The method of reading a HVAC chart depends on the chart you use, but for a square-footage-range-by-climate-zone chart, locate your climate zone from the map and the square footage of the home for determining the tons of cooling capacity the home requires. For a more accurate determination, use the Manual-J charts. These charts are more complex, and HVAC professionals often use software incorporating them to determine the unit size.
Manual-J calculations consider many factors, including the R-value of the insulation in the home, the number of windows and the size of the room. The calculation considers the individual heating and cooling load for each room in the home and uses the total of all rooms to determine the size unit the home requires.
In a true Manual-J calculation, the technician also determines the number of vents each room requires to meet its heating or cooling load. However, HVAC companies do not always use this information when replacing a system. Adding more ducts is time-consuming. Replacing the unit and using the existing ductwork is much easier for the company.
While installing an undersized does not provide adequate cooling, an oversize unit is also a problem. Because the oversized unit cools the house quickly, it often does not provide adequate dehumidification. Consulting the manual or charts ensures adequate provision of both processes.