Furnaces are designed to provide specific amounts of heat energy per hour. The term “BTUH” refers to how much heat can be produced by the unit in an hour. Before you can determine what size furnace you’ll need, you must have a heat loss/heat gain calculation done on the structure. From that, an accurate determination can be made of the size of the system you’ll need. Most fossil fuel furnaces are substantially oversized for heating requirements, resulting in increased operating cost and unpleasant temperature swings.