I can remove the cartridge and it seems like it is in fine shape. There is no balancing screw that I can find.
I think there is something strange about the plumbing in this house. When I close the valve that provides cold water to the hot-water heater, and then turn on a hot water tap somewhere in the house (not necessarily in this shower) water still comes out at full pressure…only it is cold. There is no shut-off valve for hot water coming out of the hot-water heater. How is cold water getting into the hot water pipes? Is this a normal setup for house plumbing or is there some cross connection that shouldn’t be there?