Water pressure issue caused by PEX elbow / tees?

[britgeezer]

Have a problem at my daughters house.
An "apprentice" plumber did some work at my daughters house a while back, Kohler mixer valve to bathtub /shower has been leaking for a long time.
I have replaced the cartridge and seemingly fixed the leak, but the valve doesn't operate as it should. It should start at cold and go to hot, it starts at hot, but when opened more than a little goes cold. There are flow adjustment valves and by turning cold almost off you can get a low pressure hot shower, daughter says it always been quirky. I don't see how the cartridge can be the wrong way round (I tried inverting it without any change) and the hot is fed from the left of the 4 port valve cold from the right.

She has well water and pump, the feed to hot water tank is 3/4", the 1/2" copper pipe from the tank was replaced with 1/2" PEX and I count about 5 right angle or T joints over the 40 ft horizontal and 3 ft vertical run to the valve. The cold-water run is still 1/2" copper.
In my mind the PEX joints are limiting flow rate to the shower mixer valve - could this be true?

If so, should I:
1. Replace the joints with shark bite joints - or similar that don't reduce internal diameter.
2. Make a new 3/4" inch run from hot water heater like a manifold, with 1/2" branches to the kitchen sink, dishwasher and ultimately to mixer valve?
3. Your better suggestion.

Thanks for your insight.

 

[GReynolds929]

The cartridge is backwards. Has it always started on hot? Did you replace just the cap or the pressure balancing unit also?

 

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[britgeezer]

The cartridge is backwards. Has it always started on hot? Did you replace just the cap or the pressure balancing unit also?

The plumbing store called the part on the right the cartridge, I replaced both, but can see now the black only part (balancing unit) is reversed - didn't seem to want to fit that way around. Will try again. Thanks.

Any comment on water pressure - daughter says it was never good compared to upstairs shower which is 1/2 " copper. Do the multiple smaller diameter internal tee and elbows reduce 1/2" COPEX pressure or not.