We just got a insta-flame FSDV-30 (TN) corrected vented and installed. How does one control the thermostat. Is it preset at the manufacturer or can it be adjusted. Right now, in cold Canada, it goes on to about 26 degrees C, turns off and the fan goes for 15-25 minuters.

First refer to the owners/operators manual. If you can't find an answer there, you should contact the company who installed &/or sold the heater.