You don't say where your boiler is, if its DOWN STAIRS the length of pipe to upstairs basin will have a LOT LONGER to travel all this water has to be cleared before you get hot water upstairs .This problem would be the other way round if the boiler was UPSTAIRS ie the sink/bath hot, sink downstairs taking longer. Also the ambient water temperature coming out of the ground winter is a lot cooler now compared to summer boiler takes longer to heat the colder water
wirecutterman, January 2008
