What is a tundish?

TundishA 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 overpressure or overtemperature in the system) to be passed from the valve to a downstream pipe.  Also, it allows the user to see the water passing, via an open-sided design.

Firstly, normal designs consist of an inlet, connected to the body of the tundish. containing an open-sided construction (normally by the use of two arms). Secondly include an outlet that connects to the downstream waste pipework. Up until the advent of dry trap tundish, the waste pipe would normally be terminated safely at a point located outside of the building.


What does a tundish do?

On all sealed heating and hot water systems, it’s mandatory for any water passing from the safety device (either a combined Temperature and Pressure Relief Valve (T/PRV) or Pressure Relief Valve (PRV)), to be conveyed to a place where any passing water can be; observed, discharged safely, and doesn’t cause danger to a person or building.

A standard tundish provides an industry-acceptable way of allowing water to be observed from within the building. It gives the occupiers a way of seeing the water. Using this as a warning something could be 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 the building.

This becomes problematic, for an installer in the following situations. A landlocked application (e.g. a vented cylinder is replaced with a combi boiler in the old airing cupboard) and high-rise application. The only solution is to utilise the internal drain.

Also, a standard tundish doesn’t prevent smells from coming back up from the drain, causing odour to circulate on the premises. The solution – is an open-sided tundish trap with a non-return valve. So what is it?


What is a dry trap tundish?

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

dry trap tundishWet traps cannot be used for (T)/PRV applications. Water discharge from PRVs happens infrequently and a wet trap would dry out, becoming ineffective. This also allows foul odours and gasses to enter the building via the foul waste pipe.

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

The tundish non-return valve is held in position by a light spring. The valve is strong enough to be held shut against gasses but light enough to open when there is a small quantity of water sitting above. When this occurs the valve springs open and water passes into the lower waste pipe before closing.


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.image of boiler installation

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 a 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 it.

On large, unvented hot water cylinders (above 15 litres 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


large unvented installation diagram

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 pipes for 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

installation process document for a tundishIt 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 the market leader, in the removal of T/PRV wastewater 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 about 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 of RA Tech UK Ltd

Get In Touch

Registered Address

RA Tech UK Ltd
6 Nottingham Road
Long Eaton, Nottingham, UK

T: 01332 702678
E: info@ratechuk.co.uk



In one simple fitting, hotun solves the problem of compliantly running a PRV valve discharge from a pressurised hot water cylinder relief valve or boiler pressure relief valve to a local waste or soil pipe.

hotun combines a tundish and a non-return valve to make a dry trap tundish which acts as a visible early warning device and prevents foul odours from escaping from the waste/soil pipe.


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