Got a new Toto TSTAR thermostatic shower valve to go with my copper re-pipe. Sort of worked for a couple days, then I could no longer turn it up over 104F. When I run the shower in the adjacent sink the water comes out of the tankless HWH at 125F no problem. But when I do that the water actually cools coming out of the shower!!

This makes no sense at all. I cleaned the screens in the shower valve, the brass body gets plenty hot from the hot water coming in, the stupid thermostat just doesn't let it out. I 'calibrated' according to Toto instructions, but all that does is set the maximum temp, which it is incapable of achieving. And what is this with the shower cooling when the hot faucet is turned on?? The shower is 1" to 3/4" pipe, and the faucet comes off the same 1" pipe to 1/2". No way I have a significant pressure drop in the pipe or a lack of hot water supply with the biggest Noritz HWH.

That thermostat isn't working. $600+ valve and trim and this is what I get? Grrrr . . .