Formulation of Manual J
The ACCA Association (Air Conditioning Contractors of America) developed a method called the Manual J Residential load Calculation for determining the ideal size of an HVAC system for a home.
what “Manual J” means?
Single-family detached houses, small multi-unit structures, condominiums, townhouses, and manufactured homes all have their HVAC equipment sizing needs addressed by the ACCA-recommended Manual J 8th Edition.
In a nutshell, Manual J is the process that is followed to figure out how much warm air must be supplied to a home in order to keep its occupants comfortable, and how much cold air must be supplied in order to cool it when necessary.
Manual J calculations can be challenging without a solid grasp of the methodology underlying them. That's why the BTU load calculator you see up there is just a simple rule of thumb developed by contractors.
In order to properly size your equipment and make accurate calculations for determining the HVAC load of a residential building, we advise you to try out this free trial HVAC Load Calculator.
HVAC Load Calculator
There are a plethora of free alternatives to this HVAC Load Calculator on the web.
The aforementioned HVAC calculator is a stripped-down version of such tools, allowing for quick and easy estimation of the necessary machinery size.
Divide the sum of the cooling loads you calculated above by 12,000. This will give you the ideal size for your equipment (12,000 BTU make 1 ton).
How to calculate HVAC load
Keep in mind a few common factors that must be considered when determining the HVAC load for a residential building.
1 occupant = 100 additional BTU
1 window = 1000 additional BTU
1 exterior door =1000 additional BTU
The above HVAC calculations were performed using the following formula.
Assume there are three occupants in a 1000-square-foot home with an 8-foot ceiling, five windows, and two exterior doors; this would require an HVAC load calculation.
The HVAC load calculations would look like this:
Multiple the square footage of your home by the height of your ceiling (in feet) to get the square footage of your attic (in square feet).
1000 sqft (house surface) x 8 ft (hight of the ceiling) = 8000
3 (occupants) x 100 = 300
5 (windows) x 1000 = 5000
2 (exterior doors) x 1000 = 2000
Total HVAC load = 15,300 BTU
Knowing that for every 12,000 BTU you should add 1 ton to the HVAC equipment, the correct equipment size for this particular house would be 1.5 ton.
Disclaimer
All estimates made in the above formulas and calculations are made in good faith and provided for illustrative and educational purposes only. Not all of this data has been verified by us. The suggested BTUs could also be influenced or distorted by other external factors. A professional HVAC service or engineer should be consulted for precise numbers.
