I have no hot water to my upstairs bathroom. I have hot water to other areas in my house. I have hot water upstream and downstream of the branch that goes upstairs. Yesterday when I noticed this I thought the cut off valve was bad so I replaced it. In order to replace it I drained the line and then turned the main back on. At that point the hot water started working upstairs. Problem solved. Then this morning the same thing, no hot water upstairs. I am cunfused as to what the problem could be. Please help.

