No hot water in upstairs bath/shower

[HaLo2FrEeEk]

The upstairs sink gets hot water just fine, but the bath/shower barely gets lukewarm. When I first turn the hot water on in the bath it's cold while it purges the pipes, then it's hot for about 3 seconds, then it gets cool for about a minute and slowly warms to lukewarm.

The potentially odd thing about the setup is that there is a shower head with a single handle to turn it on, and on the other side there's a bath tap with individual pipes and knobs for hot and cold water. Both the shower and bath taps have the same issue. I've noticed that the hot water has considerably less pressure and was wondering if it's possible that the bath side pipes are routed through the shower lever on the other side. I took the handle and wall plate off of the shower side, but I'm only seeing what looks like a physical stop for the handle, no valve controlling the actual hot water input. Could anyone give me any advice on what to look for here? I can post pictures if necessary. Thank you!

Edit: on closer inspection with a light, it looks like there's a pipe (whitish translucent flexible type) coming into the shower valve from the left, then another going out to the right, in addition to the copper pipe going up to the shower head. That seems to support my theory that the bath side is routed through the shower valve. Still not sure what to do though.

Here are some pictures:

This is the bath side. Individual pipes coming out of the wall with hot/cold knobs.


Shower side with handle wall plate removed.


Shower valve.

 

[HaLo2FrEeEk]

I hate to bump this, but I really do need some help with this. I'd like to take baths upstairs without having to carry up buckets of hot water from downstairs.

 

[HaLo2FrEeEk]

Please help y'all! I don't know what to look for back there, so any advice would be helpful!

 

[phishfood]

My first thought is that the hot and cold are somehow crossing at the two handle bathtub faucet, but thinking it through, I am not sure just how that could happen.

Without being there to look at it in person, I would try changing the balancing spool in the Kohler single handle shower valve.

 

[frodo]

my thoughts,

when was the mixing valve installed ? who did the installation ?

do you still have the literature ? if not, what is brand name and model name ?

is the wall behind the valve still open or has it been re sheet rocked ?

 

[Mr_David]

I would start by replacing the pressure balance cartridge in the shower.
I believe phish is right about it being a Kohler.
lots of videos on YouTube on how to replace