Unvented hot water systems are now more popular than traditional water heating solutions. An unvented cylinder is connected directly to the mains so there is no need for a cold water tank in the loft to bring in cold water from the mains. There are lots of advantages to having this kind of system. For example, you can save space in the loft by not having a tank, and the system delivers high water pressure even when multiple appliances or taps are turned on. However, because the system operates under high pressure, some safety devices are required, including a tundish.

A tundish sits on the piping just outside the cylinder near the temperature and pressure relief valve. The pressure relief valve (PRV) will ensure that the pressure inside the tank doesn’t get too high as the hot water goes in.

The tundish is a conical-shaped piece of plastic that sits within two pieces of pipe. It contains a clear window that allows you to see into the pipe and look out for any drips that shouldn’t be there. It must be positioned within 600 mm of the PRV in a safe and visible position so that it can be checked. The pipe from the tundish should never be smaller than the tundish outlet. 

What does a tundish do?

When the pressure in a hot water tank gets too high, a PRV relieves some of that pressure by allowing steam or water to exit the tank. This water should be drained correctly into the proper pipe. The tundish needs to be put in place so you can see if there is a problem with your water tank. The clear window allows you to see when water is exiting the pipe, which could be a sign of an underlying problem.You shouldn’t be able to see water leaking through the tundish. If you can, this could suggest a problem with the PRV or the expansion vessel.

The tundish is also able to supply an air break. The air break is a literal break between two pipes and is used to ensure that any waste water is taken away from the appliance completely. Without the air break, the water that exits the hot water tank may be sucked back into the tank because of the high pressure. An air break prevents this from happening and ensures that the waste water drains correctly. This helps to prevent any cross contamination, particularly if an overflow pipe is connected to a drain or sewage pipe. 

What is a tundish used for?

A tundish is used in many different appliances, not just heating systems.

It can be found in air conditioning units. The condensate that is created by the unit needs to go somewhere. It is discharged to a drain pipe, which is why a tundish is required. The same is true for condensation that is created during refrigeration.

In some cases, a tundish may be required for a shower drain. A shower gully is used in wet rooms or shower rooms. This gully may be connected to a central wastage drain. To prevent the drain from becoming blocked with sewer water, a tundish is fitted with an air gap to ensure that the water from the shower is draining fully. Some other water appliances that require water to drain may need a tundish for the same reason, such as dishwashers, washing machines and even sinks.

It’s important to check the tundish to ensure that it’s working correctly. First, you should check it for signs of water. If there’s water in your tundish, then there could be an issue with your PRV.

What are the regulations for a tundish?

When a tundish is installed, the suitably qualified heating engineer should position it in accordance with the relevant building regulations. The required installation and position of the tundish is explained in the Building Regulation G3 Approved documents.

The tundish should be located close to the PRV, and no more than 600 mm away from it. The minimum straight drop below the tundish should be 300 mm. This means that the pipe that exits the tundish should remain straight for at least 300 mm before bending or turning in a different direction.

The tundish must be vertical and it shouldn’t be located more than 500 mm horizontally from any valve that discharges to it. It needs to be in a place where you can easily see it, as it’s there to help you determine whether there are any leaks, but without becoming a hazard. 

There are also certain regulations to follow for the discharge pipe that leads out of a tundish. This pipe should be made of metal and it should be wider than the inlet pipe. The inlet is usually around 15 mm, whereas the outlet should be around 22 mm.

For a diagram that demonstrates all of these regulations, you can visit the Upperplumbers website.

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