How to manually calculate heat load

Written by mike smith
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How to manually calculate heat load
A building's size and shape affect its heat load. (house blueprint and house model studio isolated image by dinostock from

The amount of heat in a building that an air conditioner is expected to cool is called the heat load. Heat is generated inside a building by a variety of sources including lights, appliances and even people, but the largest source of heat for any building is radiant heat from the sun. Accurately calculating the heat load for your home or building will allow you to choose the most appropriate cooling equipment. You can use a simple series of measurements and calculations to arrive at a useful estimate.

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  1. 1

    Multiply the number of square feet to be cooled by 31.25 then write down this number. If you do not know the square footage of your home, multiply the length of the room by the width when measured in feet.

  2. 2

    Measure each window in your home and find its area in square meters by multiplying its width by its height. Multiply the area you find for each window by 164 if the window faces north, east or west and by 868 if the window faces south. Multiply each unshaded window's result by 1.4. Add all of the windows' final results together, write down the number and set it aside.

  3. 3

    Multiply the number of people who commonly occupy your home or building by 600. Write down this number and set aside.

  4. 4

    Add the total wattage of all of the appliances commonly used in your building or home. The wattage of televisions, ovens and computers can usually be found in the owner's manuals. Multiply the total by 3.4.

  5. 5

    Add the total wattage of each light commonly used in your home or building. Most standard and compact fluorescent light bulbs are stamped with their wattage, but you may have to consult the fixture rather than the bulb for bar-style fluorescent lights. Multiply this total by 4.25, write it down and set aside.

  6. 6

    Add together the numbers you wrote down, to calculate the heat load estimate for your building. This estimate reflects the number of British Thermal Units any cooling system would be responsible for in your building. You can use this number to determine whether a specific air conditioner will meet your cooling needs.

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