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

  • TUNDISH used in PLUMBING - CONTENTS: definition of tun dish or "tundish" and explanation of how, where & why this safety feature is used on pressurized tank relief valve discharge tubes & similar locations on water heaters, cylinders, geysers, and hydronic heating boiler relief valves
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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 tun dish 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).

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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

  • Air conditioning / heat pump condensate discharge drain piping where the piping is connected to plumbing waste system piping
  • Discharge tube serving the pressure/temperature relief valve on water heating equipment: cylinders, hot water heaters, geysers
  • Floor waste gullies
  • Refrigeration condensate discharge in localities where the condensate discharge is (or must be) connected to sewer system piping
  • Evaporative cooling drain discharge when the drainage is routed to a disconnector gully.
  • In-wall tundish devices may be used to receive a variety of discharges (with suitable piping) including even clothes washers or dishwashers, water softeners, and other water conditioning equipment, all of which may require an air gap in their drain piping.

Clothes washing machine drain connection not quite a proper tundish (C) Daniel FriedmanOur 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.

Watch 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

  • Australia Plumbing Codes (PCA)
    • "Technical Solution 3.04, Refrigerated Cabinet Condensate Drains Connected via Tundishes", AS/NZS 3500:2 Plumbing and Drainage provisions for below-gr0und sewer lines, Part 2: Sanitary plumbing and drainage (discussion of floor waste gullies and the connection of tundishes) requires that the maximum un-vented drain length can be 10m, and that a sealed gully inside the building must be protected as per Clause 4.6.5
    • AS/NZS 3500:2 Clause 11.21 - air gap requirements for refrigeration condensate drain connections
    • AS/NZS 3500:2 Figure 4.3A clause of 3500:2 - Retrieved 5/21/2014, original source: _Refrigerated_cabinet_condensate_drains_connected_via_tundishes.pdf
    • "Technical Solution 7.08, Split System Air Conditioning", (2012) Plumbing Code of Australia (PCA) 2008, AS/NZS 3500:2: Plumbing and drainage Part 2: Sanitary plumbing and drainage is a "deemed to satisfy" document listed in part CI of the PCA and contains a section on the connection of tundishes that is also cited for split system air conditioners - Retrieved 5/21/20145, original source
    • "Technical Note, Evaporative air conditioners waste water drains and water supply", Plumbers Licensing Board, Government of Western Australia, Regulations 2000, specifies use of a tundish to provide a visible air gap that is 2 x the internal diameter of the receiving drain of discharge water from an evaporative cooling system routed to a gully

  • New Zealand Plumbing Code
    • "Drains from hot water cylinder relief valves", - retrieved 5/21/2014,original source - excerpt:

      AS/NZS 3500.2 (cited in G13/AS3) says the range of temperatures likely to discharge to the plumbing and drainage system must be taken into account when selecting materials for use in the plumbing system. Therefore, if the drain line discharges over a tundish into the plumbing system, the plumbing system has to be constructed of a material that will take the high temperature discharge from relief valves.

      Watch out: Failure to comply with these simple requirements could cause the PVC-U plumbing system to melt when a relief valve discharges, resulting in a failure of the plumbing system.

  • Scotland, Mandatory Standard 4.9, "Every building must be designed and constructed in such a way that protection is provided for people in, and around, the building from the danger of severe burns or scalds from the discharge of steam or hot water.", referring to unvented water heaters states:
    the tundish and discharge pipework should be correctly located and fitted by the installer and the final discharge point should be visible and safely positioned where there is no risk from hot water discharge.

    Section 4.9.4 "Discharge from unvented hot water storage systems" contains specifications for the connection of the tundish to the relief valve and piping from the tundish to its final discharge point. Also see our example tundish installation text given below.
    - retrieved 5/21/2014, -original source:

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

  • a temperature controlling thermostat,
  • a high temperature cutout, and
  • a temperature and pressure relief valve or TP valve - in the U.K.,, Australia, New Zealand, for example, Temperature and pressure relief valves are designed to keep the temperature below 99°C and to relieve excess pressure to make sure valve-vented storage water heaters do not explode.
    • Acceptable Solution G12/AS1, and Standards AS/NZS 3500.4: 2003 (cited in G12/VM1) and NZS 4607: 1989, all require:
    • the drain line from the relief valve to be in copper
    • discharge to an appropriate place that does not cause damage to the building. - op .cit. New Zealand (2014)

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.

