Last winter we had a new furnace installed--Coleman model #DGAH077BBSB. I turned it on for the winter this October and it was working fine until last night. When I got home, the house was cold, and the furnace wouldn't switch on. Flipped the reset off and on and turned up the thermostat. The motor started running but then clicked off--no air. I checked the LED code and got the 3 blinks that indicate that the combustion air switch failed to close. My boyfriend detached the ends of the tubes that lead to the pressure switch and blew into them. The furnace ran and blew for one cycle, then clicked off with the same code. He tried it again, and we turned the thermostat up to 72 because we weren't sure how long it would run and we wanted to get it as warm as possible. The furnace then ran and blew normally throughout the night and through this afternoon. However, I don't like to leave my thermostat that high, so I turned it down. The next time the furnace came on, it came on only briefly and then clicked off as before. So, I put the thermostat up to 72 again and now it's fine. My question is, how can I turn the thermostat down without causing it to click off and not blow? Sorry for the long explanation. Would it work if we turned it down to the temp I'l like it to be on permanently and then blew out the tubes again? Why would turning down the thermostat cause a problem with the combustion air switch if the initial problem seemed to be a tube blockage? Do you think I need a new switch?
Thanks in a advance for any help you might offer.