I don't think my propane furnace is igniting every time. I say "I think" because it is difficult to tell. It is a combo unit placed outside. I was lying in bed last night and heard the rumble as it attempted to come on, but then it just stopped. When I woke up this morning, the house was warm and it seemed to work during the day. As i sit here now, I notice the house is a bit chilly and the furnace doesn't seem to have kicked on for a while. Why would the propane furnace work most of the time but not all the time? Is this a sign of serious trouble? Does it just need to be cleaned?