In my early years of designing HVAC systems, I had come to learn key budget values to check my engineering data and continuously benchmark these rules of thumb. If I saw an out-of-line budget value, such as an improper cfm, I could go back and review the calculations to double-check how the total cfm for a floor was calculated. An example of this would be to have a computer program complete the heat gain for a single-story building with the results showing 3 cfm per square foot. Knowing my rule-of-thumb budget value to be 1.5 cfm per square foot, this check indicated there were errors in the computerized heat gain calculation. I then would go back to find where the error occurred to bring the simulation down to a value closer to 1.5 cfm.
A side note to this check and balance of my 1.5-cfm rule of thumb was a pre-energy-crisis value. Once the oil embargo of the 1970s hit, and energy conservation took control of design engineering, my rule of thumb check figure dropped to 1-1.25 cfm per square foot. Those buildings that had less than 1 cfm per square foot were designed to be incredibly energy efficient. This often resulted in “sick building syndrome.” The computerized heat gain calculation told the designer what to engineer the cooling capacity at, but the program didn’t have a default minimum ventilation rate. Over time, we hear less about sick building syndrome from lessons learned as designs take into account the space required for adequate ventilation when the area is occupied.