Thermostat Controlled Shower Valve

[63splitray]

My Delta DT17097PN, BRPN MC 17 Serv Valve Trim Valve is set to max hot (H against the stop block), but when the temps in AZ get down to 40's it still barely delivers hot water at all. The supply for this mixer comes straight from the separate tub thru only about eight feet 8' of PEX. The tub water gets very hot instantly. Two years ago I did a complete master bath remodel (to the studs) and did not reroute any plumbing, I simply used the existing tee off the tub to run to the new Delta fixtures/components and added a spray wand. (it was always too cool long before I rebuilt everything)

To be fair the water in AZ does go up and down quite a bit with the outside temperature extremes. But the shower is pretty much unusable at these temps. I just don't understand why I cannot get even 70 degree water through the 3-way vale to the shower head when its 120 degrees only 8' away? The mixer adjustment does work because when I installed it originally it was too cool so I set it 3 notches to the hot side and it was better (maybe the time of year). But when winter came it was too cool again so I adjusted the control the remaining 3 notches to the stop block. It was better but it has never gotten warm enough in winter.

Kind of at a loss as to where to go next?

 

[63splitray]

Ok, so I figured it out. The thermostatic control handle goes on TWO ways, it shouldn't but it does. One way the plastic clip has about a 90* sweep (maybe a bit less) and the handles sit just like the picture on the instructions (about 30* apart form each other in the off position). The other way the therm. handle sweeps 180* and now fully opens the hot water port. Now, In full cold the handles are over the top of one another (not shown in the instructions), and full hot the two handles are 180* out from one another (also not shown in the instructions). But it works!

 

[Geofd]

Not that familiar with delta shower valves we have. a highrise dorm with 18+22 story
towers we almost never have to touch those valves looks like a sime cartridge thou