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  1. #1
    Join Date
    Feb 2007
    Posts
    999

    question about Manual J

    Mea Culpa--sorry for my inadvertent hijacking, so let me repeat my post:


    The only way I could get a heat calc done was to do it myself. My loss came to 60,000 BTU. According to the author (of the software), one should allow a safety factor of 20 or 25%.

    One furnace that I am considering, a Rheem Mod 75,000 unit (upflow), has a high stage output of 72000 which would be exactly 20% oversize at design. My understanding of the furnace is that it would probably not operate at full capacity except maybe for recovery from setback (11 degrees). While this hasn't been the coldest winter in memory, we had a cold spell which was 4 degrees below design, and the Mod could handle a difference of 10+ degrees below design.

    I had originally considered two 80,000 units but because excess sizing, and of blower specs, I'm leaning toward the Mod.

    Since we normally keep our occupied temperaure at 66 degrees, it would be necessary to lower the setting in the event of temperatures in excess of 10 degrees below design. In looking at the discussion here, I haven't seen any reference to a safety factor and am wondering what emphasis you put on this, or to put it another way: how much safety is reasonable?

    AM

  2. #2
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    all equipment larger than necessary is wasteful -- for first cost, maintenance, operation expenses --

    the answer depends upon your desire & monies--

    do you also drive a 300hp vehicle? when a 90hp would do the job?
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  3. #3
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    Quote Originally Posted by cem-bsee View Post
    do you also drive a 300hp vehicle? when a 90hp would do the job?

    Oh hell yes, the bigger the engine the happier I am.

    ain't no callousus on my inner fore arms.

  4. #4
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,115
    Quote Originally Posted by cem-bsee View Post
    .
    do you also drive a 300hp vehicle? when a 90hp would do the job?
    Heck I would drive a 1000 Hp. vehicle even if 90 hp. would do if I could
    afford one, not a good comparison, try again.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  5. #5
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    Right you are mrbill

  6. #6
    Join Date
    Apr 2007
    Location
    charm city
    Posts
    18

    Rheem Mod

    I had originally considered two 80,000 units but because excess sizing, and of blower specs, I'm leaning toward the Mod.


    AM[/QUOTE]
    The Rheem Mod... with the WR electric modulating valve, while delivers a good number on the AFUE rate, is not much of a miser when modulating. They blow a lot of excess air on that WR gas valve to keep it safe. Hook an analyzer up to one and check it out over it's modulation range. It's also more of a stepper valve than a true modulating one unless you buy their pricey thermostat to control the furnace/valve operation. There are better appliances out there. I've heard the inducer fan make a nice whine sort-of noise as well...

  7. #7
    Join Date
    Dec 2006
    Location
    Arkansas
    Posts
    292
    Pay attention to the author of Manual J, not the author of the software. There are only three ACCA approved software packages for Manual J:

    Elite Software RHVAC
    Wrightsoft Right-J8
    Nitek HVAC Wizard

    I would suggest looking into Manual J and follow that if it is not one of those above.

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