  • Install the tundish in the same space as the un-vented hot water storage system
  • Install the tun dish by fitting it as close as possible to (and below) the temperature/pressure relief valve outlet opening, with no more than 600mm of pipe between the valve outlet and the tundish device.
  • Install the tundish and related equipment, meaning the water heater or storage tank itself in a location and position where the air gap provided by the tundish will indeed give a view that permits observation by building occupants that water is in fact spilling through the device, should that condition occur.
  • In buildings where occupants are vision impaired or mobility impaired, additional suitable warning devices should be installed to indicate that the relief valve has opened
  • The discharge pipe receiving water from the tundish should extend vertically (down) at least 300mm below the tundish before any elbows or bends in the discharge pipework
  • Discharge piping must slope continually downwards on a slope of at least 1 in 200 (for the U.K. or 1/8" U.S.) and can be piped "upwards"
  • Discharge piping both before and after the tundish should be made of metal or other material that has been listed as capable of withstanding the hot water temperatures that may occur (e.g. in the U.K. part of BS 7291).
  • Discharge piping should be at least one pipe size larger than the nominal outlet size of the temperature / pressure relief valve unless its total equivalent hydraulic resistance exceeds [sic] that of a straight pipe 9m long, i.e. for discharge piping between 9m, and 18m the equivalent resistance length should be at least two sizes lager than the nominal outlet of the safety device; between 18 and 27 m this size increases to 3x, and further, bends must be taken into account when calculating the flow resistance. The citation given below provides additional details.
  • Discharge piping from a tundish and temperature/pressure relief valve should not be connected to a soil discharge stack (plumbing vent piping) unless it can be demonstrated that the discharge stack can safely tolerate the temperature of water discharged therein, includes fittings to prevent sewer gases from passing back up and out of the tundish air gap, has no sanitary devices connected (toilets for example), and if plastic pipes are used ... they should be either (PB) to Class S of BS 7291-2:2006 or cross-linked polyethylene (PEX) to class S of BS 7291-3:2006 and finally, the pipes must be continuously marked with a warning that no sanitary appliances should be connected to the pipe.
  • Additional required installation and safety details are in the citation below.

"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.

  • 3P Technik, UK, Email: Tel: 01239 623506 Tundish Article No 5000411/ 5000410 with brackets and fittings. Quoting from the company's 3P Tundish brochure, the device is ideal for use as an air gap to prevent back-flow.
    Tundish with DN50 outlet and DN40 overflow, with wall fixing bracket and ½“ elbow allowing the tundish to act as an unobstructed air gap to prevent pollution of potable water by backflow; meeting BS EN13076. Specification
  • Ariston Boilers, Artisan Building, Hillbottom Road, High Wycombe, Buckinghamshire, HP12 4HJ, Technical Advice: 0333 240 7777, Sales: 01494 459775, Email: Technical Advice:, Customer Service:

    Ariston produces the Ariston Kit C 15 x 22 mm Tundish, Product Code 79926 available from various vendors including Ariston Kit C 15 x 22 mm Tundish, Product Code 79926 ScrewFix Direct Ltd., Website:, Tell: 0500 414141
  • Britex, Britex Place, Mira Crt, Bundoora VIC 3083, Tel: 03-9466 9000, Website:, "In Wall Tundish", the company has other locations in Australia including in Sydney, Perth, Brisbane, and Adelaide. Except from the company's product literature:

    The BRITEX In Wall Tundish is manufactured from 1.2mm satin finish stainless steel and comes complete with a removable inspection window allowing for visibility to detect blockages and ensure ease of cleaning. It is primarily used as a discharge point in refrigeration and air conditioning wastes for conditioning wastes for condensation and waste water. The In wall is commonly used in hospitals, apartment buildings, medical centres and aged care facilities.
    - retrieved 5/21/2014, original source:
  • Intatec Limited Airfield Industrial Estate Hixon Stafford ST18 0PF U.K., Tel: 01889 272180, Website:, provides a tundish 15mmx22mm, code TUN23711522. This product is distributed through local plumbing suppliers
  • McAlpine Plumbing Products, Kelvin Avenue, Hillington, Glasgow G52 4LF , Website:, McAlpine White One Piece Tundish, 3/4" Product Code TUN3, available from plumbing suppliers in the U.K.

    The company provides tun dish products including vertical and side-inlet devices in sizes up to 50 mm. in inlet diameter in both plastic and chrome plated products. McAlpine also produces non-return valves ("check valves" in some terminologies) and other plumbing products. McAlpine products are distributed through your local plumbing supplier.


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