Fan Speed

[dzd9fy]

Need help understanding Speed Tap. I have volume at static pressure values, but canot determine how this relates to air flow speed. I see on HVAC Calc they recommend 900 FPS for supply and 700 FPS for return.

[dash]

Need help understanding Speed Tap. I have volume at static pressure values, but canot determine how this relates to air flow speed. I see on HVAC Calc they recommend 900 FPS for supply and 700 FPS for return.


Volume ,should be cfms(cubic fet per minute)

FPS ,should be FPM (feet per minute,velocity)


The relationship to air flow(cfms) is the higher the static pressure(resistance of the duct system and external cooling coil),the lower the cfms will be.

[dzd9fy]

why would the heat want to be at 900 FPM and the return lower?

[gfp777]

Not sure what you are asking?

[dash]

why would the heat want to be at 900 FPM and the return lower?


It goes back to an older method of Manual D,where more "Availabe Static " from the indoor fan,was allocated to the supply side then the return side.

The feet per minute recomendation,is mainly about noise,not system perfromance.


If you really know the static pressure,you then use the manufacturers "fan data chart",to select the speed tap that will deliver the required airflow,assuming the static is not over the limit to do so.