How to Calculate Heating Load

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Heat loads help determine air conditioning needs.

Heat loading involves measuring and evaluating all of the heat sources that contribute to the temperature in a room or office. Many sources apply, including the sun, equipment running, lights and body heat from occupants. For homes, solar radiation from the sun is by far the biggest source as the sun bears down on the roof and walls. To this end, to calculate heat loading, you need to calculate the contribution from each source and add them together. You can then use the information to determine the size and rating of the air conditioning unit you need to cool the room or home.


Step 1

Calculate the Area BTU using the formula: Area BTU = length (ft.) x width (ft.) x 31.25

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Step 2

Calculate the heat contribution from the solar heat radiating through windows. For the north facing windows, use the formula: North Window BTU = Area of North facing windows x 164. Find the area of each window by multiplying the length by the width. Add the areas of all north facing windows together and multiply by 164. If the north facing windows have no shading, multiply the final value by 1.4. For the south facing windows use the formula: South Window BTU = Area of South facing windows x 868. Follow the same procedures as with the north facing windows, except multiply the total area by 868 followed by 1.4 if no shading. The Total Window BTU will be the sum of the North Windows BTU and the South Window BTU.


Step 3

Determine the number of occupants that normally occupy the space. Calculate the Occupant BTU using the formula: Occupant BTU = number of occupants x 600

Step 4

Locate all of the equipment and appliances that occupy that space such as computers, printers, appliances, etc. Refer to the manufacturers tags and find the power in watts for each item. Add the power of all items together and multiply by 3.4 for the total Equipment BTU.


Step 5

Find the contribution by the lights in the room or space using the formula: Lighting BTU = total lighting watts x 4.25.

Step 6

Calculate the Total Heat Load BTU using the formula: Total Heat Load BTU = Area BTU + Total Window BTU + Occupant BTU + Equipment BTU + Lighting BTU.



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