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.

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

Restaurant Guy wrote:
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.


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

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

On 10 Dec 2006 06:38:43 -0800, "Restaurant Guy"
wrote (with possible editing):


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.

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

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

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.

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.

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

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:
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.

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.