I used to have two showers with separate hot/cold dials. These were the old-school kind with no scald-guard.

I upgraded one to a generic brand thermostatic mixer a few years ago. I noticed that it would never let the water get "hot" and it would turn ice cold in 5-7 minutes. You can only run 1-2 inches of semi-warm water before it turns ice cold. Previously, I never ran out of hot water. Naturally, I never really used this shower.

I chalked this up to a cheap mixer, so when I was forced by code to replace the faucet in the other shower, I agreed to have an expensive Delta mixer installed. Same problem happened.

My plumber says that this is just how these thermostatic faucets are. He said that code prevents him from installing the old independent systems. Since both of my showers are basically useless and my water heating bill has exploded, I am "this close" to just ripping this thermostatic gimmick out of the wall and reinstalling the old system myself.

Any ideas on what's going on?