“Load Calculations” are the only accurate method for determining the amount of Heating and Cooling required in a given space. The amount of heating/cooling needed for the entire home, a specific zone in the home, and for each specific room in the home.
Decades ago, the amount of heating/cooling needed was based on a simple square footage rule of thumb (400 sq. ft. of living space needed one Ton of air conditioning). This “one size fits all” formula was highly inaccurateat best. Older homes with lots of glass were terribly under-conditioned, and heated or cooled poorly during extreme temperatures. Newer homes with good insulation and good windows had systems much bigger then they needed, which made the home seem like a wind tunnel when the system was running, with drafts, noise, short cycling, and reduced system efficiency.
Instead of simply measuring the square footage of floor space in a home, we are actually adding up the amount of exterior wall space, ceiling areas, non-slab floor areas, glass areas, type of glass, insulation values in the walls-floors-attic, and north/south exposure of the home. Also factored in, are any appliances, lighting, skylights, amount of occupants, and any other items that may give off heat into the space.
Choosing the proper size unit is one of the most important parts of comfortable, efficient, heating and cooling system.