Backflow protection for shower mixer AND shower hose water inlet?

[Marritza]

We are refitting our bathroom and need a new shower - planning for a shower mixer with an overhead shower and a shower rail set, all concealed. Found the right combination but am stumped regarding the choice being offered of backflow protection being integrated into the shower mixer or not, so I am not sure which version to buy. It's a hansgrohe mixer "with integrated safety combination EN 1717" or without one, which I found out refers to this backflow protection device (grohe rather than hansgrohe but probably similar) - https://www.grohe.co.uk/en_gb/safety-combination-en-1717-29008000.html.

So, having looked it up online and checked with my plumber I decided to be safe and get the version with the backflow protection despite it making the mixer about £100 more expensive. However, they are also offering two versions of the shower water outlet - with or without a non-return valve, which is presumably the same valve as their "backflow protection" for the shower mixer and which seems to cost about the same except that the one with the valve has a plastic connector rather than metal.

without - https://www.hansgrohe.co.uk/articledetail-fixfit-wall-outlet-s-without-non-return-valve-27453000
with - https://www.hansgrohe.co.uk/article...non-return-valve-and-synthetic-joint-26453000

There does not seem to be such an option for the overhead shower (also hansgrohe).

The plumber says I only need one backflow protector although he's covered himself by saying I should ask hansgrohe, which I have and having got nowhere, I am wondering whether: 1) I need backflow protection at all and 2) if I do, then which one should I get - the one in the shower mixer or in the shower water inlet? Do I need both?

We have an unvented system boiler with a hot water tank and good water pressure.

Would be grateful for any advice - the retailer haven't been particularly helpful either, unfortunately, and I have neither plumbing qualifications nor experience as you can probably tell...

 

[CBW]

Backflow protection is required to prevent undesired water from entering the potable (drinking) water supply. For showers this is generally achieved by use of a retaining being to prevent the shower head from being allowed to fall into anything it shouldn’t including the bath spillover level, a nearby basin or wc pan. As for the incoming feeds, if they are unbalanced supplies then iirc single check valves are required, if they are balanced supplies then no backflow prevention is required. Hth?

 

[Marritza]

Thanks for your reply. The shower head is in no danger of falling into anything and I'm not sure our incoming feeds are unbalanced - we have a system boiler with a hot tank and we've never had problems with water pressure. Does this mean I don't need any backflow prevention then?

 

[cjcoffin]

If you have an unvented hw cylinder you should check it has a cold water outlet pipe connected to the pressure reducing valve, if so you have balanced supplies and don't need backflow prevention. If you have a vented hw tank then you do.

 

[Marritza]

Thank you for the clear reply, cjcoffin - I actually have an unvented cylinder, so I won't go for the backflow protection.

Incidentally, I managed to get through to a very helpful member of the technical staff at hansgrohe, who told me that both versions of the shower mixer have a non-return valve, with one of them having additional backflow protection. I definitely won't need the further backflow protection that is in the water inlet valve for the shower.

 

[Madrab]

if so you have balanced supplies and don't need backflow prevention

Unfortunately whether the supplies are balanced or not has no bearing on whether backflow protection is required or not. Backflow/backsyphoning protection is to ensure that contaminated or non CAT1 water isn't drawn back into the system and contaminate the mains water, if there were to be say a pressure drop in the supply.

If I may also add, whether there is a vented cylinder or not doesn't affect the requirement to have backflow protection either. If a system is unbalanced, if say there was a mains cold and a gravity hot, then that's when NRV's would typically employed in a shower and are fitted in most cases but they are 2 different beasts.

 

[Marritza]

So, would you advise me to get the shower mixer with the additional backflow protection or is it not required because there is a non-return valve in the standard version already?

 

[Madrab]

All mains water systems should really have backflow protection. The balance is between having the water pressure to overcome the restriction without an impact to the flow. If you have unvented HW then you already have backflow protection built into the combination valve and I would hope it has been specified correctly therefore the mains pressure and flow is adequate.

No real need to have a valve that has inbuilt backflow protection if it is costing you an extra £100 and the shower outlets can't get to the bottom of the tray/bath/toilet and be immersed in contaminated water, worst case is a double check valve could be fitted to the cold supply - £5 for one @ 15mm

 

[Marritza]

That sounds reasonable, thank you Madrab!

 

[Madrab]

My pleasure