Oil smell from furnace (when fan is on but flame out, I think)


We've had an oil smell coming through our vents for about a year. Our local heating service company has come out twice and found nothing. Neither the smoke detectors nor the carbon monoxide detector protest and we haven't had any apparent health effets. Yesterday I realized that the smell only comes through only after the temperature exceeds the setting on the thermostat and contines for a while until the fan stops blowing. I don't know if it works this way but I'm guessing that the fan keeps going for a little while after the flame is out. If so, that seems to be when the oil smell occurs.
I would appreciate your advice/thoughts.
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Restaurant Guy wrote:

If the service co. inspected and found no leaks in the heat exchanger, it's possible that the burner emits a "puff" of unburnt oil vapor upon shutdown. The fan does continue to run after the burner cycles off and there may be a small leak on the return ducting near the furnace which pulls in the "puff" from flameout.
All conjecture, but this kind of thing does happen.
Jim
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wrote (with possible editing):

Well, I don't know so much as a tame crow, but it certainly sounds like there is a hole in the heat exchanger. That separates circulating air and combusting oil. The fan runs while the heat exchanger is warm enough to actually heat the air so it (the fan) will continue to run for a while after combustion stops. The hole might be small enough or in a place which evades detection. You might find it yourself by removing the gun, putting a strong bulb in the combustion chamber and looking in the circulator.
While I am not terribly knowledgeable in this field, if it were mine and I found a hole, I would replace the furnace or at least the combustion chamber as I would be afraid it would eventually get larger and spread noxious fumes.
--
Larry
Email to rapp at lmr dot com
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I truly doubt all this.
It's likely that there are a few spots in the combustion chamber or the oil burner assembly (fuel pump, combustion air blower, ignition transformer and spark gap) that get a little oil on them that isn't burned. When the burner is operating there is a strong draft that takes the fumes from the warm but not burning oil right up the stack. When the burner switches off the fumes from this un-burned oil will diffuse back into the living space. This may be made worse if the burner actions puts the living space under a slight negative pressure relative to the outside air. When the burner stops, there would be a reverse flow through the burner.
A "cure" might be to bring in outside combustion air to the furnace. A "box" around the burner assembly that's connected via duct work to the outside would stop the negative pressure problem and also keep the oil smell from penetrating the living space.
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Thanks all for the replies. Furnace repair is over my head, for sure. I've decided to call a different service company (and hand them a summary of your responses.) I'll report back here for your interest on what they say/find/do. Wish me luck. I don't want to be spending big bucks!
Brian John Gilmer wrote:

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