Fan laws and fan curves are a deep subject with a lot of nuance and variation. Just to get our heads around the subject, let's focus on two different types of fans that we see all the time in HVAC: the prop/axial type and the radial/centrifugal type.

Centrifugal/Radial

As we increase the pressure differential across it due to any variety of factors (small ducts, dirty filter, dirty coils), a centrifugal/radial blower with a typical PSC blower motor (non-variable/ECM) moves less air. Therefore, it uses less power. The easiest way to test this is to measure the amperage of a PSC blower with a blower door off, and then you measure again with it on. The current will be higher with the panel off because the static pressure is LOWER, and the blower moves more air. Take a look at this chart that shows the huge impact static pressure (and input voltage) have on airflow.

If you do the same test (door on then off) on an ECM constant torque or constant airflow motor, the amperage will go DOWN with the door off. However, that happens because of the motor characteristics ramping the RPM down, not because of the blower wheel properties.

Take a look at the above chart for an air handler that uses an ECM motor. The lines of airflow to static pressure are pretty constant until the static gets above the 0.5″wc. If you were to check amperage on an ECM blower, you would notice that it draws higher amperage when there's higher static pressure across it. That is due to the motor ramping up to maintain the designed torque or flow. The PSC motor is the opposite; if we increase static, the airflow, amperage, and wattage all drop due to the characteristics of the centrifugal blower.

To summarize:

A blower wheel decreases in power used as static pressure increases UNLESS there is a ECM motor changing the RPM to compensate.

Axial/Prop

A prop fan performs oppositely in relation to pressure. As pressure differential across it increases, the power used INCREASES even as the airflow it produces decreases. If you block a condenser coil, the fan will move less air and draw higher amps—illustrating again why keeping condenser coils clean has a big impact on performance.

To summarize:

A prop fan “loads” more based on pressure, while a centrifugal fan “loads” based on mass flow.

Again, this is a simplification. However, understanding these relationships can help you diagnose and understand system issues. Read specific system fan charts and curves for a better look at how a particular fan performs.

—Bryan

P.S. – If you would like to read an article that dives deeper into fan laws and fan curve charts, please check out this thorough article by Tim De Stasio.