I aquired this furnace used and had it installed this year. Ther fan only runs for about 20 second and then shuts down. The fan limit control is showing that the tempeture drops very fast. I have had a man from my propane company look at it and he said it's drawing too much cold air but there is no ducting outdoor or in the crawlspace so I do not understand where the cold air is coming from. I was thinking that possibly it would be alright this way until now that the very cold weather has come(below 10 degrees). Yesterday I came home to a 50 degree house after about 8 hour of the furnace alone without woodstove.

I notice in an instruction manual I read online that this unit may need a two stage thermost, I think mine is single stage. Could that be a problem?