I've recently noticed an issue with the hot water in my shower. After I start the shower water flow and wait for the warm water to start coming out, it stays warm for about a minute, then it gets luke warm. At this point, it doesn't matter what I do with the control knob (I have a Mixet valve, FYI) - all the way hot, but it stays the same temperature. I thought it was the water heater, but as a test, I left the tub hot water on during my shower and the hot water flowed the entire time. It's like some other draw of hot water needs to be on in order for it to stay warm. So then I thought it was the valve, which really made no sense to me but I replaced it anyway because it's easy and cheap. The problem remains. Any help is very much appreciated.