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80% Flue Temps

Jim Davis_3
Jim Davis_3 Member Posts: 578
Flue temperatures should be proportional to what we are heating and in the case of 80% furnaces plenum air should be at least 130 degrees. We use a 170 degree added to air water or steam temperature on 80% equipment. 300 to 400 degrees would be the maximum ranges for 80% furnaces. Under 300 you are underfired over 400 you probably have poor heat tranfer caused by airflow problems.
These are high fire temperatures. On two-stage if the flue temperature gets much below 250 degrees some of the flue gases will probably cool and condense before they get out.
Boilers would run higher. Forget the DOE chart!!


  • Home Depot Employee
    Home Depot Employee Member Posts: 329

    I know that many things affect the flue temps on a 80% furnaces such as connector lenght, chimmney heights & types, BTU of unit and gas, Blah, Blah, blah.
    But, can anyone give me an approximate flue temp range of a 80M, 80% N/G furnace?
  • Mitch_4
    Mitch_4 Member Posts: 955

    about 300-425°F..ballpark.

  • Doug_7
    Doug_7 Member Posts: 233
    Combustion Efficiency and Flue Gas Temperature

    See attached.

    Proper combustion tune-up to control excess air are important factors.

This discussion has been closed.