Pressure balancing valve problem

With guidance from a local design person we remodeled an old tile shower.The plumber installed a two outlet half inch Grohe pressure balancing and mixing valve which provides user temperature control. The tub outlet is capped and the other outlet feeds a Grohe volume control above leading only to spray handle on a flexible cable on a bar (no fixed shower head).

After installation was complete, the plumber said he was surprised to be told by a Grohe USA rep that the downstream volume control had to come out because the pressure balancing valve would be damaged by any back pressure (for example shutting the volume control valve first). Is this typical for combined pressure balancing/mixing valves? It seems as if illustrations in the Grohe catalog and flyers show what effectively we have installed (mixing valve with volume control/stop downstream). The design person said she never heard of this and puts volume controls downstream of either pressure balancing or thermostatic mixing valves in most jobs (though not always Grohe).

The bathroom hasn’t been put into use but the pressure balancing valve bleeds a little water when shut off so it is necessary to close the volume control to avoid sending a couple gph down the drain. Presumably some backpressure is then inevitable with the volume control closed.

What are our options? I'm not looking forward to tearing out the tile.

Any advice welcome Part numbers available if that helps.
Thanks.

valve

1. They make two kinds of valves. One is temperature control only and needs a downstream shut off to control the flow. The other has the on/off function built into the valve. Which do you have.
2. If yours does have the on/off function, then why do you have the second control valve, and the main valve should not leak any water unless it needs to be repaired.
3. When a multiport diverter is installed after a valve with its own on/off function all the ports have to be used even if it means connecting two of them to the same shower head device, to prevent using it as a shut off, but it is more to prevent using that control as the shower on/off valve than to prevent back pressure.

Thanks for your response.
1. The valve is a Grohesafe 35 253:
Diaphragm cartridge, Handle limit stop (temperature control),Built-in by-pass,
Check valves,Service stops 4-port (2 inlets, 2 outlets).
It does have on/off function (though it currently leaks)

2. The on/off function of the pressure balancing valve is binary and occurs at the cold end of the temperature range. To get warm or hot water, that valve must be on at full pressure. In a region with water conservation issues, the intent of the downstream volume control was to allow the user to reduce or temporarily halt water flow while showering, without changing the temperature markedly. To a non-plumber, it seemed like a reasonable plan from the designer and, until it was installed, apparently no-one learned that the Grohesafe valve was somehow incompatible with this.

3. This install has only one outlet, so no multiport diverter in the system, I don't know why we ended up with a two outlet rough-in, since Grohe makes an equivalent (?) single outlet, but it may have been what was available to the plumbing sub from the local distributor when they placed their order. Again to a naive observer, capping the tub outlet didn't seem unreasonable.

valve

The "temporarily stop the flow" is the problem. That would allow it to be used as the on/off valve for the shower once you set a comfortable temperature level, which means you could leave the shower valve on "permanently" which is a no-no. For that purpose the "shut off" valve should always have a small drip so the main valve would have to be used for the actual shutting off process.

valve

Thanks again, that was very helpful.

If we can correct whatever currently causes the mixer to leak, your suggestion of adjusting the volume control so it can't fully close seems like a much more useable solution than tearing it out. How much of a trickle to do we need to not damage the upstream valve?

Is it correct that even temporarily stopping the flow for a minute or two damaging to the mixing/pressure balancing valve? What happens that makes having the downstream valve closed when the mixer is open a no-no? Is this a general trait for this type of valve or something specific to Grohe's implementation?