When I did my research in selecting shower fixtures I couldn't tell the difference between pressure balance and thermostatic valve and after much reading I came to this conclusion. I'm only talking about the Pfister line since that's what I purchased. Pressure balance valves Normally work at a maximum of 2.5 gallons per minute which is what a standard shower head needs. They normally operate at full pressure when turned on and the single handle valves only adjust the temperature, not the volume, unless you purchase a higher end fixture. The temperature is controlled by way of adjusting the water pressure and attempts to maintain it with within +- 3-5 degrees and you will notice a drop in water pressure when a toilet if flushed etc., since it's attempting to balance the pressure to maintain temperature. There will not be a temperature setting on the trim kit as since the pressure balance valve has no idea what temperature the water is, it only knows where you adjusted the handle. Summer and winter the temperature to the valve will be different so you adjust accordingly. Based on the 2.5 gpm flow rate you can normally operate one item at a time unless you don't mind a reduction in water pressure. Thermostatic Valve; Pfister has a 7.0 gpm and a 12 gpm flow rate at 70 psi, to accommodate multiple shower heads, rain cans body spray to operate at once. They generally come with at least one volume control and you can add onto it as well as add diverter valves. The temperature is monitored by way of a wax ring and when it senses a change in temperature, not water pressure it adjusts up to 1-3 degrees without changing the pressure. Regardless of the water temperature coming into the valve, you adjust the temperature on the indicated ring and it remains that way any time of year. In other words, it has a temperature setting on the trim kit. Since shower heads are normally 2.5 gpm, body sprays 1.6 gpm, you can add up what you want to operate at one time as long as you don't go over the either 7.0 or 12.0 gpm. Again this is based on the Pfister pressure balance custom line.

To a certain extent it's not really all that different whether acrylic or tile. However, what I have seen is with some acrylic/fiberglass/whatever units is when a too large of an opening had been cut through the acrylic was whoever did the work used a larger piece of plastic...sort of like a piece of corian, 1/4" to 1/2" thick and whatever size or shape was needed...to act as a larger cover plate to hide the hole, then the valve and trim kit were installed over that larger cover plate. The "corian piece" (I'm using "corian" just to describe, I saw one that actually used a piece cut and shaped from a plastic kitchen cutting board) was sealed to the acrylic wall with a bead of silicon, then the valve's trim pieces installed over that. I think someone on this forum showed photos of doing something like that in a tiled shower. The demo'd too large of a hole when doing valve repair, so they covered the too-large hole something like a square 12" tile set on a diagonal and siliconed in place, then installed the valve trim kit over that tile. I will backtrack and say that if light "demolition" could be carried out, then an acrylic unit could be different. If you could peel back or remove the wall piece on the valve wall that might give you better access to the valve. Then reinstall that wall piece. Just an idea, but it depends on the structure of the unit. Some are one-piece molded (rigid and probably more difficult to do), others have walls made up from individual pieces. But with an acrylic unit you could still go through the back side of the wall if need be to access the valve.

Thank you. I'll stick with Delta. The brilliant brass finish looks nice. The one I have appears to me to be brass plated only, and thus not cleanable. But my little magnet doesn't stick to it. Could it be aluminum? When I look at Delta stuff currently for sale it's all brass. Did they make plate in the old days? Maybe my set just has some kind of coating on it that makes it look fakey yellow? If it's real brass, then some corrosion has gotten between the coating and the brass and the coating on the edges of the trim around the control. I know the shower drain cover was plate, because the plating is more than half gone. That's what got me thinking the other stuff was plate too. My husband just took some brasso upstairs to see what happens.

After 5 years of trouble shooting, $500 in plumber's bills, calls to Tagaki and Moen, I was delighted to find other suffering souls. Based on what I think I know, I would have to agree with "aidan m's" approach. However, in my experience it is NEVER the easiest or least expensive solution that ultimately works! So get ready to buy that thermostatic valve. Here's why I think this: Our upstairs bath/shower (a result of a remodel) is supplied by a Tagaki heater and a Moen "Posi-Temp" pressure balancing valve (model T2343). It is supplied by a 1/2 inch water line. At the other end of the house is a second Tagaki supplying the "level" ground---three showers, two tubs, sinks, etc. The shower that works flawlessly is in the master bath. I insisted on a 3/4 inch water feed to the shower and a Grohe thermostatic valve (second knob for temperature control). Compare that set-up to the upstairs unit (less water pressure and a valve that is trying to do the same thing as the Tagaki in terms of water temperature), and the solution seems pretty clear. Of course, it wasn't that easy figuring it out. Tagaki sent their expert. He didn't see the problem. Moen was no help. Our plumber/philosopher got close. Throw in a huge variance in water temperature coming into the house during summer months and winter months, and you have a problem with so many variables that, well, you're willing to spend $500 on the thing. My next step, and last step, will be to put in another Grohe upstairs. Then I'll clean the shower head!!