The 2nd floor bath in our vacation home in NH sat idle most of the winter. Heat stayed on in the house and the other areas were used (kitchen, main floor shower and bath). There is no hot water in the 2nd floor shower, even though there is hot water to the sink. Dismantled the shower control to see if any blockages, etc. and all appears open. Tracing the H and C risers, it appears that the pipes split in an interior wall and run horizontally through the floor, under the tub and up to the shower control. Any suggestions on how to diagnose the problem? Could it be mineral build up in the horizontal HW pipe running to the shower?