Whitfield pellet stove fan

I just bought a Whitfield Advantage II WP 2 everything works fine except the circulating fan. When the stove is started the fan is on high, this panel has the knobs and the rheostat knob will not slow it down. I have checked all of the snap switches and they are OK, the trouble shooting said it could be the one on the exhaust fan but it is OK also. I have checked the rheostat with an ohm meter and it shows higher ohms and lower ohms when the dial is turned. I unhooked the black wire from the motor that went to the power terminal and put in an in line rheostat and things work fine. The thing that gets me is if the in line rheostat is about 3/4 turned up then the one on the control board works fine. But if I turn the in line one up to full the one on the panel has no control over the motor . It does have the thermostat jumper wire. Thanks for any help