smell coming from furnace has me concerned

After having a reliable oil burner for nearly 30 years, it was recommended to me last year to have a new one installed. I agreed because the blower was making a strange sound and my hvac tech said that, due to age, the blower would be difficult/ impossible to replace. Not only that but there was a strange smell coming from the ducts every time the heater was running. It's hard for me to describe since I don't have a good nose, but all I can say is that I could hardly stand the smell. I wouldn't say it was a soot smell exactly- more like "rotten" soot is the best way I could describe it. I figured a new oil burner installation would cure this. Before the cold set in this year, I had a new unit installed. Everything was replaced with new except my heater ducts. The heater runs great and is much quieter than my old unit, but the same strange smell has persisted and it has not become less over time. I have concerns about breathing whatever this is and I am posting here looking for solutions to cure it. Any idea what could be causing it and how do I get rid of the odor? As long as the furnace isn't running, I don't smell anything but the minute air starts coming from the vents, the odor is nearly overwhelming.
Thanks, Al
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Al Roarke wrote:

Sounds like you have a mold growing in the ducts.
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Al Roarke wrote:

I have to agree with Chuck. Have someone check the ducts, including the returns.
I am not a fan of cleaning ducts, but there is a limited number of situations that call for cleaning and it sounds like you have on of those situations.
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Al Roarke wrote:

<SNIP> As long as the furnace isn't

If you can, open a section of the supply duct and swab a sample off the metal surface. See if it's sooty/smelly.
You don't say, but if the furnace has central A/C attached, the evap coil could be fouled and producing the odors.
Jim
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Sounds a lot like "Dirty Sock Syndrome" That occurs mostly with heat pumps but can also occur on AC coils. Clean the AC coil with detergent type coil cleaner. Keep spraying the coil with the cleaner so that it stays wet for at least 10 minutes. Use a wet/dry vac to continuously suck the suds produced out the condensate drain line. Then rinse with water spray.
Use a pump type garden sprayer to clean the coil.
If you don't have AC also, please disregard my post.
Good luck
Stretch
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Speedy Jim wrote:

FWIW, I had something similar, and it turned out to be a clogged drain from the A/C drip pan that caused some standing water in the pan. Clearing that problem eliminated the smell.
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Maybe the chimney is blocked.
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Might be chimney partialy blocked or with poor cold draw or mold in ducts, get it checked out, A digital Co meter might register, a good idea anyway, have it looked into .
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