What is my "Max CFM" I'm supposed to enter into the box near the bottom?

posted Dec 11, 2014, 6:22 AM by Greenhill EnviroTechnologies Inc.   [ updated Dec 11, 2014, 6:22 AM ]
The Max CFM you are supposed to enter is the actual CFM of your fan when at max fanspeed, while operating your solar collector. (It is not the max CFM rating of the fan on the fan data-sheet.)

The standard fans we use and deliver with our Okapi Systems have a data-sheet rating of 110 cfm: this is when the fan has zero airflow impedance.

When a fan is pulling air through any solar collector, there will always be some level of airflow impedance, making the max CFM when the fanspeed is at 100% a little less than 110cfm, for instance. As each collector design is different giving different levels of impedance, this 100% fanspeed CFM must be measured.


To get this measurement, you will need an anemometer (-They are easy to find and quite inexpensive... also fun to play with!)

  • On a full-sun day, when your collectors are running full blast and Okapi Viewer tells you your fanspeeds are at 100%, take a few anemometer readings at your outlet(s). 
  • Then, Download the Solar Heater Output Calculator app (-the file is located at the bottom of the page link below). As you have already updated your Java to get the Okapi Viewer App, you will not need to update your Java, again. Simply download this calculator program:



 Once you have downloaded the Solar Heater Output Calculator App, use "open with" your "Java(TM) Platform SE binary".

1. Ignore the other input boxes, for now...
2. Type in the FPM measurement you make using your anemometer, when fanspeeds are at 100%, into the FPM box for both heaters (they may give different readings being of different design, if you have two heaters... and you may get a few different readings depending on how and where you hold the anemometer... getting a nice estimated average is good enough!
3. then type in the diameter of your outlet vent, or if it is square, you can type in those dimensions...
4. and finally, look down at the CFM results box, and that is your calculated "max CFM" for your fan for your solar collector, right there for you!

(Oh, and have some fun playing around with all the other features of this calculator app, too!)