Arvin Humidity Control

thoscon

I have a house built in 1966. Raised ranch, two story masonry chimney with three flues, wood burning fireplace upstairs and down. The house has electric heat with thermostats in each room. In the middle of the house there is a control mounted on the wall next to the thermostat for that room marked "ARVIN HUMIDITY CONTROL". It has a rotary dial that goes from "ON" through 80, 60, 40, 20 to "OFF". There is a duct that goes from the chimney to the outside. The fireplaces are both heatilator units that take in air at the hearth and put out air at the mantle. I have been unable to find any blower in the ductwork of the ARVIN HUMIDITY CONTROL system.

Can anyone educate me on this system?

Thanks