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.