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  1. #1
    Join Date
    Jul 2008
    Location
    Nebraska
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    11

    I Need Manual J help

    I am taking a contractors test and I have to do a Manual J 7th edition. I am getting quicker but I need some help this weekend. When I get to the equipment summery, how do I get the Cooling CFM and Heating CFM? I cannot find a description in the Manual J book. I am not a service guy, I am a tinner. After this test I will probably never have anything to do with residential again so don’t think I do not deserve the license for this one question. I can get the answer Monday but thought one of you fellas could help me out before then. I would like to complete two more Manual Js this weekend at home and start on a fresh one Monday. Is there a table in Manual J or is this something from the code book or this just something I should now from experience? Thanks for your time.

  2. #2
    Join Date
    Dec 2003
    Location
    Citrus County, Florida
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    1,547
    find your SHR by taking the sensible BTUH and adding the latent BTUH. This is the total BTUH. Now divide the sensible by the total. This will give you SHR. If above .85, use TD of 17. If SHR is .8 to . 85, use 19. If below .8 use 21.

    Then take sensible divided by 1.1 times Td answer is CFM
    Doug

  3. #3
    Join Date
    Jul 2007
    Location
    Chicagoland
    Posts
    475
    Im a novice in terms of system design,but this seems like a manual S issue.

    I looked@ man.J 8th v2 and found in sec.11-21 "procedure for evaluating zoning requirements". Preliminary estimates for block cfm

    Aprox.Heating cfm=heating load/(1.1 x ACF x (supply air temp - room air temp))

    Aprox.Cooling cfm=sensible load/(1.1 x ACF x (room air temp - supply air temp))

    It says 30* is typical heating TD, 20* for cooling

    it also states these approximations are ok for zoning decisions but use man.D for dist. system
    Last edited by AccurateHT; 07-13-2008 at 01:10 PM. Reason: add

  4. #4
    Join Date
    Jul 2008
    Location
    Nebraska
    Posts
    11
    Quote Originally Posted by Gunslinger View Post
    find your SHR by taking the sensible BTUH and adding the latent BTUH. This is the total BTUH. Now divide the sensible by the total. This will give you SHR. If above .85, use TD of 17. If SHR is .8 to . 85, use 19. If below .8 use 21.

    Then take sensible divided by 1.1 times Td answer is CFM
    Thanks. You are really helping me out. Let me run this past you.
    I have a Sensible gain of 17671 Btu (Calculation Procedure D)
    I have a Latent gain of 6044 Btu (Calculation Procedure D)
    Add these together I get 23715 Total
    Divide 17671 by 23715 = SHR .74514 so I use 21.
    21 divided by 1.1 = 19.09091.
    19.09091 X 75 TD = 1431.818
    So my answer is 1432 CFM Cooling?
    If I did this right I really appreciate your help. Now how do I get the heating CFM?

  5. #5
    Join Date
    Jul 2008
    Location
    Nebraska
    Posts
    11
    Delta T:

    I am not allowed to use anything past 7th edition for this exam manly, I am told, because 7th edition was still long form. To tell the truth I do not even own anything past 7th edition. I am going to look at manual “D” since you brought it up but I am only allowed to take the Manual “J” into the exam. Thanks for the help.

    Post Script: Actually the more I look at it, Your qouted formula may be just what I need. Let me put this to pencil some more.
    Last edited by gypsieking; 07-13-2008 at 02:50 PM. Reason: spelling

  6. #6
    Join Date
    Dec 2003
    Location
    Citrus County, Florida
    Posts
    1,547
    17671 plus 6044 =23715

    17671/23715 = .75 which is SHR

    use delta T 21* 1.1 X .75 = 23.1

    cfm= 17671/23.1 = 765 cfm

    use heat rise for heating
    Doug

  7. #7
    Join Date
    Jul 2008
    Location
    Nebraska
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    11
    Quote Originally Posted by Gunslinger View Post
    17671 plus 6044 =23715

    17671/23715 = .75 which is SHR

    use delta T 21* 1.1 X .75 = 23.1

    cfm= 17671/23.1 = 765 cfm

    use heat rise for heating
    Thanks fellas: I may have a chance at this yet.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,058
    Quote Originally Posted by gypsieking View Post
    Thanks. You are really helping me out. Let me run this past you.
    I have a Sensible gain of 17671 Btu (Calculation Procedure D)
    I have a Latent gain of 6044 Btu (Calculation Procedure D)
    Add these together I get 23715 Total
    Divide 17671 by 23715 = SHR .74514 so I use 21.
    21 divided by 1.1 = 19.09091.
    19.09091 X 75 TD = 1431.818
    So my answer is 1432 CFM Cooling?
    If I did this right I really appreciate your help. Now how do I get the heating CFM?
    Something wrong with that formula.
    Your not going to get a 21°F delta moving 1432CFM with a 2 ton unit. Which is what your sensible load is indicating you need.
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  9. #9
    Join Date
    Jul 2008
    Location
    Nebraska
    Posts
    11
    Okay. I have just slipped into the darkness again. Let me tell you what I have. I have a winter design temperature difference of 75. A summer design temperature difference of 20. Room Relative Humidity of 55% Daily range of 24m. Design heating load requirement of 70604.5. Cooling is 17671 sensible and 6044 latent. Total cooling load of 23715. Design temp swing of 3 degrees. Ventilation CFM = 100. Can someone walk me through the cooling CFM and Heating CFM of the equipment summery? I guess I really need it spelled out. Gunslingers last formula really spelled it out but I can not get the third line to = the same. Where am I getting off the path?

  10. #10
    Join Date
    Dec 2003
    Location
    Citrus County, Florida
    Posts
    1,547
    3rd line.

    use a delta of 21*

    1.1 times 21 equals 23.1

    sesible of 17671 divided by 23.1 equals 765 cfm
    Doug

  11. #11
    Join Date
    Jul 2008
    Location
    Nebraska
    Posts
    11
    Thanks for everyone's help. I am feeling more confident. I take the exam Wed and should know the result by Friday. I will let you know how I did.

  12. #12
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,183
    Good luck with your exam!
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  13. #13
    Join Date
    Jul 2008
    Location
    Nebraska
    Posts
    11
    The exam was tough. I believe all my figures where acurate. I got a little rushed the last hour and it looks like a child drew in all the systems on the print but the info was there and done correctly. The exam board had a golf outing Friday so I will get my results Monday afternoon. The exam was eight hours and was worse then my wedding. I will keep you posted.

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