Problem with mixet shower valve. Suddenly my hot water flow has drastically decreased. Cold flow is fine. Affects both shower and tub flow. I replaced the cartridge. Same result. I'm guessing the problem must be an obstruction on the hot side but hate to tear out the tile. 20 year old house without flow problems anywhere else. Pretty hard water and I have had problems with filters on faucets clogging but now I have a softener so that's not a problem... I can't imagine that would affect large volume flow to the valve anyway. Any other thoughts?