric pressure | Pressure and temperature relief valves discharge through a tundish into a discharge pipe |
| Likely source of a wet patch | Loft tank, float valve, overflow or vent pipe, with water tracking down to the cylinder | Relief valve, tundish, expansion vessel, immersion heater seal or cold feed valves |
| Who may do the work | Plumbing work; anything on the gas side needs a Gas Safe registered engineer | Notifiable work under the Building Regulations in England, restricted to a competent person |
A vented cylinder is easy to recognise by the cold water tank in the loft and the vent pipe that curls over it. Water from that tank can run down joists and pipes and only become visible at the cylinder. Our guide on water coming out of the vent pipe into a loft tank covers that pattern. An unvented cylinder has no loft tank and carries valves, a tundish and an expansion vessel on or beside it.
Why a dripping tundish or discharge pipe leaves water under an unvented cylinder
A dripping tundish is one of the most common reasons for water appearing below an unvented hot water cylinder. A tundish is the small open gap built into the discharge pipe under the relief valves. It exists so that water leaving a safety valve can be seen rather than hidden inside pipework.
Vaillant states that a dripping tundish shows a fault in the heating system and that action is needed. The cause is either a faulty pressure relief valve or system pressure that is too high. Vaillant is clear that you should call a qualified engineer when the tundish drips.
The discharge pipe must stay open to atmosphere so hot water can leave safely, and Heatrae Sadia notes that occasional dripping from the discharge outlet can occur by design. A steady drip, a wet tundish or a puddle below the pipe is not that. A spent air charge in the expansion vessel is a frequent cause, explained in our guide on why an unvented cylinder expansion vessel loses its charge.
Which parts actually leak when you find a wet patch under the cylinder
The steel or copper body of a hot water cylinder rarely fails first. Water that gathers at the base has usually come from a valve, a seal or a joint higher up and run down the outside. Every item below is work for a qualified engineer, not a householder task.
- An exhausted air charge: Heatrae Sadia states that intermittent discharge from the expansion valve shows the air volume can no longer take up the expansion of heated water.
- An internal air bubble that has dissolved into the water over time. Recharging it is normally done during an annual cylinder service by a trained and suitably qualified installer.
- The immersion heater seal. The O-ring or gasket at the boss is a known leak point, and manufacturers treat fitting or replacing it as installer work.
- A relief valve that has not reseated after discharging. Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 requires such valves to close automatically after a discharge, in England and Wales.
- The strainer, pressure reducing valve or stopcock on the cold feed, which manufacturer fault-finding charts list as leak points on unvented systems.
- Pipework and compression joints behind or below the cylinder, where water runs to the lowest point and collects under the base.
What you can note yourself before an engineer looks at the cylinder
What you can do with a wet patch under a hot water cylinder is observe and record it. Good notes help the engineer find the source faster, because many leaks only appear at certain times.
- Where exactly the water sits: under the base, below the tundish, near a valve, or in a trail from another room.
- Whether the water is warm or cold to the touch.
- Whether it is constant or only after the cylinder heats up, or only when a shower runs.
- Whether the tundish or the discharge pipe outside is wet.
- Photographs of the cylinder, its data plate and the valves, taken from a safe distance.
A wet patch under a hot water cylinder is a fault signal, not a quirk to live with. The safe checks a householder makes are limited to boiler pressure, thermostat and programmer settings, the reset button and bleeding radiators. Relief valves, seals, the immersion heater, the expansion vessel, the discharge pipe, gas and flues are all work for a qualified engineer, and on an unvented cylinder the law in England restricts who may carry it out.
Who is allowed to work on a leaking unvented hot water cylinder in England?
In England, Requirement G3 of the Building Regulations 2010 restricts unvented hot water storage work to a person competent to carry it out. Approved Document G sets out what the system must do. Stored water must be prevented from exceeding 100 degrees Celsius, and any hot water released by a safety device must discharge safely and visibly. It expects at least two safety devices that work independently of the thermostat, such as a non self-resetting energy cut-out and a temperature relief valve.
Installing or replacing an unvented cylinder is notifiable work. An installer registered with a competent person scheme for unvented hot water systems can self-certify, and a compliance certificate follows within eight weeks of completion. Without that registration, the building control body must be told before the installation. Our guide on who can install an unvented hot water cylinder in England goes through the routes.
Where a gas boiler heats the cylinder, the gas side is separate. Work on gas, on the flue or inside the boiler casing is for a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998 as amended, which apply in England.
Does an annual cylinder service reduce the chance of a leak at the base?
An annual service checks the parts that cause most wet patches under a hot water cylinder. Heatrae Sadia sets out five points for a competent installer to cover: electrical safety, the operation of the relief valves, any blockage of the tundish or discharge pipe, the condition of the sacrificial anode, and the condition of the seals. The air charge inside the cylinder is also restored at that visit, because air slowly dissolves into the water.
A service does not make a leak impossible. It does mean a tired relief valve or a flat expansion vessel is found before it drips onto the floor. If the leak under your cylinder turns out to be the cylinder or its controls, you can arrange a free home survey and a written fixed price for Megaflo and unvented cylinder work.
Sources
- Vaillant — What is a tundish?
- Viessmann — A guide to vented vs. unvented cylinders
- HSE — Scalding risk from domestic hot water systems (safety alert)
- GOV.UK — Approved Document G: sanitation, hot water safety and water efficiency
- legislation.gov.uk — Water Supply (Water Fittings) Regulations 1999, Schedule 2, hot water services
- GOV.UK — Building regulations approval: use a competent person scheme
- Heatrae Sadia — Love Your Cylinder FAQs
- Heatrae Sadia — Importance of servicing your cylinder
- Heatrae Sadia — Megaflo Eco Plus installation manual
- Baxi — unvented direct and indirect cylinder manual