Sizing comes from a load calculation, not the old system's label
A contractor works out the right size by figuring the heat loss and heat gain of the house room by room: the windows, the insulation, the ceiling height, which way the walls face. The old furnace or AC size is not a safe stand-in for that, since whoever installed it may have just matched whatever was there before. A heat pump sized off a real calculation matches the house instead of guessing from history.








