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  1. #1
    Join Date
    Feb 2008
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    Kalamazoo, MI
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    2-stage 80 vs. 1-stage 90

    Is it true that an 80% 2-stage is more efficient than a single-stage 90%?

    I've heard this but I'm looking for some sort of solid proof (documentation of sorts) that this is correct or incorrect.

    Thanks in advance for all help.

  2. #2
    Join Date
    Dec 2007
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    Ohio
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    A 2 stage 80% ........

















    ........ is still an 80%
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  3. #3
    Join Date
    Nov 2003
    Location
    Rochester, New York
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    No, its not true. 2 stage is a comfort thing. 90 % will always be more efficent all things being equal.

  4. #4
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    Depends on how you define efficient. If by efficiency you mean fuel utilization, then the 90% will be more efficient.

  5. #5
    Join Date
    Jan 2004
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    Lancaster PA
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    A single stage 90% is more efficient the a 2 stage 80%.
    A 2 stage 80 can provide better comfort then a single stage 90%.
    But it will use more gas.
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  6. #6
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    Nov 2000
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    Quote Originally Posted by Frostie View Post
    Depends on how you define efficient. If by efficiency you mean fuel utilization, then the 90% will be more efficient.
    Just how else could you define efficiency?

    80% is and will always be less efficient then 90%. As mentioned; staging is only for comfort level.
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  7. #7
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    Jul 2006
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    Southern Ca, Elkton Md
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    Quote Originally Posted by RoBoTeq View Post
    Just how else could you define efficiency?

    80% is and will always be less efficient then 90%. As mentioned; staging is only for comfort level.
    Just because it says 90% on the label dosen't always mean that it is running at that efficiency.

    An 80% running properly could be more eff than a poorly running 90%.

    But, for the op questions, no an 80% 2stage is less then a 90%, but that 2 stage might make you feel like your getting more for your money because of the comfort level.
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  8. #8
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    Nov 2000
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    Quote Originally Posted by weber View Post
    Just because it says 90% on the label dosen't always mean that it is running at that efficiency.

    An 80% running properly could be more eff than a poorly running 90%.

    But, for the op questions, no an 80% 2stage is less then a 90%, but that 2 stage might make you feel like your getting more for your money because of the comfort level.
    I understand what you are saying, but for a badly operatiing condensing furnace to operate at a lower efficiency then the best running 80% furnace, that condensing furnace would be shutting down on one safety or another.

    Besides, when we get into these scenarios we must assume that the equipment is operating as it was intended to do so.
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  9. #9
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    Feb 2008
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    Quote Originally Posted by RoBoTeq View Post
    Just how else could you define efficiency?

    80% is and will always be less efficient then 90%. As mentioned; staging is only for comfort level.
    90% efficient furnaces use only one parameter to define its efficiency, and that is fuel use. But IF a person chooses to look more closely at the costs associated with owning a 90% AFUE furnace, the "efficiency" will include many more factors than just the fuel consumption. Time associated with breakdowns /service requirements/ manufactoring materials used / lifespan of the unit/ should all be considered, not just fuel use, when determining how suitable a furnace is to your situation.

  10. #10
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    Quote Originally Posted by Frostie View Post
    90% efficient furnaces use only one parameter to define its efficiency, and that is fuel use. But IF a person chooses to look more closely at the costs associated with owning a 90% AFUE furnace, the "efficiency" will include many more factors than just the fuel consumption. Time associated with breakdowns /service requirements/ manufactoring materials used / lifespan of the unit/ should all be considered, not just fuel use, when determining how suitable a furnace is to your situation.
    You have my apology. Even though this is not normally considered an "efficiency" factor but rather an "economical value" factor, I do see what you mean and how it can be more cost effective to use an 80% furnace rather then a 90% furnace.

    Thank you for explaining your intent.
    Government is a disease...
    ...masquerading as its own cure…
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  11. #11
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    Quote Originally Posted by Frostie View Post
    90% efficient furnaces use only one parameter to define its efficiency, and that is fuel use. But IF a person chooses to look more closely at the costs associated with owning a 90% AFUE furnace, the "efficiency" will include many more factors than just the fuel consumption. Time associated with breakdowns /service requirements/ manufactoring materials used / lifespan of the unit/ should all be considered, not just fuel use, when determining how suitable a furnace is to your situation.
    Please provide evidence which suggests that high efficiency furnaces are more prone to breakdowns and service problems.

    A 90%+ unit is just a mid-efficiency furnace with a secondary heat exchanger.

    Just about all modern units contain crappy chinese controls and hot service igniters, high efficiency or not.

  12. #12
    Join Date
    Feb 2008
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    Quote Originally Posted by amd View Post
    Please provide evidence which suggests that high efficiency furnaces are more prone to breakdowns and service problems.

    A 90%+ unit is just a mid-efficiency furnace with a secondary heat exchanger.

    Just about all modern units contain crappy chinese controls and hot service igniters, high efficiency or not.
    Generally I find that the electronic controls are very reliable(with a few unique exceptions), as well as the nitride ignitors.

    Plugging of secondary heat exchanger due to dog/cat hair the same way an evaporator coil plugs. Collector box cracking, since they are mostly made of plastic. Drains plugging, could be kinked hoses, could be worms growing in the piping. Drains leaking. Drain pressure switches, doubling the amount of pressure switches doubles the chance they could fail. Ice/frost build up on vent intake. The list goes on, and nobody could rationally argue that high efficient furnaces are more dependable than mid efficient furnaces. They have more parts in them and they condense the flue gas which by this very fact provides for more problems.

  13. #13
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    Mar 2002
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    Concord, CA
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    The list goes on, and nobody could rationally argue that high efficient furnaces are more dependable than mid efficient furnaces.
    But they can argue irrationally.


    I agree with you. "ipso facto" comes to mind, though I'm not certain if that would actually be correct usage. Anyway, all things being equal a more complex device must fail at a higher rate than a simpler one. You'd have to start breaking laws of physics to do otherwise.

    However, all things are not equal. For example, if it were the case that furnace failure rates in general are pretty low; and if were the case that hacks tend to install 80+ furnaces as where most 90+ furnaces are installed by good contractors; then you might see 90+ furnaces showing lower repair rates. That would not prove 90+ furnaces are more reliable, but it would make it seem so because of who installed it.

    In any event, I would love to see repair rate statistics generated from a large sampling of furnaces and air conditioners across all brands and types.

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