The Manual J Calculations program is used as the initial step in designing air conditioning and heating systems, allowing HVAC installers to determine the amount of heat lost through a home's exterior during winter months and the amount of heat gained during summer months. Using the system, a designer examines the thermal characteristics of the home, such as the walls, ceiling, floor, doors and windows, taking into consideration the home's location, which direction it faces, duct leakage and appliances.
Manual J also takes into consideration how much heat and humidity the people living in the house add to the home's interior, as well as the home's envelope tightness, lights and its direction relative to the sun.
The types of Manual J load calculations include Whole House, or Block, Load Calculations and Room-by-Room Load Calculations. Whole House calculations give the loads for an entire home and are used when the existing duct system does not need modification. These calculations can assess the proper equipment size required to replace the system in an already-constructed house. Room-by-Room calculations, on the other hand, give loads for individual rooms in the house. These calculations provide the air amount required to heat or cool each room, which aids in figuring out duct sizes as well as overall layout.
Most professionals use the Manual J software packages, but some independent home owners use online tools that provide the same calculations by collecting information about the home and its location.