It depends on the fan motor. Most typically, blowers need a solid-state speed controller, not a rheostat (a simple variable resistor). I don't know if there is a way for you to extract the motor's specs or get a part number.

I was informed by my hood mfg. (Prizer) that their 3 speed motors are not compatible with variable speed controls, and advised against it. They stated shorter motor life, and humming were two possibilities, besides voiding the warranty...

Thanks for the feedback.

Yeah I figured the warranty would be kaput if we did this, but would be willing to trade off a 1 year warranty against a risk-free conversion. Doesn't sound like it's risk-free however.

Air-King's model number is B900 for the the 900 CFM blower. It's specified as a permanently split-capacitor motor. I went to Wikipedia to try and get some info on this terminology, and it talks about changing the number of taps on the run windings of a permanent split-capacitor motor to get different speeds. I'm guessing this is what the 3-speed control would do at each setting.

At best, my imagination sees the 3-speed control used for on-off and cranked up to 900 CFM when on, in combination with a solid-state speed controller introduced somewhere in the circuit for continuous control from slower speeds up to the 900 CFM max. Not the most convenient even if it did work.

At worst, I see trying to rig this up and getting all the nasty side effects ctycdm mentioned.

Thanks again.