Shower temp rises when adjacent body sprays turned on

Folks
I'm hoping you can help solve this. It seems complicated and it will be way too expensive to have a stream of plumbers come by the check it out, nor practical to start demo looking for a problem.


Setup:
This is a double shower. I'm going to call one the LEFT and one the RIGHT.

Background:
The RIGHT was the original shower. The LEFT was added on the opposite wall 7 ft away. New pipes were laid from the RIGHT under the shower pan and up the outside wall to feed the LEFT. The RIGHT's components were replaced with those described below.

Components:
LEFT: Artos-Westover -- 2 - F906-2 vol ctl's (1 for shower head; one for 3 body jets); 1 - F904-3 Thermo vlv (controls temp output from one or both vol ctl's)
RIGHT: Same except 1 hand held spray instead of body jets.

Problem 1: LEFT shower: after approx 10 min the water temp slowly decreases. Turning the temp control toward hot, or to the hot stop, makes no difference. While running, turning on the body jets' vol ctl causes (1) spitting and sputtering for 5-10 sec from the three jets (2) followed by a very large water temp rise from both the shower head and body sprays, so much so that I have to get away from the water. Then, turning off the body jets vol ctl causes the temp to drop to its original level after approx 10-15 seconds.

Problem 2: RIGHT shower never reaches the temp of the LEFT with the LEFT shower and jets on. This is the case whether LEFT's shower and jets are runnung AND the RIGHT's shower and hand held are running OR just when any component of the RIGHT is running.

Notes: 1. These thermo vlv's have some kind of complex sensor inside that regulates temp. It's possible that the RIGHT thermo vlv is set so it cannot put out as high a temp as the LEFT's. 2. When the jets are turned on, they spit air and water for a bit. This occurs whether they are being turned on for the first time or after being turned off for a few seconds and then turned on again.

Finally, it appeared we were running out of hot water and needed a new water heater. Now it seems something in the way this plumbed is the problem.

If you need more info, don't hesitate to ask.

Thanx in advance for the help!

Has this been a problem since the day the new shower was added or did it work fine for a while, then start acting up?
What you are describing sounds like a problem with the equalizing of pressure and volume due to a poor piping arrangement.

Unfortunately, I don't know.The jets aren't used often and then only in the summer when there is no problem running out of hot water. The installation is two years old but I only discovered the problem last week.

I have attached a piping pic that shows what I BELIEVE what the contractor did.

Can you explain more about "equalizing of pressure and volume due to a poor piping arrangement. "

When setting up dual showers with multiple heads, pressure balanced valves and body sprays volume and pressure are important, especially adding on to an existing shower. Size of the pipes to accommodate the demand of volume will effect performance when you turn on multiple heads and having adequate and balanced pressure is just as key to ensure one side isn't causing the other to starve. Without seeing the exact design, pipe sizes, etc. it's only a guess. Unless other suggestions come in from the forum from members who are familiar with the brand of valves, my guess would be design issues with the pipes, since that is what I find to be the most common problem.
I'll try to find a picture of an example for piping arrangements and post.

Thanks, I appreciate that.

Additional information...

This morning I used the RIGHT shower. Hot water output was fine, but after approx 7 minutes it seemed to be cooler. I turned the temp control to get more hot water and, in fact all the way to the end of its travel, and there was a minimal increase in temp. While the RIGHT was still on, I went to the LEFT shower and turned on both the shower head and body sprays. After a few seconds, very hot water came from both. Output from the RIGHT became ice cold.

In the last email I included a sketch showing what I believed was the plumbing arrangement for the LEFT. However, today I found notes given to the contractor that showed the body spray design should have been a continuous loop, as shown in Artos documentation. This is now what I BELIEVE was installed.