I had the same issue happen whenwe went to turn furnace on for the first time in our new 2012ST25RB. Easily remedied via the one controlling thermostat. I think its set up that way with the A/C fan kicking on by default. If you read the thermostat instruction booklet that came with the RV, it tells you exactly how to disable the A/C fan coming on. I think if my recollection serves right, the thermostat fan is set to "Auto" which has fan come on automatically. If you can't find your instruction book, let me know and I can quote the exact instruction from mine. You should have received all the instruction manuals for all the appliances, radio, tv, etc. when you picked up your trailer from dealer.