Quote Originally Posted by everythingair View Post

If the blower chart for a psc motor blower rate the required 1000 cfm of air at the standard SP of .5, then at .7-.8 SP that psc blower will only be providing about 700-800 cfm of air. With a psc motor, this can be a problem during humid days when a wet coil from low airflow will create even more resistance which will lower the cfm ability even more until the coil eventually freezes up.

With the same system using a VS blower, the VS motor set at 1000 cfm is going to continually ramp up to provide that 1000 cfm of air. There is no SP differential chart for a VS blower because the motor continues to attempt to achieve the amount of air it is programmed despite varying SP conditions...within reason.
Some have a derate percentage to use above X amount of static.

If a current system with a PSC blower is moving 800 CFM at a .75" static. Then increasing CFM to 1000 would raise the static to roughly 1.17"
They end up using about the same amount of electric in that type of an install, and making the system loud.
VS blowers don't have CFM ratings at that high of a static.