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  1. #1
    Join Date
    Feb 2012
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    Michigan
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    87

    efficiency of two stage LP furnace

    Let's say I have a 95% two stage furnace, like a Goodman GMH95 series, which uses a constant speed combustion air blower with a two stage gas valve, and I'm burning propane in it.

    Will the furnace be more efficient on high stage or low stage, or will the two heat settings be equal in efficiency, with more comfort on low stage?

    I realize the GMH is only quasi-two stage, but set for automatic stage switchover, it runs about 13 minutes in low stage before switching, which means it spent all of September in low stage.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,761
    It'll be slightly less efficient in first stage. but better comfort usually makes up for it by allowing people to set the stat a little lower from the constant heat. At least those 2 stage that are controlled by the thermostat work that way.
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  3. #3
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,425
    It's kinda six of one and 1/2 dozen of the other. You'll use less gas at lower stage, but at a slightly lower efficiency. I've got the same furnace in my house and love it set on the auto time 2nd stage. Seldom notice 2nd stage coming on.

  4. #4
    Join Date
    Feb 2012
    Location
    Michigan
    Posts
    87
    Slightly less efficient because it's running more combustion air than necessary?

    I'm glad I didn't let anybody talk me into a 70k furnace. My 46k is running 9 minutes out of 27 on low stage in 40° weather. And I haven't gotten around to blowing more insulation into the attic yet.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Becasue they all are less efficient in first stage. has to do with the size of heat exchanger, and lower amount of radiant heat the lower flame puts off.
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  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,761
    Quote Originally Posted by wahoo View Post
    It's kinda six of one and 1/2 dozen of the other. You'll use less gas at lower stage, but at a slightly lower efficiency. I've got the same furnace in my house and love it set on the auto time 2nd stage. Seldom notice 2nd stage coming on.
    There have been members here that had 2 stage furnaces installed that were using the furnace timers. Said they loved them. Then when they were finally convinced to get a 2 stage stat to control staging. they said they couldn't believe how much more comfortable they were now that the thermostat was controlling staging.

    I think one of them was mayguy.
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  7. #7
    Join Date
    Feb 2012
    Location
    Michigan
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    87
    How much is the efficiency drop, is it like having a 92% instead of a 95% furnace?

  8. #8
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,614
    Could also add a heat pump to it and save some propane
    We really need change now

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,761
    Quote Originally Posted by Homeowner314 View Post
    How much is the efficiency drop, is it like having a 92% instead of a 95% furnace?
    A 2% or so steady state efficiency drop.
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  10. #10
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,918
    No such thing as constant heat with the GMH. Get a REAL 2 stage furnace

    Of course if you listen to the Captain, low fire is extremely inefficient!

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,761
    Quote Originally Posted by BaldLoonie View Post
    No such thing as constant heat with the GMH. Get a REAL 2 stage furnace

    Of course if you listen to the Captain, low fire is extremely inefficient!
    If you listen to him, a 95% furnace is only 70% efficient.
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  12. #12
    Join Date
    Feb 2012
    Location
    Michigan
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    87
    Quote Originally Posted by beenthere View Post
    If you listen to him, a 95% furnace is only 70% efficient.
    It's got to be way more than 70% when the flue gas leaves my house at 82° in autumn weather. I'll go with your 93% figure.

  13. #13
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,158
    Best bet is to really look into if the 2nd stage is needed at all. In most cases a smaller furnace will do the job.

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