Name the part before you name the fault
The top of an unvented cylinder is crowded. Almost every fitting that can weep is up there, and the insulation foam moves water sideways before it drips, so where you see the drip is rarely where it started. Before anything else, work out which of these you are looking at.
| What is up there | What a leak from it looks like | What it usually means |
|---|
| Temperature & pressure relief valve (T&P) | Hot water arriving in the tundish and running out of the discharge pipe outside. Often intermittent, often after a long heat-up. | The water is too hot or too high in pressure. The valve is working. Something else has failed. |
| Expansion vessel connection, or the vessel's own tee | A weep at a threaded or compression joint, usually on a short branch off the cold side. Cold or lukewarm water. | A joint that has relaxed, or a vessel that has corroded at the connection. Sometimes a vessel that has been hanging unsupported. |
| Immersion heater boss | A slow drip that appears under the immersion flange and tracks down the side of the case. Water is hot. | A perished flange gasket, or an element refitted without a new seal. |
| Secondary return connection | Damp on a small-bore pipe entering the upper third of the cylinder, dripping at the lowest bend. | A joint, not the cylinder. Common where a return has been added after the original install. |
| Thermostat or thermal cut-out pocket | Damp behind the control plate, sometimes with corrosion staining. | A pocket seal, or water that has run down from something above it. |
| The cylinder shell itself | Foam insulation soaked through, water emerging from under the outer skin at a seam with no wet fitting above it. | Perforation, usually at a weld. Not repairable. The cylinder is replaced. |
Telling them apart without guessing
This takes about ten minutes and it is worth doing before anyone quotes you.
- Dry everything. Wipe the whole top of the cylinder, every fitting, the tundish and the pipe stubs. Put kitchen roll under each fitting separately so you can tell which one wets first.
- Look at the tundish. That small open funnel in the discharge pipe is the diagnostic point of the whole system. If water is running through it, a safety valve is discharging, and no joint is leaking at all.
- Feel the temperature. Hot water points at the T&P valve, the immersion boss or the secondary return. Cold or lukewarm points at the cold inlet group, the expansion vessel branch, or the expansion relief valve.
- Time it against a heating cycle. Water that appears only while the cylinder is reheating, then stops, is an expansion problem. Water that drips steadily whether or not the system is running is a joint.
- Press the foam. If the insulation under the outer case is saturated and nothing above it is wet, stop looking at fittings.
If you cannot reach a fitting or cannot see the tundish, leave it. The point is to gather information, not to start undoing things on a vessel that is under mains pressure.
A discharging T&P valve is a message, not a fault
This is the part that gets people into trouble. The temperature and pressure relief valve sits in the top of the cylinder with its probe in the hottest water. It exists to open if the stored water approaches boiling. It is the last defence in a chain, and by the time it opens, two other things have already failed: the cylinder thermostat that should have switched the heat off, and the thermal cut-out that should have caught the thermostat.
So a T&P valve that discharges hot water is doing its job while telling you the temperature control has gone. Replacing the valve, or worse, plugging it, removes the only thing standing between you and a cylinder that keeps heating with nowhere to go.
The expansion relief valve on the cold inlet is a milder version of the same story. When it passes, the usual cause is an expansion vessel that has lost its air charge or filled with water, or an incoming mains pressure the pressure-reducing valve is no longer holding down. The water has expanded and had nowhere to go, so the valve dumped it. Again, the valve is the symptom.
If either is discharging now: switch off the immersion heater at its isolator, switch the hot water off at the boiler or programmer, leave the cold supply on, and keep people away from the discharge pipe outside, because it can run scalding. Do not drain the cylinder and do not touch the valve.
Never cap, plug or valve a discharge
We will say this plainly because it is the single dangerous mistake on this topic. The discharge pipe from the tundish is not an overflow and not a nuisance. Nothing may be fitted to it that could close it: no cap, no plug, no tap, no isolating valve, no blanking nut on the valve outlet, no bucket taped over the tundish, no bend that can hold water. The pipe has to run continuously downhill to a place where a discharge is visible and cannot scald anyone.
If your discharge pipe has already been capped by someone, treat the system as unsafe and get it looked at before you use the hot water again. Our guide to leaking overflow and discharge pipes covers what a discharge is telling you in more detail.
What the cylinder is actually storing
An unvented cylinder holds a couple of hundred litres of water at mains pressure, heated to around 60 °C for legionella control. Unlike an open-vented tank, it has no vent pipe to the atmosphere and no cold water storage cistern above it. There is no escape route except the ones designed into it.
The reason that matters is what happens if temperature control fails and the water is driven above 100 °C while still held under pressure. It does not boil, because the pressure suppresses it. It becomes superheated. If that vessel then loses its containment, the water flashes to steam more or less instantly, and water turning to steam at atmospheric pressure expands to roughly 1,600 times its liquid volume. That is the whole reason the safety chain exists, and the whole reason the relief valves are not optional extras.
Why it is a controlled item
Unvented hot water storage above 15 litres is not ordinary plumbing. It is controlled work under Part G3 of Schedule 1 to the Building Regulations 2010, which sets out how the system must be protected against the water exceeding 100 °C and how a discharge must be conveyed safely. In practice, installing, altering or replacing the safety components is notifiable building work unless it is carried out by someone registered under a competent person scheme for that work.
Two practical consequences. First, the valves are matched to the cylinder, not generic parts — the T&P valve has a specific set temperature and pressure, and fitting a spare that happens to thread in is not a repair. Second, an unvented cylinder is meant to be serviced, which is when the expansion vessel charge is checked and the relief valves are exercised. A cylinder that has never been serviced is the one that discharges.
What we do, and what we do not
We are a leak repair firm. On a cylinder that is wet at the top, that means: finding which fitting is actually losing water, testing whether the loss is a joint or a valve responding to a real fault, repairing the pipework, joints and connections around the cylinder, and putting right the floor, ceiling or plaster the water has already damaged.
What we do not do is pretend a failed safety chain is a plumbing weep. If the T&P valve is discharging because the temperature control has failed, or if the cylinder shell itself is perforated, the work belongs to someone qualified on unvented systems — and for a perforated cylinder it is a replacement, not a repair. We will tell you that on the day rather than sell you two hours of looking.
Where the water has been running for a while, the damage is usually below: a stained ceiling, a wet floor in the cupboard, sometimes a neighbour's ceiling in a flat. That part is ours, through boiler and heating leak repair and reinstatement afterwards.
What it costs
£150 per hour, agreed before we attend, with no call-out fee — the full breakdown is on our pricing page. If we cannot find the leak you do not pay for the time we spent looking. Most cylinder cupboards are resolved well inside an hour, because everything is accessible and the tundish answers half the question on its own.
If the water has damaged something and you intend to claim, the visit includes a written report that sets out what was found and where, prepared to support a trace and access claim. The insurer decides what your policy covers, not us; what an insurer-ready report contains explains what we put in it.
