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.

How cold is where you live? Any possibility that this line could be going through an exterior wall and freezing?

The lines go through the exterior. I live in Rhode Island. It is no colder than usual and we have lived here for four years and never seen this before. It has been a lot colder.

The lines go through the exterior.
If lines go through exterior walls there is always the possibility of freezing. Surprised that plumbing in exterior walls would be permitted in RI. Not permitted in VA.

If lines go through exterior walls there is always the possibility of freezing. Surprised that plumbing in exterior walls would be permitted in RI. Not permitted in VA.

So it is not as cold as usual. It has been colder before. The cold water is fine, it is just the hot. And you still are leaningtowards a frozen pipe? Could it be anything else?

Could it be anything else?
Not that I can think of. The hot water line will freeze before the cold. Wind has to be taken into consideration if the cavity that the pipes are in, is not air tight to the exterior.
If possible increase the heat in the bathroom. Heat lamps, keep light on. If there is a vanity or cabinet on the exterior wall, open cabinet doors, put light in cabinet.

The real concern is not the inconvenience of no hot water but the possibility of pipes bursting.

The hot water just started working, but has very low pressure. I do see a lot of calcium build up on the temperature valve on the boiler and on the hot water outlet of the boiler. Both the outlet connection and the temp vamve need to be replaced. Could the buildup be affecting the pressure upstairs, but not affect the pressure downstairs? Either way I am going to replace the "crusted up" connections and valves near the boiler. I hope this helps.

Could the buildup be affecting the pressure upstairs
Don't think so but if build up of mineral deposits were loosened by replacing the stop valve it could have moved to the upstairs valve and be obstructing the flow. Suggest removing the cartridge or stem and flushing the valve.

If flow just started on it's own, still think it may have been frozen and is now thawing. However, it would have had full flow very quickly after thawing.