What is a tundish?

what does a tundish do

What is a tundish?

A tundish, for sealed heating (boilers) and unvented hot water heaters, is a device that allows water released from a combined T/PRV or single PRV (caused by an overpressure or over temperature in the system), to be passed from the valve to a downstream pipe and allowing the user to see the water passing, via an open sided design.

It normally consists of an inlet, connected to the body of the tundish. It has an open-sided construction (normally by the use of two arms). It then has an outlet that connects to the downstream waste pipework. Up until the advent of dry trap tundishes, this waste pipe would normally be terminated safely, at of point outside of the building.

What does a tundish do?

On all sealed heating and hot water systems, it is a mandatory requirement that any water passing from the safety device (either a combined Temperature and Pressure Relief Valve (T/PRV) or Pressure Relief Valve (PRV)), is conveyed to a place where any passing water can be observed, but that it is discharged safely, not causing any potential danger to persons in or about the building.

A standard tundish provides an industry acceptable way of allowing this water to be observed from the inside of the dwelling. It gives the occupiers a way of seeing this water as a warning that there is something wrong with the system (Under normal operating conditions, water does not pass out of the PRV).

However, although it can be seen safely, it still needs to be conveyed to a point of discharge outside or, in the alternative, to a local drain.

If the installer has, for example, a landlocked application (e.g. a vented cylinder being replaced with a combi boiler in the old airing cupboard) or in a high-rise application, getting to a point of discharge other than to internal drain can be really problematic.

A standard tundish does not prevent smells coming back up from the drain. This is why I, invented an open-sided tundish trap with a non-return valve.


What is a dry trap tundish?


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The hotun dry trap tundish does what a standard tundish does plus it also stops smells or odours and foul gasses from entering the building via the drain connection.

Wet traps cannot be used for (T)/PRV applications. Water discharges from PRV’s happen infrequently, and a wet trap would dry out and become ineffective. This would then allow foul odours and gasses to enter into the building from the foul waste pipe.

The hotun tundish has a familiar design to that of standard tundishes. The inlet is connected to the body in an open-sided manner via two arms. Making the difference, it has, within the body of the tundish, a patented, one-way sprung valve as part of its construction.

tundish valve

The tundish non return valve is held in position by a light spring, strong enough to be held shut against gasses, but light enough to open when there is a small quantity of water sitting above the valve, at which point the valve spring opens and the water passes into the lower waste pipe and then closes again.

What does the hotun dry trap tundish do?

The hotun tundish non-return valve is designed to open (or trigger) at between 5ml and 30ml of water.

The trigger point is the same for both a boiler tundish and/or an unvented water heater tundish.

Small quantities of water drip into the tundish and are visibly held in place by the valve until water builds to sufficient quantity. Because of hotun’s open-sided construction, the water can be easily seen by the building occupier. They will now be aware of a potential fault and can then take remedial action. They would call in their local plumber, who can then determine the cause of the fault and repair.

On large, unvented hot water cylinders (above 15 liters in capacity) it is a requirement that they come with a device that prevents the water in the cylinder to reach above 95°C. They achieve this by combining a pressure relief valve with a temperature relief valve or a T/PRV. This valve is designed to open under pressure (to relieve the pressure safely) or when the water reaches 90°C. If the temperature in the cylinder starts to climb higher the valve is fully open at 95°C.

At this point, a large quantity of water is being discharged. The tundish and downstream pipework must be designed to handle this high temperature water without being compromised by the heat.

Materials suitable for use with high volume, high temperature T/PRV’s

Many in our industry confuse the need for “high temperature” waste pipe on different applications. They do not distinguish the different methods of operation between a relief valve operating just on pressure (PRV) and that of Temperature (combined T/PRV)

RA Tech UK Ltd, specifically advises on MI’s that only pipework suitable for T/PRV applications should be used. We do not manufacture waste pipework, but we do give guidance as to acceptable pipework materials. PolyPropylene (PP), Modified Plasticized Polyvinyl Chloride (MuPVC) or High-Density Poly Ethylene (HDPE) are all acceptable materials. However, they must still be correctly supported at 300mm centers.

It is important to remember that the volumes discharged by a PRV are totally different from that discharged by a T/PRV. The former with only drips being released and the latter, potentially high volumes. The configuration of pipework leading up to and away from the tundish are vital parts of the system to get correct. Once done it will allow for the successful discharge of any water released.

Service/Maintenance of the hotun dry trap tundish

It is best practice to service boilers or unvented cylinders once a year. At this point, an installer should check the hotun for its correct operation. They can also check and report on any suspect historical discharges.

On unvented hot water heaters, this can be observed as slight water residue on the plastic, or (in hard water areas) a small build-up of limescale. With hotun, this limescale can be easily cleaned off by pulling the valve up through the valve aperture (which self cleans any deposits off). Wipe the valve with a finger/cloth and then harmlessly pushed back into position. All of this can be done without having to dismantle any part of the product or remove it from the pipeline (unlike some other products). On servicing, this saves the installer time and money.

At the same time, the installer can feel for the valve’s smooth and unobstructed movement, proving correct operation. The service engineer should also pour a small quantity of water into hotun to check for the correct trigger point operation.

About hotun and RA Tech UK Ltd

hotun is a revolutionary dry trap tundish. RA Tech UK has pioneered and is market leader, in the removal of T/PRV waste water to drain. Because of hotun, for the first time ever, boiler manufacturers have accepted this type of installation. Saving installers and contractors, time and money, and, sometimes, on large high rise multi-apartment buildings, thousands of pounds!

For your project or development, choose the hotun tundish, plumbing at its best.

hotun, it’s not just a tundish.


To find out more for boiler applications please click here

For unvented hot water heater applications (large over 15 liters capacity) click here for under 15 liters capacity click here

To buy one of our hotun dry trap tundishes or products please see our trade suppliers list here

hotun was designed and invented by Russell Armstrong, a professional plumber, and Managing Director or RA Tech UK Ltd


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