Airflow is typically measured by cubic feet per minute (CFM)—a cube of air 1 by 1 by 1 foot flowing by every minute equals 1 CFM. That cubic foot of air weighs about 1.2 ounces. At higher elevations, air becomes less dense (for instance, Denver air—neglecting particulate pollution—weighs about 0.99 ounces). Since fans blow air by volume (irrespective of air density), a 1,000 CFM fan placed in Denver will still blow 1,000 CFM. The problem is that 16% less air mass is being moved, and the mass is what does the work of heat transfer.
A common misconception concerning whole-house fan use is that it can cool down the house in 15 minutes and then you can “turn off the helicopter.” But “quick cooling” a house with a blast of outside air defies the physics of heat flow. The weight of air in a 2,000-square-foot house is approximately 1,200 pounds, with an aggregate thermal capacity of 288 Btu per pound per degree Fahrenheit. According to demolition studies, a typical 2,000-square-foot house will weigh in at 222,000 pounds. Based upon the typical material mix, the weighted average specific heat of that house is 0.39 Btu/lb°F, which means that it would take 85,000 Btu (0.39 x 222,000) to raise or lower the temperature of the whole house by 1°F. Of course, the house doesn’t heat up and cool down uniformly, which is one of the reasons why nothing in building heat transfer is simple. Since air is relatively easily cooled and heated, the goal must be to cool the high mass of the house. The physics of heat transfer prevents this from happening quickly.
Although some houses are situated to capture breezes, most houses do not “self-ventilate,” necessitating some form of mechanical ventilation. Fresh air requirements are often in the range of 0.35 air changes per hour (ACH) or 20 cubic feet per minute (CFM) per person. To put this in perspective, a 1,700 CFM whole-house fan would yield more than 6 ACH when placed in a 2,000-square-foot house with 8-foot ceilings [(1,700 x 60 ÷ (2,000 x 8)].
Older criteria simply recommended an airflow of 3 CFM per square foot. But this overly simple formula comes from a time when houses were not well-insulated—and whole-house fans were the only form of cooling available. Since natural cooling depends upon Mother Nature, technical factors, and personal preferences, it’s not possible to have a definitive size for a particular case. However, customer feedback combined with engineering knowledge has resulted in developing empirical formulas for airflow that take into account cooling strategies, location, and house construction.