When a heating contractor receives a set of plans from a homeowner or builder, the first thing he does is perform a heatgain calculation. He will calculate the ability to retain cooling of every window, door, ceiling, floor, and outside wall in every room of the house. This calculation will determine how much cooling each room needs. Once he knows how much cooling is required, in BTU per hour, he will select a system which produces an amount closest to that quantity.

Air conditioning equipment consists of the air conditioning condenser which creates the cooling, and a furnace or air handler to deliver the cooling to the rooms. The furnace or air handler will also contain a cooling coil thru which the air will flow. Air conditioning BTU is also called tonnage.

A ton of air conditioning is 12000 BTU and usually requires 400 CFM of air flow in a low velocity air conditioning system.. CFM meaning the cubic feet per minute of air that the blower will deliver. He will divide this proportionally between the rooms and then design the ductwork needed to properly deliver the correct amount of air to each room.

The contractor must take into consideration the direction the room faces, southern or northern exposure, and if there are many electrical appliances or a large number of people in the room. The exposure of equipment and ductwork to high attic temperatures is also important.