A tundish is a funnel-shaped fitting with a visible air gap, installed in the discharge pipe from a hot water cylinder's relief valves — in UK practice within 600 mm of the temperature & pressure relief valve. It does two jobs: the air break stops discharged water being siphoned back into the system, and the open window makes any discharge instantly visible. A healthy cylinder shows a dry tundish.
What does a tundish do?
A tundish provides a visible, protected break in the pipe that carries relief-valve discharge from a sealed hot water cylinder to a safe outside point. Water falling through its open air gap cannot be siphoned back, so the potable system stays protected from backflow contamination, and because the gap is windowed, anyone walking past can see whether the safety valves are passing water. On a correctly working unvented cylinder the tundish stays dry; any drip is diagnostic information.
The fitting is deliberately simple — no moving parts, nothing to service — which is exactly what a last-line visibility device should be. European unvented products to EN 12897 and national rules such as UK Approved Document G3 require discharge pipework to run via a tundish, typically installed close to the valve group (within 600 mm of the temperature & pressure relief valve in UK practice) and with the downstream pipe falling continuously so discharge can never pool.
Why is my tundish dripping?
A dripping tundish means a relief valve upstream has opened or is letting by — the tundish itself is never the fault. The pattern of the drip usually identifies the cause:
| Symptom | Most likely cause |
|---|---|
| Drips only while the cylinder heats up | Lost expansion vessel pre-charge — expansion water has nowhere to go |
| Steady drip at any time of day | Expansion or pressure relief valve held off its seat by debris or wear |
| Drips mainly overnight | Mains pressure rising at night above the pressure-reducing valve's tolerance |
| Hot water or steam discharge | Over-temperature event — the temperature & pressure relief valve has opened; isolate and investigate immediately |
The expansion vessel is the leading culprit in practice: its diaphragm pre-charge bleeds down over time, and once the cushion is gone, every heat-up cycle pushes the expanded volume — roughly 2% of the stored litres — straight out through the expansion relief valve. Re-pressurising the vessel is a routine service task. The full diagnostic walk-through is in why is my unvented cylinder discharging through the tundish.
Where should a tundish be installed?
A tundish belongs close to the relief valves it serves, in a position where discharge is visible to occupants, with the pipework arranged so water always falls away from it. The upstream section from valve to tundish should be short and the downstream discharge pipe must fall continuously — a vertical drop below the tundish before the first bend, then a graded run to a safe, visible external termination. Discharge pipes are sized up a diameter for longer runs so that a full-bore steam-and-water discharge can never back up through the air gap. Exact dimensions and termination rules are set nationally, so follow the local code and the cylinder manufacturer's instructions.
Is a dripping tundish dangerous?
An occasional cold drip is not dangerous — it is the system telling you a service item needs attention, most often the expansion vessel charge, and it wastes water and heat until fixed. What it must never be is ignored or, worse, capped off: plugging a discharge route disables the safety chain of a sealed cylinder. A continuous discharge, or one that is hot or steaming, indicates a control failure and calls for isolating the heat source and calling a competent installer the same day. Checking the tundish is dry takes two seconds and belongs in every annual service alongside the checks in common problems with unvented cylinders.
Frequently asked questions
Should a tundish ever have water in it?
No — in normal operation the tundish is completely dry. Any drip means a relief valve has opened or is letting by, most commonly because the expansion vessel has lost its pre-charge. Treat a wet tundish as a service call, not a cosmetic issue.
Can I just cap off a dripping tundish or its discharge pipe?
Never. The tundish and discharge pipe are the escape route for the cylinder's safety valves; blocking them turns a nuisance drip into a genuine safety hazard on a sealed vessel. Fix the cause of the drip instead — usually the expansion vessel or a fouled valve seat.
Why does my tundish only drip when the water is heating?
Because heated water expands by about 2% and needs an expansion cushion to absorb it. If the expansion vessel's air charge has bled away, each heat-up forces that volume out through the expansion relief valve and past the tundish. Recharging or replacing the vessel normally cures it.
Does a vented cylinder have a tundish?
No. An open-vented cylinder relieves expansion up its open vent pipe to the header tank, so it has no relief-valve discharge to make visible. Tundishes belong to sealed, unvented systems.