Simple 'Rule of Thumb' Sizing

Obviously, conditioned floor space, expressed in square feet, is a chief factor when calculating air conditioning capacity (tons). Depending on local experience, contractors may allow a certain amount of airflow per square foot. In this example of a 10,000 square-foot residence, let's set a 'rule of thumb' at one cubic foot per minute (cfm) for.

10,000 ft2 X 1 cfm/ft2 = 10,000 cfm

Next, we set a second 'rule of thumb' of 400 cfm per ton refrigeration.

10,000 cfm/400 cfm = 25 tons

A much more conservative value of 300 cfm/ton can be used to evaluate the margin.

10,000 cfm/300 cfm = 33 tons

A real-life example: a 26,000 ft2 Texas residence features a 100-ton 'chiller' that circulates refrigerated water to 24 fan-coil units. (Read more about fan-coil units)

At 400 cfm/ton, an ambitious low-bid contractor might estimate 65 tons. A more conservative contractor, using 300 cfm/ton might come up with 86 tons. In this real-life case, the ratio is 260 ft2/ton.

What's the correct tonnage? Would you believe even the most conservative calculation proves to be inadequate?

Why?

Simple calculations may work for the small system situation where it is actually hard to go wrong. Structures, residences over 3,000 ft2 need to be scientifically evaluated. Various software products exist for this purpose and these software products are intended to be used by professional air conditioning contractors and mechanical engineers. You will not find a "How To" book at Home Depot to solve this problem -- "you can't do it and they can't help".

What to Do -- Leave it to the Professional

Professionals measure dimensions, create a floor plan and evaluate several important features of the structure.

  • Windows -- type, size, relative compass direction they face, solar screens, reflective film, etc.
  • Insulation -- type (glass, cellulose, foam), thickness, condition
  • Roof -- type shingle, radiant barrier, attic ventilation, construction (gable, flat, cathedral, etc.)
  • Ceiling height
  • Geographical location (latitude, altitude, ASHRAE design temperatures)
  • Use of spaces -- game rooms, gymnasiums, indoor spas and pools, trophy rooms, bedrooms, media centers, etc.
  • Computer rooms, home office, etc.
  • Garage spaces
  • Vestibules and passage ways

Symptoms of Poor Design

Some problem signs are patently obvious -- one register in an 800 square-foot space is a good one. Loss of control on hot summer days is another. If it's 100 degrees outside, I expect my system to hold the inside around 73 degrees, and it does, because my system was professionally designed and professionally installed.

My Choice for Air Conditioning

Geothermal easily comes in first over air-cooled conventional residential equipment and it definitely surpasses commercial 'chillers' for cost of ownership and overall reliability. (Read more about geothermal...)