My brother-inlaw has problems with his hot water supply. The furnace
heats the house OK but the hot water, supplied by the furnace, seems to
work best only when the furnace is also heating the house. It seems like
the furnace has the wrong control, one for a furnace that only
supplies home heat. Assuming that the furnace has the proper control
does it seem that the control has failed in some way such that the
furnace will cool all the way down when there is no demand for home heat
and thereby allowing the heat exchanger coils to no longer heat the
domestic hot water?

Thank you for any ideas,
andy