What is a Tundish?

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Last updated: October 31, 2023

There are two types of hot water cylinder available on the market: vented and unvented. While both of these hot water systems have their many advantages (and some disadvantages), the unvented hot water solution is more popular as it’s more flexible in terms of space restrictions, plus it delivers higher water pressure. However, because the unvented system operates under high pressure, certain safety devices are required, such as a tundish.

But exactly what is a tundish? In short, the tundish is a safety device that provides a window into the overflow system, allowing you to see any water drips that shouldn’t be there. In other words, the tundish helps keep the unvented hot water system safe and sound. Not sure if your system needs a tundish? Read on because in this article, we cover everything you need to know about tundishes, including exactly what they are, what they do and how you can install one.

What Is a Tundish?

A tundish is a piece of plastic that is placed close to the pressure relief valve. Canonical in shape, it looks a bit like a plastic cup sitting between two pipes. With a clear window, the tundish allows you to see if any drops of water have escaped the system as a result of high water pressure.

For safety reasons, the tundish should be placed vertically and at least 500 mm from the pressure relief valve (above the discharge pipe). It should also be in a visible position so that you can easily and safely check it for leaks.

What Does a Tundish Do?

A tundish is a handy little safety device with two purposes. The first and most important one is to prevent cross-contamination between two pipes – the overflow pipe and the drain pipe. The second task of a tundish is to serve as an air gap through which you can see if water is dripping from the pressure relief valve.

  • Prevents cross-contamination

Under normal operating conditions, water shouldn’t flow out of the pressure relief valve, however, when the water pressure in a system gets too high, the valve must relieve some of that pressure (hence the name!) by allowing the water or steam to exit through the overflow pipe. This unsanitary water should be drained from the system completely and not sucked back into it. This is where the tundish comes in.

  • Serves as an air gap

By providing a break between the overflow and drain pipe, the tundish gives residents of the house where the heating system is located the opportunity to see if there is any leakage, which is usually a sign of an underlying problem. It’s worth mentioning again that under normal operating conditions, you shouldn’t be able to see any water dripping through the tundish. But if you can, you should report the issue to a Gas Safe Registered engineer.

How Do I Check My Tundish?

Checking your tundish is very simple and doesn’t require any special tools or skills. All you have to do is find the tundish on your heating system and then check for water discharge. If you don’t know where to look, simply locate the temperature and pressure relief valve; the tundish should sit right there, just outside the cylinder.

If you notice any water drippage flowing into the tundish, this is a sign that your pressure relief valve is leaking. This is usually a result of one of two problems: either there is high pressure in the unvented cylinder or the pressure relief valve itself is faulty. Either way, to fix the issue, you’ll need the help of a heating engineer.

To keep your heating system in good shape, we recommend checking your tundish on a regular basis, as a part of your normal home maintenance. Remember, if you discover the problem early, you can save yourself a lot of time, money and headache!

Is a Dripping Tundish Dangerous?

A dripping tundish can be dangerous if not dealt with on time by a professional.

As mentioned, there are usually two problems with the hot water cylinder that can lead to water leakage. These are excessive unvented cylinder pressure and/or a faulty pressure relief valve. If your heating system is experiencing the first issue, the pressure needs to be adjusted, which should only be done by a professional boiler plumber or Gas Safe registered engineer (even tiny mistakes in pressurized systems can be dangerous). If the problem lies within the pressure release valve and the system is not over-pressurised, the valve could fail completely, leading to a whole lot of mess. Either way, the issue should be fixed as soon as possible by a heating engineer.

Do I need to be Qualified to Install a Tundish?

Yes! Do not attempt to install a tundish if you’re not qualified to do so.

The tundish must meet certain building regulations and it should be installed by a qualified heating engineer. Both the proper installation and positioning of the tundish is detailed in the Building Regulation G3 Approved documents.

According to the regulations for unvented hot water systems, the tundish should be installed at least 500 mm (or within 600 mm) from the pressure release valve. It should also be placed in a visible position so that the house occupants can easily and quickly check for any signs of dripping. There are also certain rules for the discharge pipe: according to the regulations, it should be constructed from metal in order to resist hot temperatures, and it should be wider than the inlet pipe.


  1. What is a tundish? – Vaillant