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Sizing Air Conditioning Systems

Sizing a central Air Conditioning System for you home or business can be a complicated task.  There are a few common misconceptions that need to be avoided so as to no make a costly mistake.  Air Conditioning Systems are sized differently all over the country, so some of the rule of thumb simplifications may only apply to Southern Arizona. 


  • Cooling is equipment is measured in "tonnage".   Units are measured from 1 ton to 5 ton in residental applications.  Commerical systems go much larger.

  • 1 ton of cooling is equal to 400 cubic feet of air per minute.  2 ton of cooling is 800 cfm and so on.  So a 4 ton unit will have 1600 cubic feet of air passing through the system every minute.

First of all, you need to understand a couple concepts:
The Rule of Thumb

Each home in Arizona has different insulation properties.  But the general rule of thumb is to have 1 cfm per square foot of living space.  So a 1600 square foot home would have 1600 CFM of cooling capacity, or a 4 ton unit.  Said another way, each ton of air conditioning will keep cool 400 square feet of living space.  Southern Arizona is very hot, so 400 cfm per ton is probably the smallest number you will find in the country.  Numbers may go as high as 800 cfm per ton in cooler climates.  Also very well insulated homes may also experience higher numbers.

Over-sizing your Air Conditioning System Is a Problem

If your thinking about increasing the size of the unit, because you think it will be better. You should know, that if you get a unit that is too big (1 ton more than needed), it will cool the air in your house too fast. If you cool the air too fast in your home, humidity doesn't get a chance to be removed from the air. High humidity will cause discomfort just as well as high air temperature. Also, a bigger than needed unit will cause larger swings in temperature.

Two stage compressors can be oversized because they have a slower speed they automatically adjust to as demand decreases.  Two stage compressors are usually only avaiable on the top of the line products. 

Under-sizing your Air Conditioning System

Most of us know what happens when you under-size your Air Conditioning system. It runs all the time, and the temperature rises beyond the comfort level. If your current system is doing this, you might consider going up 1 ton over your current system. If only certain rooms are getting hot, then you might have an air duct problem. You should have a technician come out and evaluate your system before making a choice if this is the problem.  Also, if one room gets warmer than the rest alot of the time, it is best to keep the fan blowing more often, this keeps the air circulating.

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