Tundish Dripping: What It Is Telling You

What is a tundish and why is it dripping?

The tundish is the small funnel-shaped fitting, often with a clear window, fixed in the discharge pipe a short way below the relief valves on an unvented cylinder. It has no moving parts. Its purpose is to make any discharge visible and to leave an air gap, so that waste water cannot be drawn back towards the cylinder.

Because it only shows what arrives from above, a dripping tundish is a symptom with more than one source. The cause sits at one of the valves, and the two articles on the TPRV and on the expansion vessel deal with those. This page is about reading the tundish and the route it sits on.

What should a correct discharge route look like?

Knowing the intended arrangement lets you spot a poor one. Approved Document G describes it in outline. The tundish is upright, in the same space as the cylinder, fitted as close as possible to the safety device and lower than it, with only a short run of pipe between the valve outlet and the tundish. Any discharge has to be visible at the tundish.

Below the tundish, the pipe should drop vertically for at least 300 mm before any elbow or bend, then fall continuously, at least 1 in 200, to a safe termination where nobody can be harmed by hot water. The pipe is sized from the valve outlet size, and a longer run or more bends call for a larger pipe. The cylinder manual and the document set the exact sizes for your valve.

Two faults show up often. A tundish hidden behind a boxed-in panel, which nobody can see, and a discharge pipe with a flat or rising section, which can hold water and back up. Both are worth noting for the engineer. If yours is out of sight, a drip can run for weeks unnoticed.

What does the pattern of the drip tell me?

Read the tundish the way an engineer reads a gauge. Four patterns cover most calls.

• A short run that starts a few minutes after the cylinder begins heating and stops when it cools: expansion is not being absorbed, which points to the expansion relief valve and the vessel.
• A slow constant drip day and night, unchanged by the heating: a valve seat that no longer seals.
• A drip that appears mostly overnight or in the early morning: mains pressure rising when street demand falls, which concerns the pressure reducing valve in the inlet group.
• A heavy flow, or hot water or steam: the cylinder is overheating. This is a stop-and-call situation, not a diagnostic one.

Which valve is discharging into my tundish?

Two valves usually share the tundish and the window cannot tell them apart. The expansion relief valve is on the brass cold inlet group where the mains enters. The temperature and pressure relief valve is on the cylinder. Their opening pressures differ and are printed on the valves and in the manual, so do not rely on a typical figure.

You can look without touching. Follow the short pipes from the valve outlets towards the tundish. A wet outlet on the inlet group points at the expansion relief valve, and a wet outlet on the cylinder side points at the TPRV. Do not unscrew, lever or tighten anything while you do this.

Once you know the valve, the other articles take over: the temperature of the water and the control faults on the TPRV page, the pressure and the vessel on the expansion vessel page.

Why does a dripping tundish cause damp and limescale?

The drip is rarely the whole problem. Water that has been heated, even a little, leaves limescale where it evaporates. A tundish that has dripped for months builds a crust on its funnel and window and often on the valve outlets above it. That scale can stop a valve from sealing, so the leak helps to maintain itself.

The second consequence is moisture. If the tundish is in a cylinder cupboard, the discharge pipe often runs down inside the same space. A pipe joint that weeps, or a tundish that overflows because the pipe below it is restricted, wets the cupboard base and the timber under the cylinder. Rot and mould are common reasons a small valve fault becomes a larger repair.

If the tundish itself is heavily scaled, or its window is clouded and the funnel no longer drains cleanly, it is replaced as part of the repair. A blocked tundish cannot do its job, and a full one can overflow back into the cupboard.

What should I do while I wait for the engineer?

Keep the discharge pipe clear and uncovered. Never cap, plug or restrict it, and never fit a tap or valve to it, because a blocked discharge pipe removes the last safety path out of the cylinder.

Note the pattern. A phone photo of the tundish with a time stamp, and a short note of when it runs (on heat-up, overnight, constantly), saves an engineer guesswork.

If water is landing in the cupboard, switch the heat source off at the isolator rather than leave a hot cylinder pushing water out for a week. A diagnostic visit starts at the cylinder, not at the tundish, and the order is described on the hot water cylinder diagnosis page, with the repair side under unvented cylinder fault repair.