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  1. #1
    Join Date
    Jun 2005
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    New England
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    906
    Hi - I want to thank everyone for the input on the "did I buy the wrong furnace" thread - lots of good data, and the thread went in more directions than I anticipated.

    Lost in the discussion however, was one question I was really really trying to get an answer to...

    If I compare a single-stage FHA furnace, and a two-stage furnace of the same size....

    1) Compared to the lowest fan speed of the single stage, roughly how much slower are the 'extra-low' fan speeds in the 2-stage (the speeds that are used when only one stage is on)? 20%? 50%? more?

    2) When both stages are on in the two stage, will the 2-stage designed to move -more- air than the single stage?

    This information (which I should have asked for before the install) will go a long way to help me gauge whether the recommendations I received to avoid a 2-stage for my unusual, archaic duct system were correct.

    Thanks, I really appreciate the help
    /j

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,367
    On a York PT9 120000 input, low spped CFM is approx 55% of high speed. This is a 2 stage unit.

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  3. #3
    Join Date
    Jun 2005
    Location
    New England
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    906
    Thanks - and I would expect the 'high-speed' of the two-stage to not be much better than the regular running speed of a 1-stage?

    (That suggests that a 2-stage would have -not- worked for me, as 55% of my current airflow probably would -not- pump properly through my 3.5" ducts).

  4. #4
    Join Date
    Jan 2004
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    Lancaster PA
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    I've never installed a 2 stage on the 3.5 systems yet.

    I've also never seen one that the original furnace wasn't over sized either.


    Why not put some heat into the basement, to lower the air flow to the rest of your house.
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  5. #5
    Join Date
    Jun 2005
    Location
    New England
    Posts
    906
    Hate to put too much heat in the basement, people are in it much of the time...

    by the way - I'm a certified Moron (or CFM). I just went to Bryant's site. The CFM for Heating on my furnace is 1720CFM. The CFM for the closest 2-speed furnace of about the same size (actually 7% larger) is 1770CFM on high, and 1215CFM on low - which means the low-speed would be 30% lower than what i have now.

    Now - what muddies the waters a bit is that my current furnace is a 4-speed, and they only list 1 CFM for heat (Which I think is the 2nd-to-lowest speed, and I'm already on the lowest, which could be like 1600 CFM). But, it boils the question down to - given my duct system as described at the top of this post, is 1215CFM too little in any case? I suspect it is....

    /j

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    At 1215 cfm, and 112,000 btu output, that puts your heat rise at 85°. I believe you'll find thats alot higher then your furnace is rated for.

    With the size of your supplies, your furnace may not be able to deliver 1215.

    The more restrictive the duct, the less volume of air moved by the furnace.

    The noise you hear, is velocity noise, not volume.
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  7. #7
    Join Date
    Jan 2004
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    Lancaster PA
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    Originally posted by beenthere
    At 1215 cfm, and 112,000 btu output, that puts your heat rise at 85°. I believe you'll find thats alot higher then your furnace is rated for.

    With the size of your supplies, your furnace may not be able to deliver 1215.

    The more restrictive the duct, the less volume of air moved by the furnace.

    The noise you hear, is velocity noise, not volume.
    Oops, misread your post.
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  8. #8
    Join Date
    Jan 2004
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    Lancaster PA
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    From here I can't tell what you need for air flow for your house.

    I think your furnace is over sized by 20,000btu's..

    Did you have this temp imbalance with the old furnace.
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  9. #9
    Join Date
    Jun 2005
    Location
    New England
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    906
    Hi Been there - first Thanks again, I'm learning a lot from you.

    Not sure what you mean by 'temp imbalance'. the old furnace (nominally the same output BTU) had longer cycles and had less 'velocity noise' (which I still think is -related- to cfm, no?) than the new one.

    It's possible that my 45yr old, (112KBTU out) 80% efficient furnace was really less than 80% efficient, so that the new one, which really DOES put out 112KBTU, is over-sized, but I reserve judgement on that until we get some 0DegF weather.

    I was thinking - it's possible that if I opened all the ducts all the way the 1215CFM might move enough air through the system so that the the temp at the furnace didn't go too high, and it would run longer cycles and be quiet. HOWEVER 8-}, when the 2nd stage cut in - whoosh - the noise would be annoying, and the system would be unbalanced (some rooms would cook waiting for others to get warm...).

    Having an older stupid duct system sucks (sigh).

    (btw - according to my somewhat accurate oven thermometer, the air temp right over the furnace is about 155DegF)

    [Edited by jeffw_00 on 02-02-2006 at 08:34 PM]

  10. #10
    Join Date
    Mar 2005
    Location
    burlington county n.j.
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    9,756
    your 45 year old furnace on its best day was no more than 50-55% efficient. new furnace is grossly oversized. is it possible to add more ducts if replacing what you have is not possible.

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    A 2 stage 100,000 input, 92,000 output second stage, and 65,000 in and 59,800 out first, could have a 60° rise with 923 cfm first stage, and 1419 cfm second stage.



    Do an acturate heat load, and see what size furnace you really need.

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  12. #12
    Join Date
    Jun 2005
    Location
    New England
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    906
    Yup - I have a plan for the weekend -
    check the bottom of here..

    http://www.hvac-talk.com/vbb/showthr...3&pagenumber=5

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Good.

    And yes, if the air is moving faster, you have less temp drop from one end to the other.

    Along with the rst of your busy weekend schedual, why not do your own heat load, its 50 bucks, and well worth it.

    Click the hvac tab.

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