I cannot figure this out ... we got a new hot water heater and now my master shower doesn’t get cold water. The only way to control cold water is to turn on one of the sinks in the master. We’ve replaced the cassette in the shower without any change. My other shower works just fine. I have a video but it’s too large to upload. When the house was built (we’re second owners), all plumbing was sheet rocked over without access panels. I don’t know where to start ripping out Sheetrock.