A pretty big plumbing outfit installed my tub/shower. The valve is an American Standard "Central Thermostat with Volume Control". A twin ell feeds the tub/shower. I attached some pics. The problem is that the shower diverter on the tub spout will pop up all by itself after a minute or so, especially when the flow/volume is turned about half-way or more. Otherwise, the diverter does its job, with no leaks in either up/down mode.
After reading through your forum, it seems like a pressure problem is at work, but I could use some advice. The plumbing company has already come out once and replaced the tub spout/diverter. Actually they replaced it twice… first with a temporary unit (totally different style) that actually worked perfectly. Then, with a proper replacement, which seemed to work, but maybe wasn't tested long enough, because the same symptom returned. The plumbers were dumbfounded at the initial problem, and apparently so was the American Standard tech they called, so I'd appreciate any advice to pass along before I contact the plumbers again. (I got the feeling these plumbers spend most of their time doing commercial work and hospital/lab stuff, not residential.)
Clue#1: Initially there was a pressure problem to the shower that the plumbers tried to explain away as a shower head design. Ultimately I took it apart myself and found blue stuff (pipe dope or shredded T tape?) plugging the restrictor in the shower head. After cleaning it the shower pressure seemed adequate. I think this might be a clue because: Where did that stuff come from?
Clue#2: My pics might show reducers at all three valve connections, (although I don't exactly know what they look like.) If so, maybe the plumbers had to adapt 1/2" copper pipes to a valve with 3/4" connectors? The valve comes in both 1/2 and 3/4 inch versions. I specified the 1/2" version, but I never verified which part they actually provided.
Clue#3: I took off the shower head and noticed that when I put my finger on the hole, the water pressure to the tub drops nearly in half. When I take my finger off, it immediately surges to the tub again. With the shower head off, I managed to trigger the ghostly diverter by covering the hole about 75% (but strangely not at 0% or 100%). I can hear/feel the hole sucking a lot of air. Bonus physics question: How does the shower pipe regulate the tub flow? Or in other words, why does water flowing through the tub spout create a vacuum in the shower pipe?
FYI, the walls were all finished before this problem was noticed, so I can't get any new photos.