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  1. #1
    Join Date
    Aug 2010
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    Manual J - Speed Sheet from


    We are in the market for a new furnace and air conditioning system and, purely out of curiousity, I wanted to see if I could determine what size furnace our home needs. I downloaded the Manual J Speed Sheet from the website and I have viewed the tutorial. However, I am trying to follow along with the example but I am not able to come up with the same numbers from the MJ8AE table.

    For example, for the example "Glass" tab:

    *1C-cm, Clear, Single Pane/Storm, Operable, Metal no break, fixed Horiz. Blinds at 45
    U-Value: 0.087 (Why? From Table 2A I would have used 0.87)
    Cooling HTM (N, NE/NW, E/W, SE/SW,S): 16, 33, 46, 40, 24
    (Again, why? From Table 3A-1 I would have used: 26, 46, 61, 54, 35 because the Design CTD is 21).

    *1E-c, Double Pane, Clear, fixed Horiz. Blinds at 45, rounded CTD of 20
    U-Value: 0.560 (I agree with this number).
    Cooling HTM (N, NE/NW, E/W, SE/SW,S): 13, 30, 43, 37, 21
    (Why? From Table 3A-1 I would have used: 16, 33, 46, 40, 24)

    Any assistance you may be able and willing to provide would be greatly appreciated.

    Thank you!

  2. #2
    Join Date
    Jun 2010
    Philadelphia PA
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    Suzie, it would be really difficult to guide you through this great resource you found on your own. I would like to suggest you share this with the contractors you ask for bids and tell them you consider this as needed to be included along with their equipment choices and firm price proposal.
    If one of them tells you they would have done the more thorough room by room MAN J if they got the job, you might waive that initial spread sheet because they have already agreed to the room by room loads that you should demand anyway.

    Ask them to look at your ducts and give you some comments and for sure If you are having any problems holding temperature in any rooms/areas tell them now and ask them to give you an idea of the duct modifications.

    This should do a couple of things. It sends a message you've done your homework and their response will give you a quick idea if these people know what they are doing. If they try to dissuade you. Thank them for their time and show them to the door Do Not Call them Back
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

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