What is a Manual J?
A Manual J calculation is a formula that determines the size of the Heating and Cooling equipment for residential use. The calculation is a standard set by the Air Conditioning Contractors of America (ACCA) and is required by most building codes.
What is a Manual D?
A Manual D calculation determines the recommended sizing of the supply and return ducts in residential buildings.
What goes in to a Manual J/D?
Insulation
The R value of walls, floors and roof
Window Information
U value (insulation factor) and Solar Heat Gain Coefficient (SHGC - reflection reducing solar heat gain) and how much area do the windows include
Shading
Shading of windows from porches or large overhangs
Orientation
How the home is situated on the lot can make a significant difference in sizing equipment. Lots of windows facing south and west will increase the heat load on the house
Air Handler and Ductwork Location
Ducts in vented attics will contribute to heating and cooling losses versus ducts located inside the conditioned spaces. Ducts in spray foamed roof lines can significantly reduce the heating and cooling losses
Why not use a "rule of thumb?"
For many years, a simple rule of thumb of 500 or 600 square feet per ton was used as the standard for sizing heating and cooling equipment. This simple rule of thumb could easily lead to grossly oversizing or even undersizing the equipment.
