Originally Posted by
everythingair
400 cfm of air per ton of cooling is the nominal amount of air that is usually preferred for general condtions. That 400 cfm per ton of air needs to be accurately measured in accordance to the static pressure of the air distribution system, which will be different for every duct system. For cooling in humid areas, less air is preferred to remove moisture from the air. This again needs to be properly calculated in order to not cause problems with the additional condensate on the coil freezing up.
For the heat mode, the blower should be set to whatever speed keeps the temperature rise of the air across the heat exchanger closest to the mid-range of the temperature rise allowances that are listed on all furnace name plates.
The bottom line is that you should never just change blower speeds without knowing what the quality of the air in the system is. This requires testing with air measuring devices such as anometers, manometers and magnehelic gauges. We wouldn't just take a guess at how much medicine we take, would we?