Farmer - I have a small greenhouse for orchids and hydroponics attached to our garage. The garage stays about 65F year round, so I use an 8" fan to move warmer air from the garage into the greenhouse during the Winter. I also use it to move cooloer air in the Summer. Right now it's 38F outside and I'm heating the greenhouse to about 60F for pennies a day.
I installed an in-line rheostat, a dimmer from Lowe's, to periodically reduce air flow. For example on a sunny winter day when the greenhouse is 75F, I don't want to cool if down with 65F air. It runs 24/7 when it's needed and doesn't seem effected by the rheostat. In fact, operating at slower speeds keeps the motor from overheating. I think it would work like a charm on your motor. Crank it down to say 10 rpm?