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Adequate Capacity – Air Conditioning Systems

Monday, October 26th, 2009

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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?


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…)

Residential Air Conditioning Systems

Monday, October 26th, 2009

Home Sweet Home

We live approximately two-thirds our lives in and around our homes. A man’s home is his wife’s castle – a simple truth I learned long ago. Our homes are safe havens from high-stress business and work environments, a center of entertainment – valuable asset. Some of us transform our ‘castles’ from casual residences to art galleries and even into mini-museums. Irrespective to fun times away on vacation or travel abroad, “there is no place like home”.


We want a healthy, comfortable indoor environment at the lowest possible cost. This is true whether your ‘castle’ is a modest structure or a mansion on a country estate. Stable temperature, just-right relative humidity, fresh odor and dust-free – these are the characteristics of the perfect indoor climate.

Prescription for Success

Key to a satisfactory air conditioning system is an alignment of numerous features:

Good News/Bad News

Any home can have near-perfect to perfect air conditioning when essential factors line up. Knowledgeable design and installation by skilled craftsmen are requisites to have this finished product. This is the good news.

Unfortunately, too many poorly designed, poorly installed residential air conditioning systems get into homes. It’s almost impossible to go wrong with the “rule of thumb” method for small cottages, but all systems over 3 ½ tons should be carefully designed. (Read more…)

Air-Cooled vs. Geothermal

Geothermal systems win “hands down”, when there is adequate terrain for geothermal wells or, better, an adequate pond or lake nearby. My geothermal system has nothing visible outside – no condenser coils to clean, no noisy condensing unit, simply invisible underground piping. (Read more…)