7 litre invented under sink heater

[Matt0029]

I was at a job today and notice a 7litre under sink heater was fitted. Because of the size under 10litres do these not need a pressure relief valve and expansion vessl? It didn't have any of the kind. Thanks.

 

[ShaunCorbs]

Yes they do require them

 

[Matt0029]

Thanks I thought so but then read the data bagde that 7litres I wonderd if they didn't. There was no pressure reducer, check valve or anything.

 

[SJB060685]

Doesn't all unvented regardless of how small require both? With no expansion vessel the pressure will rocket when heated

 

[Matt0029]

Doesn't all unvented regardless of how small require both? With no expansion vessel the pressure will rocket when heated

You can put a loop in the cold water pipe work between the check valve and the unit I think and is an acceptable way of not having an expansion vessle, I seem to remember?
[automerge]1578340624[/automerge]

You can put a loop in the cold water pipe work between the check valve and the unit I think and is an acceptable way of not having an expansion vessle, I seem to remember?

Though there isn't even that on this one

 

[SJB060685]

As you know when water is heated its average kinetic energy rises. Molecules move faster and if no room for expansion they bounce of system walls and generate more pressure. Personally the way I see it is it doesn't matter whether 5 litres or 500 litres, without an expansion vessel, somewhere to accumulate the expansion then the pressure will rise. Am I wrong?

 

[ShaunCorbs]

Not the best as manufacturers say straight pipe work

 

[John.g]

A few years ago a friend of mine had fitted a 10 Litre Ariston under sink heater which came supplied with a Ariston PRV, but said he was worried because this PRV also had a NRV and was worried/wondered where the ~ 0.2/0.3 litre expansion could go even though it seems (here anyway) to be accepted that it can be allowed to expand back through the cold water supply provided that the nearest cold water draw off is > 2.8M to ensure that no hot water will issue from this tap (1/2" supply) when opened, (this may be the reason for a "loop"?) (and obviously as long as there is no pressure reducing valve in the cold water supply)

I had a good look at this PRV (Ariston 571730) and here is my take on them.
This PRV is fitted directly on the top of the heater with the cold feed then connected.
They are fitted where the expansion volume is taken up by the cold feed pipe and consist of three components combined in one fitting..... the PRV itself which is in the side branch, you can see a little plastic "lifting gear" which can be used to manually open the PRV or maybe can be used as a air release if the cold water supply has failed or whatever. The second component is a “NRV” (hence the worry) which is fitted in the main body of the valve and opens (downwards) to allow cold water to enter the heater when there is a hot water draw off. I think its main purpose is to act as a anti syphon valve in the event of the cold water supply failing/draining down and keeps the heater full of water, the third component is another “NRV” which is inside the "main" NRV and opens in the opposite direction to allow expanded water to escape into the cold mains, main NRV is ~ 8mm dia and the smaller NRV is ~ 6 mm.
When cold water is admitted to the heater then the main NRV (containing the smaller NRV) will open and allow the heater to fill., the main NRV will then close and as the water expands in the heater then the smaller NRV will open at 1 bar and allow the expansion into the cold feed but will still keep the heater full of water under all conditions.

Are these PRVs allowed/available/fitted in the UK? as they are relatively quite common around here but ony on 5 to 10 litre electric heaters.

 

[moonlight]

As above follow manufactures installation instructions. If it has already been installed by someone else you can always download the instructions.

 

[Matt0029]

Thank you. Can I also ask with the prv. Is the pipe allowed to go to a hepvo trap bar a tundish and be teed in to the waste pipe system with other appliances on it. Or does it need its own waste pipe if doing it this way. Like unvented over 15litres.