I have a combo bathtub/shower unit in my condo in which I guess was probably installed in 1986 when the building became apartments. When I run the water in the tub to warm up before switching to the shower, there's a fair amount of rust present that eventually dissipates. I don't see this problem in the bathroom toilet, sink or any other water orifice in the unit. Is this a precursor to some plumbing damage, a pipe possibly rusting through? The volume of water that is allowed thru the tub spout is much greater then any other water orifice as well.