We have a hot-water-baseboard heating system that supplies our household hot water as well. About a year ago, the hot water in the shower became unavailable unless we concurrently ran the hot water in the washing machine or sink (same room). Eventually, we found that we could stop running the secondary hot water source once the shower was running well. When we turned off the secondary source, the shower ran full force. Suddenly, even this trick doesn't work. If we turn the hot water handle 1/3 on, we get a trickle of hot water. If we open 100%, we get none. This is a hot-water only problem. Cold water runs fine. Can anyone help?