Size your A/C unit Go to the unit outside your house and look at the MODEL number. DO NOT worry about the serial number. The manufacturers all put a designation in the model number to determine the tonnage. (See the label below) For the model number, you will see a series of letters and numbers mixed together. Somewhere in that group of numbers you will see a 24, 30, 36, 42, 48 or 60. (in our example it is 60) Divide this number by 12 and this will tell you how many tons of a/c that you have. e.g. 60 divided by 12 = 5 tons of a/c. (see chart for common tonages)
Determine the size filter needed in square inches The standard in the HVAC industry is to have a minimum of 144 square inches of filter space per ton of air conditioning. Our extensive testing and surveying has shown that this is the case in the majority of homes. So, for a 5 ton unit you will need 5 X 144 = 720 square inches of filter for this unit.
Measure your filter in square inches
Scenario 1 You have a single 20 X 30 filter for the unit, so 20 X 30 = 600 square inches. You are starving your A/C for air and need a bigger filter.
Scenario 2 You have two 20 X 25 filters on that unit, so there are 20 X 25= 500 square inches each, totaling 1000 square inches. Your filters are big enough for this system.
Scenario 3 You have one 25 X 32 filter, so 25 X 32 = 800 square inches. You have enough surface area for your air conditioner.