It is not a leak, and that changes who you should ring
A leak is water escaping from somewhere it was meant to stay. A discharge is water leaving by the exact route the system was built to send it down, through a valve that opened on purpose. When the tundish runs, nothing in your pipework has failed. Something upstream of it has, and the tundish is doing its job by telling you.
That distinction saves money. Water in the tundish means you do not need leak detection — nothing is hidden, and no floor needs lifting. You need someone to work out which valve opened and why, which is a diagnosis on a cylinder rather than a search through a building.
What a tundish actually is
An unvented cylinder holds heated water at mains pressure. Two things can go wrong with that: the pressure can climb too high, or the temperature can. Both have a relief valve, and both valves dump into the same place.
The pipe from the valve is short and metal. It runs into the tundish, an open funnel — a deliberate gap in an otherwise sealed run, where the water falls through open air into the pipe that carries it outside. Approved Document G calls that gap an air break, and it does three jobs:
- Backflow prevention. If the outside end blocks, freezes or sits in a flooded gully, water backs up to the tundish and spills there, in front of you, rather than pushing back through the safety valve into your hot water.
- No siphon. A continuous sealed pipe from a hot vessel to a drain is a siphon waiting to happen. The air gap makes that impossible.
- Visibility. The regulations say the discharge should be visible at the tundish. That is not an aesthetic note, it is the design intent. A safety valve that opens silently, out of sight, is a failure nobody finds until the damage arrives.
Plumbing gives you very few signals built to be read. The gauge on a boiler is one. The tundish is the other.
Read the pattern before you read the part
Almost all of the diagnosis is in when and how much, not in what the valve looks like. Watch one full heat-up cycle and note three things: hot or cold, during reheat or at other times, drip or flow.
| What you see | Most likely cause |
|---|
| A few drips while the cylinder heats, then it stops | Expansion has nowhere to go. Flat expansion vessel, or a bubble-top cylinder that has lost its air pocket. |
| Drips overnight, with no hot water being used | Mains pressure rising as street demand drops, past a pressure reducing valve set too high or failed. |
| A steady drip that never stops | A relief valve no longer seating. Usually grit or scale on the seat, sometimes a perished seal. |
| Cold or lukewarm water | The expansion relief valve on the cold inlet group, low down. |
| Hot water | The temperature and pressure relief valve, on the cylinder itself. |
| A real flow of very hot water, possibly steam | The temperature relief valve has opened in earnest. Turn the heat source off. This is the fault the valve exists for. |
The expansion story, which is the common one
Water expands when you heat it, by a few per cent. In a sealed system that expansion has to be absorbed somewhere, and on an unvented cylinder that somewhere is an expansion vessel with a rubber diaphragm and a charge of air behind it, or — on some designs — an air bubble trapped in the top of the cylinder itself.
Air escapes slowly through rubber, and air bubbles dissolve into water over months. Either way the cushion disappears, and the next time the cylinder heats, the expansion has nowhere to go but up in pressure. It climbs past the expansion relief valve's setting, the valve opens, a few teaspoons leave, the valve shuts. A drip during every reheat, nothing in between.
Same physics as a heating system with a dead vessel — that version is covered in expansion vessel failure or leak — but the cure differs. A bubble-top cylinder is re-charged by draining it in a set sequence to re-form the air pocket; a separate vessel is re-pressurised through its schrader valve or replaced. Neither is guesswork on a vessel at mains pressure.
When mains pressure is the real culprit
An unvented installation has a pressure reducing valve on the incoming cold feed, set well below the relief valve's opening pressure. When that valve drifts or fails, pressure arrives at the cylinder higher than the design allowed for, and the relief valve starts doing the reducing valve's job.
The giveaway is timing. Mains pressure is not constant — it rises when the street stops drawing water, which is the small hours. A tundish that drips at two in the morning and behaves all day is usually a pressure problem, not a cylinder problem. Hammering taps or a shower that has become uncomfortably fierce reinforce it. More under water pressure too high.
A valve that will not close again
Relief valves are simple: a spring holds a seal against a seat, and pressure lifts it. Once a valve has opened a few times, a fragment of scale or grit from the mains can land on the seat and hold the seal a fraction of a millimetre open. It never fully closes again, and you get the constant drip that ignores everything else.
People are often told to lift the lever and flush it through. Sometimes that clears it; often it makes things worse, because the seat is already scored. If the drip has not stopped an hour later, the valve needs replacing. A valve dribbling for months has also been running your hot water to a drain, which shows on a meter long before anyone spots the tundish.
A tundish you cannot see is a tundish that does nothing
The tundish only works as a warning if someone can see it, and plenty of installations defeat that:
- Boxed in. A cupboard gets a fitted panel, or a plywood casing goes round the cylinder during a bathroom refit, and the tundish disappears behind it.
- Relocated out of sight. The pipe is re-routed during a loft conversion and the tundish ends up above a hatch nobody opens for years.
- Moved away from the valve. Approved Document G wants the tundish vertical, in the same space as the cylinder, lower than the safety device, with no more than 600mm of pipe between the valve outlet and the tundish. Long horizontal runs are non-compliant and hard to read.
- Plugged, taped or capped. Blocking a discharge does not stop the fault, it removes the relief route from a vessel of water above 60°C at mains pressure. Nobody should ever do it, and where it has been done it needs undoing before anything else is discussed.
If you have an unvented cylinder and cannot point at your tundish, find it before you need it. Cutting a hatch in a boxed-in cupboard is a small job next to the one it prevents.
What the rules ask of the pipework
Worth knowing, because it tells you whether what you are looking at was installed properly:
- The pipe from valve to tundish is metal, and at least as big as the valve outlet.
- The tundish is vertical, in the same room as the cylinder, below the valve, within 600mm of it.
- Below the tundish, at least 300mm of straight vertical pipe before any bend.
- After that, a continuous fall of at least 1 in 200 all the way to the discharge point.
- It terminates where near-boiling water and steam can do no harm, and where it can still be seen.
The falls and the pipe size are not pedantry. A discharge pipe with too much resistance cannot pass what a temperature relief valve dumps, and a valve that cannot discharge freely protects nothing.
What to do now
- A real flow of hot water or steam: switch off the immersion and the boiler, open a hot tap elsewhere to draw heat out, and get an engineer to it. Do not touch the discharge pipe.
- A drip: dry the tundish, note the time, and check it before and after one hot water cycle. Cold or hot, during reheat or constant — that is the whole diagnosis.
- Check the cylinder's pressure gauge if it has one, cold and then hot. A big swing between the two is the expansion story.
- Do not cap, plug or tape it, and do not put a bucket under it and forget about it.
Work on unvented cylinders and their safety devices is notifiable, and whoever does it should hold an unvented hot water qualification. If someone offers to sort the drip without mentioning that, get someone else.
What this costs, and when it is not our job
We charge £150 an hour, agreed before we attend, with no call-out fee. A dripping tundish is usually a diagnosis and a part in one visit: test the incoming pressure, check the vessel charge or re-form the air pocket, replace a relief valve that will not seat.
The honest part: if the tundish is running you do not need our boiler and heating leak repair as a detection job, because nothing is hidden. Where we earn our keep is when the tundish is dry and the system is still losing water. Rates and what is included are on the pricing page. A cylinder that has perforated needs replacing, which is a swap rather than a leak job.
