Plumbing tundish from McAlpine 3/4 inch white plastic © McAlpinne & InspectApedia.comPlumbing Tundish
What is & why is a tun dish used on hot water cylinders, heaters, geysers & heating boilers?

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What is a tundish and what are they for?

This article defines tundish and explains where and why a tun dish may be installed on a water heater cylinder, geyser, heating boiler or other pressurized tanks. We include a history of the term tundish and an explanation of the health and safety applications of tun dish use. We describe all of the locations where a tundish is used, including some you may have already seen but didn't think to call by that term.

Our page top illustration of a side-inlet tundish shows the water flow pathway (red arrows) and the air gap (blue arrow) in a 3/4" tun dish produced by McAlpine Plumbing Products (citation below).

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

What is a Tundish & Why, How & Where are they Used: water cylinder, geyser, heater TP valve safety devices

Ariston Tun Dish air gap and safety feature used on cylinder TP valve discharge tube & similar applications (C) Ariston & Arlene Puentes, Daniel Friedman

A tundish, or tun dish in some countries, is a fitting installed on overflow pipes to provide an air-gap and a visual window into the flow of water that may pass through the overflow pipe system. The purposes of the tundish are principally to prevent an unsanitary cross connection between an overflow or discharge pipe and a sewer line or drain.

[Click to enlarge any image]

Shown at left is a vertical Tun Dish produced by Ariston (citation below). Our arrows show the water pathway and the air gap that prevents unsanitary cross-connections between potable water and building drainage piping (CROSS CONNECTIONS, PLUMBING).

A secondary benefit of a tundish when installed on a pressure or pressure/temperature relief valve and discharge tube is to provide a visual indication that water is flowing in the overflow pipe - an important safety observation where the discharge tube is piped to a termination that is not visible.

In our adapted illustrations above, the air gap (blue arrow) permits observation that water is flowing through the device. If for example a tun dish is installed on the discharge tube of a water heater cylinder, geyser, or hot water tank, and should that valve open - indicating an unsafe condition as well as attempting to avoid a catastrophe - the building occupants can both hear and observe that water flow.

A tundish is basically a funnel. The origin of the term Tundish is from the brewing industry where casks or tuns were filled from the outlet of the brewing facility by routing the brew through a pipe that discharged over a funnel, originally a shallow wooden dish with an outlet channel that directed the brew into the inside of the tun or cask. The term tundish as a plumbing component is currently used to describe any funnel or hopper that is fed by an outlet pipe and that directs a liquid flow somewhere. - Wikipedia (2014)

Puentes comments:

In the U.S. the use of a tundish is not too popular. Most heating system people claim that inserting a tube into a trapped stand up pipe is enough of an air gap but I have personally witnessed a sewage back up (I was running the water, looking for leaks) that quickly overflowed a trapped stand up pipe to which a heating system condensate tube was inserted.

Watch out: the use of a tundish may be required or even prohibited by local codes and where used is regulated by building codes such as section G3 in the U.K. as are requirements for temperature & pressure relief valves, relief valve discharge piping, and cylinder, geyser or water heater labeling.

Where is the Tun Dish Device Used & Where is it Required by building code?

Tundish in use to protect discharge of evaporative cooler wastewater Australia plumbing code - InspectApedia.comCommon uses of the tun dish include

Our photo (left) illustrates a U.S. installed clothes washing machine drain connection that almost functions as a tundish. In-wall tundish devices are provided by Britex - in our product source list given below.

Watch out: a tundish or other air gap device can be a source of sewer gas entry into a building. The connection shown at left was indeed asserted to be the source of sewer odors in that home.

Clothes washing machine drain connection not quite a proper tundish (C) Daniel FriedmanWatch out: some service providers or communities do not permit discharge of air conditioner waste to the main sewer line out of concern for the volume of water discharged.

Watch out: as noted in our New Zealand references below, the choice of materials for handling hot water discharge must take the water temperature into account.

Building & Plumbing codes citing the tundish

Other Safety Controls on Water Heating Equipment besides the TunDish

In the U.K. an un-vented cylinder (water heater) and similarly in the U.S., Canada, Australia, New Zealand, and in the E. U., must be protected by at least the following three devices

The purpose of these safety controls is to prevent dangerous BLEVE EXPLOSIONS as water under pressure in a heated cylinder flashes to steam in a tremendous release of energy should the cylinder or tank rupture.

Gas fired heating equipment requires additional combustion-related safety controls such as gas spill sensors and oil fired heating equipment requires corresponding safety devices that monitor the condition of the oil burner.

Tundish Installation Details

By far the most complete detailed advice on proper installation of a tundish for water heating equipment that we have found to date is provided by Main, a U.K. heating equipment manufacturer cited just below. Scotland, Mandatory Standard 4.9.4 also describes tundish installation specifications.

Watch out: These excerpts adapted from Main's advice for tundish installation provides useful information. But be sure to check with the tundish installation requirements in your model and local building codes as well as the installation details supplied by your equipment manufacturer and by the tun dish manufacturer.

"Unvented Mains Pressure Water Heaters 120, 150, 170, 210, 250 And 300 Litre Capacity Indirect Models And 170 & 210 Litre Capacity Direct Models Installation And Servicing Instructions", Main Heating Ltd., Brooks House, Coventry Road, Warwick CV34 4LL, Tel: 0844 871 1532, 36005994 Issue 06, Website:, retrieved 5/21/2014, original source:

Where to buy a Tundish

In Australia, New Zealand, U.K.


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