Found this information online. It may be a starting point.
QUOTE:
"The most common cause of this problem is that the LP pressure at the regulator is too low or the regulator is bad. This should be tested at least once a year. You can take the regulator off and into a shop to be tested if that is more convenient. You need to examine the combustion chamber of the furnace as this is apt to get a hole burned in it and this can be dangerous."
If you are getting hot air from the exterior vent, it sounds as though the combustion chamber does have a hole in it & the heat is escaping to the outside.
