Furnace Problem

[wdoa]

I have a forced hot air unicell oil burner furnace. My best guess is that it is between 12 and 15 years old. I've noticed recently that when the furnace turns on, especially if it has been off for several hours one gets a brief smell of diesel exhaust around the air vents. I have several carbon monoxide detectors in the house, at least one only a few feet away for the vent and none of them have issued any alarm. The smell isn't super-strong but it is definitely there for the first few seconds that the blower kicks on. If the furnace is left on and doing an normal cycle on cycle off I do not receive the diesel exhaust smell every time it turns on. The time it's most noticeable is when it's turned on after having been off for several hours.
I poked around online and saw some references that it could be anything from a cracked heat exchanger (in which case RIP the furnace), to a gasket of some sort that needs to be replaced.
Yes we have a call in to an oil dealer to have their tech come out and look at it. Just want to be as informed as possible when I talk to the repairman.

[Hube]

When a furnace has been off for say, 40 minutes or so ,the vent pipe (chimney) to the roof can get cold and the "draw" is not as good as a luke warm one. Also, the barometric damper can stick for an instant on a cold start and this too can cause a slight diesel smell in the area. Windy conditions will also contribute to a slight diesel oil smell on start up.
Having a pro come out and check things out is your best and safest bet.

ps, your exchanger is probably ok, and the problem is in an adjustment of either the barometric damper, electrode setting, etc.
Good luck.