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  1. #1
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    Jul 2004
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    troubleshooting oil furnace smell

    Imagine a hypothetical old, poorly insulated house with an '80s-vintage Williamson Oil-Saver furnace in the basement. In the fall, when we restart it, the warm air ducts emit an "oil smell" into the living space.

    Some observations:

    It smells like diesel exhaust, although when asked if it smells like fresh oil, maybe that too.

    It is far worse when firing the furnace with a cold chimney. Under steady-state operation (e.g. days and days of intermittent running), the smell largely goes away. It appears worse at the beginning of the heating season than during late winter, although maybe it's just habituation. It also appears worse in the morning when it's coldest than in the evening.

    Some of the vents smell worse than others.

    When the furnace first fires up, before the blower fan starts, you can feel air moving up out of the ducts, and it smells like oil. The whole room, and even the first floor, then smells like oil for a period of hours or longer.

    In a previous year, the problem seemed correlated with a sticking barometric damper in the vent pipe to the chimney -- but with the damper fixed and working, the problem still occurs. In fact, the oil smell comes directly from the air registers, which could not be intaking basement air, thus making this seem like a red herring.

    A carbon monoxide detector, known to have worked in other situations, and placed on the active smelliest vent does not display any warnings.

    Last year, the oil technician gave it a tuneup, replaced the nozzles and adjusted the jets, which did not remedy the problem right away -- leading me to conclude that this repair was not related to the problem resolving itself weeks later (or as we got used to it).

    No oil is leaking anywhere visible, between the tank and the regulator.

    I've taken off the door to the panel, and can see a glass window where I could look into the combustion chamber with the aid of a mirror and a flashlight.

    Even though we called for a tuneup back in August, this being a colder and place they were jammed through November. We can call for emergency service, but last time that resulted in a rejetting and no real fix to the smell.

    How can I tell if this is a cracked heat-exchanger? What else could it be?

  2. #2
    Join Date
    Dec 2006
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    We had that same problem in one of the buildings that I work with, and was a beeyatch to troubleshoot and fix. Iirc it was a conflict that was focused around the fresh air intake getting the fumes, then distributing thru the whole HVAC system. This particular building houses two restaurants, so it was disconcerting having that [often faint] odor in your nose when sitting down to eat. They did all the tests too, and there was no health concerns, just the odor...
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  3. #3
    Join Date
    Sep 2001
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    Sounds to me like it's not venting, especially the part where it's worse when the system is cold and also where you say air comes out the vents before the fan starts moving air.

    It's just a guess but check to make sure the flue is clear and there are also powered vent fans available. The weight of the column of cold air in the flue may be too much for the heated air to overcome at first, causing it to backdraft (not sure that's the correct term here) into the house. Once the system has heated up from use it would begin to draw correctly and the smell would go away.

    Hey it's a guess.

  4. #4
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  5. #5
    Join Date
    Jan 2006
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    la la land
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    You should NEVER smell oil coming out of the duct. There has to be a hole or crack in the heat exchanger for this to happen. Get it checked out STAT.

    The smell is most likely worse when it first starts due to the chimney not being hot and drawing well. If the smell was only in the mechanical room it could be the electrodes or the burner are just dirty which causes rough ignition. When this happens the fuel oil soaks the refractory before ignition, and when it finally does light, it pressurizes the heat exchanger for a second which can send exhaust gas in to the room vs. relying on the draw from the chimney to keep the heat exchanger in a negative pressure. (Note, most fuel oil burners make a rumbling sound when burning correctly. Im talking about a loud thud or woofing sound that goes away shortly after ignition.). On second thought, is there an open return air vent in the mechanical room? If there is, you could be circulating the gas smell throughout the house from there.

    Edit 4 - Iceman is on track in regards to the chimney not drawing. And edit again 4 - To look for a crack in the heat exchanger take off all the sheetmetal panels and look for oil, water spots, holes or hairline cracks on the parts that get hot.
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  6. #6
    Join Date
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    Not quite as knowledgeable on oil fired systems... but for natural gas there are special detectors used in home inspections that detect both burnt and unburnt concentrations. If they are also able to do same for oil, you might make sure which ever HVAC contractor you call has such equipment. It's odd... because most home inspectors here use this equipment, but very few of the HVAC contractors ever do. Don't recall if it's point detecting carbon monoxide and or something else or what? (Edit2: this is the kind of equipment I am talking about; http://www.toolexperts.com/tif-8800-...-detector.html - for oil I'd imagine it would only detect anything from the post combustion phase)

    The several ways/places it would be used are at exhaust fresh air vent/inlet at fire up. Its common for the readings to be high for a few seconds initially, but they should quickly drop. If it stays high, could be a blockage in damper/exhaust/chimney, and the combustion gases are spilling back instead of venting up chimney. If this has been happening for some time, you'd often find rust on the metal surfaces in that area... (instead of the rust being from water, it is corrosion from the gases). My home Inspector also then usually checks at the warm air register closest to the heating unit. If you are getting high readings there it's very likely a rusted/corroded/cracked heat exchanger.

    We have very few oil furnaces left around here... so thinking out loud. That heating oil is like WD-40, very slippery. If a fitting is even a bit loose the oil works its way around the threads looping its way out. With even a small leak you can smell it thru whole house. If there was a small slow leak inside the furnace cabinet, it might drip down and into a lower plenum somewhere you can't easily see... where it then gets into the cold air stream and makes it's way into heated air that way?

    Edit: Just wanted to add for any others who might read this later... very common to have an odd smell early in heating season for hot air systems. I've always attributed it to dust from the off season that has settled on the outside of heat exchanger getting toasted/burnt off. Doesn't sound at all like what you are describing here.
    Last edited by mock vomit; 10-22-2009 at 10:29 AM.
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  7. #7
    Join Date
    Dec 2003
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    Teton County
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    I have an oil burning furnace with forced air in my place as well. I have the same problem. After a few days of using it for the year, it goes away though. I always assumed that it was fuel that seeped into the air ducts during the summer months and then it airs itself out after it runs for a little bit. The one that I've got is from the 60's though, so I assumed it was normal. Whoever came up with using diesel fuel for heating a house anyway? Expecially out west.

  8. #8
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    My folks are up in Sweden. vacationland and had all sorts of problems last winter with their relatively new furnace. One day it started smoking so bad we had to evac the house and call the fire dept.

    I think it turned out that the oil company up there had somehow let water get into the mix or something, or it was a bad cold temp mix they got because the tank is in an uninsulated area.

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