Clayton 1800 thermostat not connected

[Salmonhound]

I just bought a house that has a Clayton 1800 wood burner with a blower. It is duct into the existing furnace already and has been used in the past. Wiring is disconnected and I can figure that out. What I dont know is the thermostat is disconnected from the back of the unit. What is its function as far as the fan function, unit has a manual switch for a 3 speed fan, and regulating heat? How do I connect the thermostat that is disconnected? Should I upgrade this thermostat? Why are there two thermostats? Any help would be great.

 

[brenndatomu]

Page 12 is your wiring diagram.

https://woodstovepro.com/store/pdf/USStoveCompany_16001800G_Clayton.pdf

The thermostat needs to mount on the wall somewhere toward the center of your living area...it controls the draft motor (fan or damper, depending on the exact model)
That "thermostat" on the furnace is your Honeywell fan control/limit switch and should be mounted on/in the supply plenum. It controls when the house/duct blower kicks on/off and also kills power to the draft motor if things get too hot.
The 3 speed control just controls the speed of the house/duct blower so to match how much airflow is needed for your house...shouldn't have to mess with it much...just keeps you from having to change speed taps on the blower motor to change blower speeds...which is normally a set it and forget it procedure.