Yes the thermostat is automatic, but the fan speed is not. With original equipment, the thermostat starts the fan on low, as temperature increases, it moves the fan speed to medium and finally to high. The stove has convection chambers completely surrounding the fire box and the automatic 3 speeds was designed to keep the fire box from over heating as the fire is burned hotter. In fact the manual says that if you lose power (no fan) that you should open the doors and burn like a fireplace (introducing cool room air in large volumes) to keep from overheating the unit. With the rheostat, the thermostat turns the fan on and off at whatever speed it is set. There is no chance for the speed to increase automatically as the unit heats up, which I think was a required design feature of this stove. With the original equipment you really had 3 separate thermostats, each controlling one of 3 fan speeds.
Good question, but I think that Buck back in the late 70's and early 80's had it figured out with their 3 speed fan and 3 temp thermostat. I have burned the 26000 since 1982 and am still going on the original bio metalic strip thermostats and 3 speed fan (replaced the fan once only because the original was making noise) and I have experienced it cycling trough the burn cycle and changing the speed thousands of times and even when I forgot it and it was really crankin, way hotter than I wanted, and it has never allowed my old Buck to get to hot. Evidenced by the fact that it shows no warped areas or any evidence of overfire after all those years. I think Buck did a good job of engineering this stove and that is the reason I recommend staying with "original". Mine is inserted in my fireplace so the only place I have to check temps is the front. The top that does stick out is cooled by the convection across the top chamber, so not a good indicator. I use two surface thermometers on mine, one on the glass (Buck's wire mesh glass cover allows me to put one there) and one on the cast door in the upper right corner. When I get it cruising, the glass therm is generally at 500 and the door therm at 400. Sometimes I go higher than that, but when I do, it is really crankin and putting out massive heat. Oh, direct answer to your question, manual doesn't say. Just to open up the doors if you lose power and have no fan or it will overheat.
Hope I have helped. The rheostat will work, you just have to realize that YOU are setting the speed of the fan and thus the cooling of the insert. If it were me, when I leave the stove alone, I would turn the rheostat up as high as it will go just in case the stove decides to take off, you are covered. When you can tend the stove, you can manually adjust it based on observing the temps on the stove.
