In this industry, CFM ratings are the “horsepower”, so to speak, that people use to compare products.

This statistic is used to measure how much air a fan can actually move. Here is the tricky part…Static Pressure. This is the number you see, usually measured in inches of water column, on those charts that show CFM for a given fan. Fine, what is it then?

In a word, resistance. That is, how much duct does the fan have to push or pull through? How many bends in the run? These are factors that can increase the Static Pressure and decrease the performance of the fan.

When fan shopping, be sure to compare apples to apples as some Manufacturers provide CFM at 0″ SP, while others may show what the fan could do under more adverse conditions such as .2″ or more. Which is more likely how it will perform when installed.

Here is a sample chart to let you see how things can change…