You may be asking “What is a Heat Load Calculation?”  A heat load calculation , also called the manual j calculation, is the starting point for providing the proper size unit for a house.  The calculation and factors are different for the different areas of the country. A home in Birmingham Alabama will need more AC and less heat than a home in Minneapolis, Minnesota.  The manual j design takes areas into account and offers different factors and heating and cooling days into the calculation.  In fact, in Alabama, homes are designed for 95 degree temperatures in the summer.  Manual j Calculations make assumptions of 2 people per bedroom in a home, this is not always true. Kitchens produce heat, the calculations have an average kitchen load in it.   If you cook a lot, have a double oven, or you are running a small bakery, this will greatly add to the heat load in a home.

Some technicians base the size of the AC on the square footage of the home. This is a large factor but not the only factor. Most people think, my neighbor down the street has the same home, isn’t that the right size? But the orientation of a home is very critical. If the majority of windows, front and back, are facing east and west, you will receive more sun into your home and that will increase heat load and the cooling needed.  Other important factors are windows type, exterior door type, and insulation.

You may be thinking, I’ve had a 3 ton AC for years and it has always done fine, why shouldn’t I skip this step?  We here this same comment all the time.  A size unit is its nominal size. The nominal size is the ability cool the air in a home while the latent size is the ability to cool or control the temperature of the items inside a home. Latent heat is the hidden heat. It’s the heat inside the furniture, carpet, drapes, etc inside the home.  An air conditioner must cool the air but it must also cool the items inside the home as well.  Many newer units with the same nominal size do not produce the same amount of latent cooling capacity.

Here is a copy of the form we use in Birmingham, Alabama.



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