I am installing a American Standard Thermostic Valve R510 in a Shower using 2 on/off volume vlaves. The instructions say to plug the "Tub Port" of the valve and tee off the other port when using more than one Volume valve. The tub port appears to be common with the remaining port and it would simplfy connections to use it for one of the volume valves. American Standard says not to, but can't tell me why.

Hi Gary

The instructions say to PLUG TUB SPOUT, but that is only if the tub spout isn't being used. Here, if the tub spout was to be used, you would need a tub spout with a diverter knob, or a on/off volume control to control water flow to the tub spout and/or shower heads.

I can't really image why it would be easier to use the tub spout for an additional shower head compared to simply installing another tee fitting off the shower (and plugging tub spout), but if you want to pipe it that way using an on/off volume control you should not have a problem with it!

Good luck!

Mark

using 2 on/off volume vlaves
Assume that means you are installing one on/off valve tub and one for the shower.

The tub port is common with the shower port. That is why you have to block it. Water only comes out the shower port if it can't go out the tub port.
Tee off the shower port with one side of tee going to the tub on/off volume valve, then to tub spout. Other side of tee goes to shower on/off volume valve then to shower head.

Thermostic Valve R510 only serves as a mixing valve and can stay set at the desired temperature, using the tub and/or shower valves for on/off and volume control.