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Tundish on Boiler – Function and Simple Diagram Explained

When doing water leak detection work at properties locally in York and around the local area, especially when people are experiencing their boiler pressure dropping (especially common in winter), one of the things we look at is the boiler tundish (or ‘tundish drain’) on the central heating system pipework. In this article we’ll help to explain ‘what is a tundish?’.

You can see on the diagram below showing the parts of the tundish and PRV (Pressure Relief Valve) on a pressurised boiler system, that is looks like an upturned funnel shape (other similar shapes exist too) with an air gap. But what is a tundish and how does it work? We will look that in more detail next.

Tundish - How It Works
Tundish – How It Works

What is a Tundish?

First of all it is worthwhile mentioning that a tundish drain can be found on various things / devices but, for the purposes of this article, we are referring to those found on or near pressurised central heating boilers. Sometimes people call them tundish valves but that’s not the common name for them.

As the diagram above shows, most often they are in close proximity to the PRV (Pressure Relief Valve) or the ‘Pressure Release Valve’ as some people commonly call them. Not to be confused with a Pressure Reducing Valve (also know as PRVs!) which are different altogether.

Tundish Drain Dripping
Tundish Drain Dripping

A tundish is effectively a device fitting to allow you to see if there is dripping or running water coming from your boiler via your PRV. If it is dripping then, most commonly, there is a problem with your boiler pressure (being too high) or a fault with the PRV itself. Generally, you should not see water dripping from your tundish under normal boiler operation, if everything is running as it should regularly.

We discuss how a tundish might show you have a problem with your boiler in our article about Christmas water leaks. It has some really useful tips in it too!

What is a PRV?

A Pressure Relief Valve in some ways is somewhat self-explanatory in that is relieves pressure build on on your central heating system. It is a safety mechanism which normally is set to trigger at a certain pressure. As you can see on the image below, this one is set to 3 bar which is fairly normal.

3 Bar PRV
3 Bar PRV – Pressure Relief Valve

Basically, because a pressurised boiler system is an enclosed / sealed (unvented) system, pressure building up can be (a) dangerous and (b) could damage the system or (c) cause a water leak as a result if something cracks or breaks for example. Therefore, should it get to the level (of 3 bar in this example), the PRV ‘releases’ to reduce pressure back to normal levels.

This water that is released often goes via the tundish allowing you to see this is happening. As an aside to this, another interesting fact is that the air gap on the tundish also serves another useful function in that it stops the risk of water coming back up the pipe, for example from drains which could be catastrophic. Especially as it could be contaminated water with fragments in it and other unpleasant things you do not want in your boiler!

Pressure Relief Valve (PRV)
Pressure Relief Valve (PRV)

Why is my Tundish Dripping?

When a tundish drain is dripping on your pressurised boiler then one of two things has happened usually. The first possibility is that there is a fault with your system causing the pressure to build up too high. This pushes against the PRV spring and seal (or seat) and allows water to flow out of the PRV via the tundish.

The other thing that can happen is that the seal on the PRV (seen in yellow above) has either perished or has something within it that is breaking the seal and allowing the water to leak through, this can make the boiler pressure drop (more on that next).

The risk of the PRV breaking is generally considered higher after it has had a ‘release’ to reduce pressure. Why? Well because water from the system has passed through it which, bearing in mind the comments above, increased wear (especially if it has happened repeatedly) and also potentially particles and contaminants stopping a clean seal of the PRV.

Boiler Pressure Gauges - Tundish
Boiler Pressure Gauge – Digital vs Analogue

Tundish Drain Dripping and Boiler Losing Pressure?

If your boiler is losing pressure, it can cause your central heating system to stop working if it drops too low (many boilers have a cut-off). As a result, when we are doing our trace and access leak detection work, we will check the tundish to see if there is water leaking from the boiler and PRV.

This is just one of the many checks we do as part of our thorough professional leak detection service. We don’t just check one thing, we do a number of checks using our bespoke process and utilising professional leak detection equipment to help maximise the chances of finding your water leak efficiently and effectively.

Wherever possible, we utilise non-destructive methods so that we do not disturb your property. This equipment, such as thermal imaging leak detection and acoustic leak detection can even help find a leak under concrete floor.

We provide this service to domestic and business customers, with our commercial leak detection.

Tundish Dripping – Top Tip
“If you have a Tundish on your pressurised central heating boiler system, familiarise yourself where it is and, from time to time, check to see if there are any signs of dripping or moisture. If it is actively dripping, or even pouring when the boiler is running get it checked”

Leak Detection Team

Hopefully you now understand more about what is a tundish, it’s function and how it might help identify the cause of a water leak in your home or business.

Tundish Dripping – Related Articles

Here are a few other articles related to this subject you may find useful to review also:

If you have a water leak contact our friendly and experienced team for help.

Rainbow Restoration Leak Detection
Rainbow Restoration – Leak Detection

Here is a useful article explaining more about our services as a water leak detection company.

At Rainbow Restoration – York & Yorkshire Coast we are experts in Leak Detection and also Water Damage Restoration, so if you need help finding a water leak or getting your property back to normal after a water damage or a flood, get in touch with our friendly local team who will be happy to help you with this. We are based in York and help find water leaks in York.

Leak Detection Logo
Leak Detection Logo

Is a tundish necessary?

Leak Detection Logo - Small

Having a tundish on your boiler / central heating system is certainly desirable as it performs some useful functions. Firstly, it is a visual inspection device to help show if water is leaking from your boiler, for example via the PRV. Plus, given it has an air gap, it can help stop waste material being drawn back into your boiler which could cause damage.

My tundish is dripping, what do I do?

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If your tundish is dripping, especially regularly or a lot of water, then you should get your boiler checked by a qualified professional as you may have a fault with it, possibly the pressure relief valve (PRV) or something which is causing the pressure to build up and triggering the PRV (correctly). This can result in your boiler stopping working.

Where does the tundish drain water go?

Leak Detection Logo - Small

Where the water from a dripping tundish drain go exactly will depend on the application and installation. However, it is common for them to drain directly or indirectly into a property’s drain pipe / drainage system, soil pipe or to an external non-return pipe to the outside of a property. This transports the water away from the boiler system pipework.