HVAC contractors rely on rules of thumb to determine the sizes of the cooling systems they
install. Usually it's based on square footage of    conditioned floor area, and contractors in
many areas generally use about 500 or 600 square feet per ton as their rule. But every house
is different. Even the same house rotated ninety degrees could vary in cooling load by 25%
or more.

In the case of air conditioning, over sized cooling systems result in:
  • A clammy house because they don't run long enough to dehumidify the air
  • Shorter lifetime for the system because it turns on and off frequently (also called short
    cycling). Since it's so big, it doesn't take long to cool off the house.
  • More expensive to install. That extra size isn't free.

When we do a Manual J HVAC load calculation, we enter all the relevant data,
such as the home's orientation, insulation levels, window types, areas of all the
surfaces that gain or lose heat, and more. The output of the calculation is how
much cooling and heating the house needs in Btu per hour for each room, each
zone, and the whole house.
BSC Ltd.