We have a natural gas furnace 7+ years old(Trane). This season I notice the pre- blower(not sure of the correct name) runs much longer than it used to. The pre blower(?) function is to ensure the exhaust fumes line(PVC) is clear before firing up the burner. Previously, the pre-blower would run for 15-20 seconds, the heat would fire up and we had warm air. This year, the pre-blower will run for 30 seconds or so then stop, then it will run again and usually on the 2nd try, the burner will fire and we'll have heat. Don't know if this is cause for concern, can be dealt with via maintenance or is a repairman and parts replacement required.

It could be in the blower safety end switch or a small crack in the rubber vac lines depending upon which system you have.

If you have not had your furnace serviced in awhile now may be the time before it gets to cold.