How to use Fan Laws for design/build retrofits

If you’ve ever had a situation where your HVAC unit had less airflow than expected, you’ve officially had a run-in with the FAN LAWS. The biggest takeaway from this article should be that sizing your ductwork is extremely critical, because if it is undersized it can be very expensive to fix!

Here are the 3 Fan Laws

The best way to go through how the fan laws work is with 2 different examples.

Example #1:

A rooftop unit was designed for 20,000 cfm at 2.0” total ESP with a 20 HP motor (running at its full load) at 1,775 RPM. Somehow the ductwork was not sized properly, and the actual airflow was measured to be 15,000 cfm at 2.2” ESP. If the ductwork is not changed, what size motor would you need to increase the airflow from 15,000 cfm to 20,000 cfm, and what would be the new system static pressure and fan RPM?

Fan law # 3 tells us that horsepower will increase by the CUBE of the cfm ratio.

So you would need to change from a 20 HP motor to a 50 HP motor!

What would the static of the system be now?

Fan law #2 tells us that the static pressure will increase by the SQUARE ratio of the cfm.

Finally, the fan RPM is directly proportional to the change in cfm.

Be careful if the static pressure and/or RPMs get too high. Your equipment might not be able to handle it!

Example 2:

How much fan energy do you save by running an HVAC fan at half speed? Let’s look at fan law #3 for this one. If the airflow is 1/2 of the original, then:

So you would be running at about 12.5% of the original HP! If you had a 10 HP motor, when you run at half speed, you would only be using approximately 1.25 HP. Talk about energy savings! Keep in mind, though, that actual motor efficiencies decay as they ramp down, and there will be drive losses with a VFD. Motor efficiency decay could easily be 10 percent, and drive losses for the VFD might be 3 to 5 percent. Also note that your static pressure would be reduced by the square.

So your system static pressure would be 25% of the original. If your system was running at 1.6” static pressure, then at half speed it would be running at 0.4”!

There you have it! You are now a FAN LAW expert!

Did we miss anything? Do you have any jobs that are experiencing airflow challenges?
Brad Telker
Connect on:
Latest posts by Brad Telker (see all)
This entry was posted in Helpful Tips and tagged , , on by .
Back to blog