Separate tub and shower problem.

[Dmt78]

I really need help figuring out this problem. We have a bathtub and shower on opposite sides of a wall in our master bath. The water for the shower barely trickles out or not at all, but if the tub is on it seems the water flows more in the shower. Can someone help me with why it’s doing this please? See photos below. Thanks.

Shower side photo with shower valve. Goes down to tub on opposite side.

Tub side. I know hot water is on the wrong side. I thought this was my only screw up. Guess not.

 

[Tuttles Revenge]

Looks like a Delta shower valve. Did it work when it was first intalled and now isn't? What did you do to fix the hot and cold reverse at the tub? Could that have affected the shower? Debris in the shower head screen? Have you removed the cartridge from the shower to check for debris?

 

[Sponsor]

 

[Dmt78]

It was working during rough in before installing cartridge and trim (shower walls are now up).
I pulled the cartridge and head last night and all was clear. This morning I thought about it, that the water from the tankless water heater outside may have been off due to a freeze we had. (Gas not hooked up yet)
Would this cause the cold water to back up into the hot water side at the valve or not come out?

 

[Tuttles Revenge]

Modern Single handle shower valves are pressure balanced. If the pressure of the hot is diminished, the cold side matches that pressure. So if your hot side has zero pressure, your cold side will have zero pressure. The tub valve likely has no pressure balance control (likely not legal) and allows water to pass.. but likely only cold.

 

[Dmt78]

Modern Single handle shower valves are pressure balanced. If the pressure of the hot is diminished, the cold side matches that pressure. So if your hot side has zero pressure, your cold side will have zero pressure. The tub valve likely has no pressure balance control (likely not legal) and allows water to pass.. but likely only cold.

Thanks for that explanation. Hopefully that’s the issue. I appreciate your help.