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Thread: FAU rating- input vs. output?

  1. #1

    Default FAU rating- input vs. output?

    I have a 1992 Lennox fau that is rated 100,000 btu input. What is the equivalent of new unit to replace it? The new ones are rated output.

  2. #2
    Engineer Furd's Avatar
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    More likely as not your unit has a rated output of about 65,000 to 70,000 BTU/hr.

    HOWEVER, if you are planning on replacing this furnace do not use this figure for the replacement furnace. You really need a heat loss calculation for your house. You can do this yourself, just Google "heat loss calculation" (without the "") and there will be lots of hits. Most of the programs will have a cost but there are a few free ones available.

    OR, call several furnace installers and ask if they will do a heat loss calculation.

    One quick and dirty method to see if your present furnace is oversized (it probably is) is to determine its cycle time during the coldest weather. A properly sized furnace will run almost constantly during the coldest weather, at least 90% of the time. If your furnace is running only a short time then it is oversized.

  3. #3
    Moderator & Master Plumber hj's Avatar
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    Default btu

    Typically the output was 80% of the input, so your unit was proably rated 80,000 output.

  4. #4

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    Thanks guys. The present fau ran pretty much continuous in cold weather.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    You can't necessarily go by the fan operation...depending on the furnace and the thermostat, the fan could be on much more than the burner...do a heat load calculation first, you'll be much better served. You sure the specs don't also indicate the output heat?
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    Plumber jimbo's Avatar
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    I usually see furnaces rated with an input BTU and an output BTU, which is the input times the efficiency. A 1992 furnace is probably an 80% , so it is about 80,000 BTU output. You could use a 100,000 BTU input 80, or even a 90,000 BTU 90% job.

  7. #7
    Rancher
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    Quote Originally Posted by Furd
    OR, call several furnace installers and ask if they will do a heat loss calculation.
    Or most utility companies will do a heat loss calculation for you.

    Rancher

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