How Manual J HVAC Load Calculations Can Benefit You as a Homeowner

sizing up your systemMost homes have HVAC systems that are oversize. This is because most homeowners do not go through the process of calculating the load capacity as it requires time and attention to little but important details. Some HVAC contractors who are supposed to offer the much needed assistance also use the rules of thumb in determining the appropriate size of the HVAC system to install.

The size of a cooling system depends on the square footage of the area to be cooled and many contractors use about 500 to 600 square feet per ton as their rule of thumb. Every house is different and as such the cooling needs vary.




The Demerits of an Oversized HVAC System

The Texan way that bigger is always better does not apply in all cases. Installing an oversized air conditioning unit may result in:

  • A clammy house because such systems do not run long enough when dehumidifying the air
  • Shorter system lifespan – The system turns on and off frequently also known as short cycling and this makes it vulnerable to breakdowns. Because it is so big, it does not take a long time to cool the house.
  • Expensive to install due to its extra size.

The Energy Star program requires that new homes be installed with HVAC systems which do not exceed 15 percent of load.

What is Manual J?

This is a set of protocols which were developed by the Air Conditioning Contractors of America (ACCA) to help in sizing AC units. Through Manual J, homeowners can be able to determine the cooling needs of their houses. Initially, load calculations were done by engineers using pen, paper and slide rules. This has however changed and the calculations can be quickly done using computer programs.

HVAC Load Calculations

When doing Manual J HVAC load calculations, all relevant data including insulation levels, home’s orientation, areas of all surfaces that lose or  gain heat and window types. The output of these calculations clearly indicates the level of heating and cooling your house needs in terms of BTU per hour for every room, zone and the entire house. Most newer homes have been found to have a load of about 800 square feet per ton and high performance homes have cooling needs of about 1500 to 2000 square feet per ton using Manual J load calculations. If you were to use the rule of thumb of 500 square meters per ton, you would end up installing AC units which are 2, 3 or 4 times larger than the required capacity. By doing a room by room load calculation, you can make the process of choosing cooling equipment much simpler and less tasking. You can also design a duct system which will operate at optimal efficiency using other ACCA design protocols such Manuals S, T and D.