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Thread: stack temp

  1. #1
    Join Date
    Apr 2003
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    81
    Oil furnace no.2 fuel oil. What would a normal stack temperature be, what would be considered too high or too low? If too high what would be causes and if too low, causes. I have been told between 450-600 but i had a furnace today pushing 675, is that normal. I had the correct nozzle in the unit, draft.04"wc, co2-12.9%, o2-4.6%, draft overfire-.03"wc, smoke-trace. Seems to me unit was running fine but stack temp seemed to be high. Is that high or is it ok?

  2. #2
    Join Date
    Sep 2002
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    Insufficient airflow across the heat exchanger perhaps?

    All vents open?
    Filter Clean?

  3. #3
    Join Date
    Jun 2003
    Location
    New Hampshire
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    How old is the furnace and burner?
    What is the make of each and model?

  4. #4
    Join Date
    Apr 2003
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    81
    filters are clean, vents open. Bard furnace, I don't have the model and serial # at this time with a becket burner, I believe the model was AG but I am not certain.

  5. #5
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    Apr 2003
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    81
    furnace approximately 8-9 yrs old

  6. #6
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    Jun 2003
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    New Hampshire
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    Is that the one with the round heat exchanger? Similar to the Thermprides' octogon with the cleanouts in the same spot but is circular instead of octogon?

  7. #7
    Join Date
    Sep 2005
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    6
    what is the condition of the chimney?

  8. #8
    Join Date
    Sep 2002
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    3,708
    .03 overfire and .04 at the breech...sure appears to be
    a poor hx design.

  9. #9
    Double check the manufacturer’s spec’s, but my guess is the O2 reading is too low and the overfire draft is too high – both of which could contribute to a high stack reading. You should probably be looking at an overfire draft of -.01 WC” and an O2 reading in the 5% to 7% range.

  10. #10
    Join Date
    Jun 2003
    Location
    New Hampshire
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    If its the same furnace I think it is, it's a poor heat exchanger design. I serviced a few of these about 5 years ago for another company. The high stack temps made me remember. I could never get better than a 78% or so efficiency.
    So don't worry this is normal for this furnace.

  11. #11
    Join Date
    Sep 2004
    Posts
    654
    If there is soot inside the heat exchanger, that will drive up stack temps. Soot is a good insulator, and it's the LAST thing you want on the INSIDE of the heat exchanger. Draft is a bit much too, I like -.02 or even -.01 if I can get a clean flame. The unit being almost 10 yrs old can easily explain the higher stack temps, too. Maybe when it was new it put out 550°F stacks, typical for those years for just about any furnace. Put the smallest nozzle the manufacturer recommends or even downfire it a bit....that is sure lower stack temps.

  12. #12
    Join Date
    Sep 2004
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    654
    Oh, forgot to say on my last post....no way would I leave a unit with a trace smoke. Units get dirtier running as they continue for the rest of the season, so leaving it with a sllightly dirty flame will make nothing but soot once the brunt of the heating season comes. Colder oil burns richer too, so consider that when you adjust a flame in the summer. Your C02 reading past 12% is a red flag, set it up with 12% max and a ZERO smoke. Having the unit stay clean the entire year is where efficiency comes from.

  13. #13
    Join Date
    Jun 2003
    Location
    New Hampshire
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    I agree with the others on the Co2 and smoke readings. On this furnace I would set it at zero smoke and 10%-12% co2 depending on smoke readings. By adding air to achieve this your stack temp may climb even higher. You may be better off to drop the nozzle one size for better efficiency.

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