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  1. #27
    Join Date
    Oct 2005
    Location
    N Metro Atlanta GA
    Posts
    70

    Warning! HO has been thinking

    When doing load calc for dual fuel systems, should you not run two calculations?
    First for the heat pump. Set the design temps to the usual summer values to figure cooling load. But set the outside temp to the balance point to figure the heat pump heating load?
    Then for the fossil fuel backup. Use the normal outside design temp but set the inside temp to ... what??

  2. #28
    Join Date
    Nov 2000
    Location
    Waco, Texas, USA
    Posts
    6,153
    Yeah and you should also run a calc for when your kids move away to go to college. And then one for when you have a lot of company coming over. And then one for when you cut down your tree in the front yard. And one for in the morning. And one for in the afternoon.

    So you start to see how these calcs leave a lot up to judgement and rely less on facts. They are a general guideline guys, could you relax a little about it please?
    "And remember my sentimental friend......that a heart is not judged by how much you love, but by how much you are loved by others" - Wizard of Oz.

  3. #29
    Join Date
    Oct 2005
    Location
    N Metro Atlanta GA
    Posts
    70

    Load Calc Update

    I received the contractor's load calculation summary.
    I ran the load again with only these changes to the design conditions of my earlier post:
    Removed original 75%-85% shading for all windows (from trees on south and SW sides).
    Removed indoor shading for all windows (blinds or drapes).

    ..........Sensible...Latent...Total......Total Loss
    First .....21,444....2,514.....23,958....31,909
    Second..12,541.......895....13,436....16,008
    Whole....33,980....3,403....37,383.....47,889[/B]
    ........................................(3 Tons)
    Previous Totals.................27,765.....47,889
    ........................................(2.5 T)

    Contractor conditions:
    ...Indoor........Outdoor
    Summer 72.......95
    Winter 70.......23

    ............Sensible...Latent....Total.......Total Loss...A/C.....Furnace.......Elec Furnace
    First.......16,504.....4,951....21,455.....27,921. .....2.0 T....27,921 BTUH.....8.2 KW
    Second...11,123.....3,337....14,460.....18,794.... ..1.5 T....18,794 BTUH.....5.5 KW
    Both floors listed Capacity Multiplier 1.00

    I don't know which program he used.
    I agree with everone on the 3 tons. Do you guys take drapes & shading into consideration? I know I keep them closed on the shady side in winter & sunny side in summer.

  4. #30
    Join Date
    Oct 2005
    Location
    N Metro Atlanta GA
    Posts
    70
    My Duct Loads (with below 120 temp and R2 insulation):
    .................. Gain .. Loss
    First........ 3,574 .. 4,162
    Second.... 2,090 .. 2,088
    Whole...... 5,664 .. 6,250
    Contractor's Duct loads:
    ................. Gain .. Loss
    First.......... 3,809 .. 6,443
    Second...... 2,567 .. 4,337
    ................ 6,376 . 10,780

    I don't know what my duct insulation value is. I didn't want to tear a hole in the covers. I guessed R2 because the metal ducts are encased in something which appears to be like the shiny barrier on attic insulation fiberglass bats.
    Does this seem a reasonable assumption?

  5. #31
    Join Date
    Oct 2005
    Location
    N Metro Atlanta GA
    Posts
    70
    Another Trane system proposal from a different company just came in by e-mail. Again proposing 100K BTU furnace.
    Which 80% furnace could I use to get close to the numbers from the load calc?

    4TWX4036
    TUD100R9V5 + RXC03783

  6. #32
    Join Date
    Aug 2001
    Posts
    220
    OK, if you had a 3 ton (I assume 3 ton straight A/C) before and you told him it would only maintain 79 degrees on a 92 degree day, he probably added 1/2 ton capacity to the system based upon this and also because you will now have a heat pump and the added capacity would be a bonus.
    That is a very nice furnace,if you are going dual fuel then I would say go with the 80 percent. The furnace will get little use anyway. Are all the contractors giving you quotes on similar setups?

  7. #33
    Join Date
    Oct 2005
    Location
    N Metro Atlanta GA
    Posts
    70
    Originally posted by AAone
    OK, if you had a 3 ton (I assume 3 ton straight A/C) before and you told him it would only maintain 79 degrees on a 92 degree day, he probably added 1/2 ton capacity to the system based upon this and also because you will now have a heat pump and the added capacity would be a bonus.
    He didn't ask about how it ran now. He ran load on the first floor & said it was 2.0 T. Then he ran the second floor & said it was 1.5 T. I think his laptop selected the models. He did almost no typing after the second load calc.
    That is a very nice furnace,if you are going dual fuel then I would say go with the 80 percent. The furnace will get little use anyway. Are all the contractors giving you quotes on similar setups?
    I have received only these two quotes. I had hoped to get 3 today (as two others said they would send it electronically today).
    I sure am glad I'm having so much fun with this purchase

  8. #34

    Talking Lucky Campers...

    Originally posted by i g norant
    Originally posted by AAone
    OK, if you had a 3 ton (I assume 3 ton straight A/C) before and you told him it would only maintain 79 degrees on a 92 degree day, he probably added 1/2 ton capacity to the system based upon this and also because you will now have a heat pump and the added capacity would be a bonus.
    He didn't ask about how it ran now. He ran load on the first floor & said it was 2.0 T. Then he ran the second floor & said it was 1.5 T. I think his laptop selected the models. He did almost no typing after the second load calc.
    That is a very nice furnace,if you are going dual fuel then I would say go with the 80 percent. The furnace will get little use anyway. Are all the contractors giving you quotes on similar setups?
    I have received only these two quotes. I had hoped to get 3 today (as two others said they would send it electronically today).
    I sure am glad I'm having so much fun with this purchase


    [Edited by AllTemp on 10-25-2005 at 02:37 AM]
    AllTemp Heating & Cooling

  9. #35
    Join Date
    Oct 2005
    Location
    N Metro Atlanta GA
    Posts
    70

    Another Proposal

    I can't find an ARI number for this contractor's combination:
    4TWX4036
    TUD080R9V4 + RXC037S3
    I found some with the XL16i. Is the proposed combination too new, just not tested or not sanctioned by Trane?

    [Edit] The combination came from Trane's Unitary Product Group CD. I guess that makes it a valid configuration for warranty purposes.

    [Edited by i g norant on 10-25-2005 at 07:02 PM]

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