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  1. #1
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    Effeciency drop of an older furnace

    I have an old (25 years) furnace. I think it is a 'keeprite' - no plate but that is what the a/c is and it is original as far as I know.

    on the label it says:

    'normal input btuh=65000'
    'rated output btuh=49000'

    so I assume that it means that it is a 65000 furnace delivering 49000 btu's - which works out to 75%. So the 'efficiency rating' I guess is 75%.

    So what I am trying to figure out is presumably over time, this number drops somewhat - wear and tear and stuff. Is there any guess what it might be after the 25 years? is there any rules of thumb as to how much these old furnaces drop in effieciency over time?

    Just kind of curious.

    Thanks in advance
    Paul

  2. #2
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    May 2008
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    If it is a pilot light furnace, it will be in the 55-62% efficiency range. If it has electronic ignition without forced draft, it'll be about 70% (at most). If it has a flue damper, it might have been rated at 75-78%, and if it has a forced draft vent fan with electronic ignition, it should be around 80%. Remeber this was when the unit was new, so existing condition could lower actual efficiency a lot! IIf the blower is dirty, and the burners are rusted, and sooted half shut, you could easily be into the 50% efficiency range.

  3. #3
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    Nov 2009
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    This is your old crappy builder version furnace. Not sure why you bring it up, but it is a 'pilot light' type furnance. It is all natural venting (no fan). We are in the process of replacing, but other than breaking down 1ce a year, it has kept us warm for many years I've never looked much inside, but rust abounds, so I assume the burners are somewhat dodgy. Did have it serviced (cleanup) a few years ago.

  4. #4
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    With proper maintenance the efficiency shouldn't drop due to age.

    Assume no higher than 60%.
    I've discovered a natural law - everything gets progressively worse. Things only break down and become depleted. Life isn't worth living and everything is going to hell.

    Death is the messiah. Everything else is irrelevant and arbitrary.

  5. #5
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    Jan 2008
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    That rated output btu # is often call bonnet capacity.
    That means that's the BEST you're ever going to get out of it.
    Strongly consider a new unit, if not for the unreliable nature of a 25 YO furnace, then for the advantage of saving some gas.
    Oh, and that heat exchanger should be examined very carefully to be sure it's not cracked.
    "Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler

  6. #6
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    Quote Originally Posted by amd View Post
    With proper maintenance the efficiency shouldn't drop due to age.

    Assume no higher than 60%.
    They had 90% furnaces 25 years ago.

  7. #7
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    Quote Originally Posted by skizot View Post
    They had 90% furnaces 25 years ago.
    Not with standing pilots and 75% steady state efficiency.
    I've discovered a natural law - everything gets progressively worse. Things only break down and become depleted. Life isn't worth living and everything is going to hell.

    Death is the messiah. Everything else is irrelevant and arbitrary.

  8. #8
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    Quote Originally Posted by rpm2203 View Post
    on the label it says:

    'normal input btuh=65000'
    'rated output btuh=49000'

    so I assume that it means that it is a 65000 furnace delivering 49000 btu's - which works out to 75%. So the 'efficiency rating' I guess is 75%.

    So what I am trying to figure out is presumably over time, this number drops somewhat - wear and tear and stuff. Is there any guess what it might be after the 25 years? is there any rules of thumb as to how much these old furnaces drop in effieciency over time?

    Just kind of curious.

    Thanks in advance
    Paul
    Since thats steady state efficiency. It will still be very close to that. Maybe 70%.

    AFUE wise, it was probably around 60 to 65%.
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  9. #9
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    May 2008
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    Smile

    Something to consider is the new energy Income Tax Credit, it will be going on next year also, but it allows you to receive a credit of 30% up to $1500 on the replacement cost of this furnace with a 95% or higher unit. If you have a gas company who also rebates, you could get a lot of $ off the actual cost by getting rid of that sucker. Yeah, I had a 1 ton Dodge van that was dependable too, however it only got 8 miles to the gallon of gas!!

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