What is a tundish?

A tundish is a small safety device fitted to the discharge pipe of an unvented hot water cylinder. It is usually made from clear plastic or metal and has a visible air gap that allows you to see if water is being discharged from the cylinder's safety valves.

Under normal conditions, there should be no water flowing through the tundish.

If you notice water dripping or running through it, it means one of the cylinder's safety valves has opened. This could happen because:

The Temperature & Pressure Relief (T&P) Valve has opened due to excessive temperature or pressure.
The Expansion Relief Valve is releasing excess pressure.
A safety valve has become worn or faulty and is opening when it shouldn't.

A small amount of discharge immediately after the cylinder has heated up can sometimes be normal. However, a continuous drip or steady flow is not and should be investigated. Besides wasting water and energy, it could indicate a fault that needs attention.

If your tundish is dripping regularly, it's a good idea to have the cylinder inspected by a G3-qualified engineer. Common causes include a failed expansion vessel, a faulty pressure-reducing valve, or a worn T&P valve.

The tundish is an important safety feature because it provides a visible warning that something isn't right. If the discharge pipe went straight into a drain, you might never realise your cylinder was losing hot water or that a safety valve was operating.

See:
https://easyflow.co.uk/articles/what-is-a-tundish-everything-you-need-to-know-about-what-a-tundish-is-and-how-to-check-it