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Flow Rated Fans - How Do They Regulate?

Bathroom-style fans like Panasonic's WhisperGreen units regulate their speed to ensure consistent airflow as restriction varies downstream. Does anyone know how this works exactly? I'm assuming it would use a pressure sensor on the exit side of the fan and have programmed speed values for differing levels of restriction, but that's just me guessing.

The reason I ask is, what happens to the flow rate if there's a restriction introduced on the intake side of the fan? These fans are probably designed to have nothing more than the included cover restricting them, so what happens if you want to filter the incoming air? If a filter was to introduce a significant pressure drop at the fan intake, would it detect this and compensate?

Asked by Lance Peters
Posted Jan 8, 2018 3:01 PM ET



Blowers with electronically commutated motors (ECMs) monitor the static pressure in the duct system. If static pressure increases, the motor ramps up to compensate. I assume that the static pressure is sensed with some sort of manometer, but I don't know the exact details.

Answered by Martin Holladay
Posted Jan 8, 2018 4:06 PM ET


Thanks Martin. I may have to contact Panasonic directly to find out how their system works. My gut says that increasing static pressure on the outlet side of the fan will tell the fan to increase its speed to compensate (as designed), but that decreasing static pressure on the inlet side of the fan will have the opposite effect and the fan will slow down excessively, reducing flow rate below where it should be.

Answered by Lance Peters
Posted Jan 9, 2018 8:56 AM ET


I'm not sure how the controls work on the "variable speed" bath fans, but on air handlers, my understanding is that "variable speed" EC motor air handlers monitor the motor rpm and torque (and perhaps power input as well) in order to make adjustments to deliver a set airflow.

Answered by John Semmelhack
Posted Jan 11, 2018 8:19 AM ET


I would think knowing the rpm and current would imply cfm. with a 'regular' motor it draws more current in open air than in a restricted space, I imagine the same is true with ECM

Answered by Keith Gustafson
Posted Jan 11, 2018 5:33 PM ET


Good points. Perhaps with a map of known airflow levels plotted on a current x RPM map the fan can assume a given airflow just by knowing its operating parameters. I hadn't thought of that, makes perfect sense to me.

If that's the case, the fan should compensate for static pressure either before or after the fan since either would reduce the mass airflow through the fan, reducing the work done and the power (current) drawn.

Thanks guys!

Answered by Lance Peters
Posted Jan 12, 2018 8:41 AM ET

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