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  1. #1
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    Nov 2019
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    80% Efficiency Furnace Flue Temperature

    Hello! I recently purchased a home earlier this year. The previous owner replaced the HVAC system (poorly!) this year prior to me buying the house. Now that it is cold out, the furnace has been running more often, and I am having issues with condensation inside the flue pipe. I’ve had some contractors take a look, but wanted to get a second opinion. This is an 80% efficiency Goodman furnace, 80k BTU, single stage, induced draft. I believe the issue is being caused because the furnace was replaced (from what i assume was an older, less efficient model), but the flue stack was not, so it may be too larger and letting the gasses cool too much. If I let the furnace run for 20 minutes, and go up on the roof and measure the exhaust temperature( meat thermometer stuck in st the top) the highest it gets to is 130F. And that was on a 60 degree day (I live in Denver-so it gets way colder). I know that determining the exact dew point is a complicated matter, but is this temperature so far away from what it should be that it is almost certainly an issue of losing too much heat somewhere between the exhaust and the roof vent?

    Thank you! Please let me know if I left out important info.

  2. #2
    Join Date
    Aug 2002
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    Southold, NY
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    You need a chimney liner. Several qualified contractor in Denver.

  3. #3
    Join Date
    Jan 2014
    Location
    New England
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    You have a inducer exhaust fan, verses maybe the old one did not, possible one reason the flue temperature is lower? Without knowing the exact model of your Goodman seen several models of the 80%, says 4” flue, but it also says depending on length.


    From Goodman install guide for one of their 80%

    Vent and combustion air diameters may vary depending upon vent length. Refer to the latest editions of the National Fuel Gas Code NFPA 54/ANSI Z223.1 (in the USA) and the
    Canada National Standard of Canada, CAN/CSA B149.1 and CAN/CSA B142.2 (in Canada).

    Do you require a high altitudes kit, seeing your in Denver?

    Take the serial number of the furnace and check Goodman’s web site warranty look up to see if that new furnace was ever registered, if never registered, register it as manufacturers warranty decreases significantly if never registered.

    https://www.goodmanmfg.com/warranty-lookup

    You may want to hire a reputable HVAC Contractor to check your system, gas pressures, static, temperature rise across HX, proper air flow, etc. etc. are all within manufacturers specifications. It’s called a start up/commissioning report.
    Last edited by Bazooka Joey; 11-06-2019 at 03:28 PM.

  4. #4
    Join Date
    Jul 2007
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    Virginia
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    Quote Originally Posted by Vinimack720 View Post
    This is an 80% efficiency Goodman furnace, 80k BTU, single stage, induced draft. If I let the furnace run for 20 minutes, and go up on the roof and measure the exhaust temperature( meat thermometer stuck in st the top) the highest it gets to is 130F.
    A properly tuned 80% efficient furnace, that is producing the rated output the manufacture states, should have a flue temp between 300º-400ºf when measured at the equipment. If this temp is met, flue gases should not condense in the flue.

    The type, condition, length, and size of the flue, along with the flue draft, will effect what the actual temp of the flue gases are when they finally exit the vent. There is no way to put an exact number on what that should be.

    A qualified technician, preferably one that is trained in combustion analysis, will be able to examine the flue for problems and test your furnace to see if it is under fired, causing low flue temps.
    Instead of learning the tricks of the trade, learn the trade.

  5. #5
    Join Date
    Jun 2012
    Location
    Northern NV
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    all the info you should need at: http://www.hartandcooley.com/files/a...Guide_0813.pdf

    Willing to bet it is not the furnace's fault...

  6. #6
    Join Date
    Jun 2002
    Location
    Dacula, GA
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    Is your flue pipe double wall required by code or is it single walled flue running horizontal for a few feet with just a little slope? This will rust out your flue pipe for sure. Thank you very much
    "I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."

    "I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution."
    Sen. Barry Goldwater

  7. #7
    Join Date
    Aug 2004
    Location
    S.E. Pa
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    Aside from performance issues, the venting system must be suitable for the class of service. If you reline the chimney a thin walled liner will heat up quickly and be less prone to condensation. Also, a good ss liner will contain the condensate to the base where a condensate trap should be incorporated to contain it. However, if you really want to stop the condensation then prevent it from occurring. Install a 4 minute post-purge. Use combustion analysis to guide the set up and firing. You can also add a double acting barometric damper with a spill switch you really need it.

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