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Gas furnace AFUE vs stack temperature

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bink
bink Member Posts: 97
I am wondering about how AFUE % affects stack temperature. In general what combustion stack temperature would you have at 80% vs 96%. Is there potential venting problems with higher efficiency gas furnaces ( natural gas ). For my daughters house it has a steel pipe running up through the house interior out the roof. Any issues with condensation with these high efficiency furnaces.
This is not discussed in any of the articles I have read so far.
Thanks.
Wishing a happy holiday to all members.

Rich

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  • JUGHNE
    JUGHNE Member Posts: 11,108
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    In a nutshell: furnace with an AFUE of more than 90% has very low temp exhaust. So low that there is considerable condensation of flue gases, (that is how it gets that extra 10% or more of efficiency). It must be vent with PVC or some form of approved plastic and possibly special SS pipe.
    The condensation will consume aluminum or steel pipe. It is acidic water.

    For AFUE of 80% there is little condensate produced and may use "B Vent" pipe, aluminum liner inside a galvanized pipe. It must not be too much over sized. You want the exhaust gases to stay hot until leaving the building. And usually you do not put an 80% into a masonry chimney unless it has an approved liner.
  • bink
    bink Member Posts: 97
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    Thank you. I really appreciate your information. I will have them stay with 80%. I do not think they have pvc pipe vent.
  • JUGHNE
    JUGHNE Member Posts: 11,108
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    90+ furnaces can easily vent out the side wall with the PVC.
    The extra advantage is a second pvc pipe that brings outdoor combustion air in for the furnace. This can make the basement warmer with sealed combustion rather than relying on air leaks and cold air being sucked into the basement.
    Is there a gas water heater there. It could still use the metal pipe for exhaust.

    In any event you do not want to use the steel pipe for a 90% furnace. CO danger.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,167
    edited December 2019
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    Reading the installation instructions for the common condensing furnaces will have venting instructions much different than those of non-condensing furnaces. The AFUE of a non-condensing furnace is anything lower than 87% Anything with AFUE over 90% is considered a condensing furnace.

    The Condensing furnace will mostly specify a low-temperature product like PVC Plastic as @JUGHNE mentioned. In the trade, we call them 90 plus 'cause it is easier to understand than "condensing". No one ever accused this trade as being an intelligent magnet

    The non-condensing furnaces are generally called 80% furnaces because the Fed EPA rules require the minimum efficiency for manufacturers to sell a furnace in the USA is 80% AFUE, so 80% to 87% is all in the 80s Simple... see!

    BUT I think you are really asking if you can install a more efficient furnace (90+%) where you have an existing 80% furnace... and use the existing vent....The answer is NO.

    You must change the vent to the proper vent specified in the instructions. If there is a water heater in the same vent as the existing 80% furnace, then you need to determine if the vent will be oversized for the remaining appliance. The Professional who installs the new furnace will know this, (or don't use that pro.)

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • bink
    bink Member Posts: 97
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    Again thanks, i really needed this information. This is middle unit condo with furnace on a middle floor. No way to vent out the side. I will stay with 80% which was in there now.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,167
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    if you have a multi-story unit you may want to consider a variable speed fan (usually 2 stage gas valve) to reduce stratification of temperatures from the top floor to the lower floor, both winter and summer air conditioning. more expensive but well worth it!

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • mikeg2015
    mikeg2015 Member Posts: 1,194
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    If the existing chimney isn’t shared with a water heater and it’s a straight shot up through the roof, you can often install 2” PVC down the existing chimney and some MFG allow the remaining air space to be set for intake air.

    IF it’s a small condo, and it shares with a water heater, you could look at a combi boiler with a hot water air handler to get hit heffeceincy water heat and space heating.

    Bit in hte end, the cost of a 80% 2 stage furnace will be so much over that it’s you best option. At some point 15% is just 15%. So if you you $400 in gas a year, that’s $60 in savings annually. Probably take 10 years or more to recover the increased installation cost. Plus reliability is slightly reduced as there is condensate drain to deal with, which can plug up.