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  1. #1
    Join Date
    Aug 2010
    Location
    West Monroe, LA
    Posts
    1,535

    Manual J is wrong for the 1st time???

    As my post stated, I have ran across a manual j that is showing me the wrong size system for the 1st time. I know what you are thinking the input details have to be wrong but I have triple checked them.

    Now let me explain. I have a customer with a 1800 square foot home that was built in the 50's which is rasied frame home with no insulation in attic or the floor. The walls are more then likely not insulated as well. The house can't be much tighter then average.

    He has been running a 4 ton system in the home for 10 plus years with no problem keeping it 72 on a 95 degree day. Some more info on the home has a good bit of windows all single wood with metal storms on them.

    My load came out to be 6.5 tons of air needed to cool the home due to no insulation but this man home has done fine for years with his existing (4) ton system as we have been doing all his work for the past 5 years.

    Other notes the system is even mismatched using a goodman a/c unit with a tempstar coil. The system has done the job. So Made a call to install another (4) ton system rated at 48,000 btus and we have installed it and per customer it is doing good. The duct system would only handle a 4 tons of air anyways along with returning it.

    Has anyone one else ever had this happen to them where rule of thumb has worked for them over manual j? I hate even saying that as I belive strongly in manual j but this time it was wrong! Any input to help me understand this would be great...

  2. #2
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    Yeah, I have gotten some comments from contractors and architects that load calculation numbers have been over estimated at times. I assume there are just factors that aren't accounted for... like shading from adjacent buildings, trees, or just curtains.

    Heck, maybe the asbestos material in the plaster give added insulation properties IDK... If the house is on a slab, then there are thermal mass properties... and there is a factor of safety in all the equations. If the 4 ton system works, then go with the 4 ton. 6.5 tons will be 2 systems in residential anyways.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  3. #3
    Join Date
    Jul 2010
    Location
    nebraska
    Posts
    1,629
    Sorry to hijack but why is there a house anywhere in the US with no insulation?

  4. #4
    Join Date
    Oct 2010
    Location
    The very humid south
    Posts
    712
    Quote Originally Posted by martyinlincoln View Post
    Sorry to hijack but why is there a house anywhere in the US with no insulation?
    here in South Louisiana we have lots of old homes with no insulation.

  5. #5
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,373
    Why not do a 3 ton and insulate the attic? Please tell me he's not going to install over 13SEER w/o installing insulation..

  6. #6
    Join Date
    Dec 2005
    Location
    Cincinnati, Oh
    Posts
    5,092
    Quote Originally Posted by martyinlincoln View Post
    Sorry to hijack but why is there a house anywhere in the US with no insulation?
    Alot of houses, actually.

    Infact, I used to do renovations that had no insulation in them, because they had no wood frame walls. Old turn of the century brick buildings. exposed brick on everything.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

  7. #7
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,373
    Quote Originally Posted by hvacvegas View Post
    Alot of houses, actually.

    Infact, I used to do renovations that had no insulation in them, because they had no wood frame walls. Old turn of the century brick buildings. exposed brick on everything.
    The thermal storage of brick is NOT accounted for in manual J !!
    Cool the house off at night and the thermal storage can carry it though a lot of the day.

  8. #8
    Join Date
    Aug 2010
    Location
    West Monroe, LA
    Posts
    1,535
    Quote Originally Posted by acsolutionsllc View Post
    here in South Louisiana we have lots of old homes with no
    insulation.
    Yes i am in Northeast Louisiana. I would say 1 out of 50 homes that I go
    To have no insulation or only about 3 inchs of blown or worse batts.

    He will be insulating in the next few months but I never go by what is there now because most of the time people don't always say what they will due...

    (4) ton already installed and working great. It almost makes you wonder why not do rules of thumb? I hate to say that but look at this home and it is prime example. I just typing out loud.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,320
    What insulation value did you use for the walls, floor and attic.


    still a lot of stone built houses here, no insulation on walls, a little bit in attic.
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    How many times must one fix something before it is fixed?

  10. #10
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,373
    Quote Originally Posted by duckman06 View Post
    Yes i am in Northeast Louisiana. I would say 1 out of 50 homes that I go
    To have no insulation or only about 3 inchs of blown or worse batts.

    He will be insulating in the next few months but I never go by what is there now because most of the time people don't always say what they will due...

    (4) ton already installed and working great. It almost makes you wonder why not do rules of thumb? I hate to say that but look at this home and it is prime example. I just typing out loud.
    He'll do it once it gets hot/cold in there...

  11. #11
    Join Date
    Aug 2012
    Location
    Silver Creek, Ny
    Posts
    94
    I just sold a system to my friend with a similar house...built in 50's with the returns on the outside wall and supplies on inside wall...minimal insulation,
    1200 sq ft on first floor with (6) 6" heat runs and a 600 sq ft loft with (2) 6" runs.

    My supplier and I came up with about 2.5 Tons with about 72,000 btu heating ( we are talking Buffalo so we have to have a good heat load).

    I will be reducing the bathroom to 4" and upping the 2nd floor to 7" runs.

    4 tons sounds like a a lot for a 1800 sq ft house unless your in an area with a large year round cooling load. you would have to have about (16) 6" runs to accomodate that CFM capacity with a 10x24 trunk line coming off the furnace...thats huge for that house.

    am I missing something?
    Perfect Practice Makes Perfect

  12. #12
    Join Date
    Sep 2008
    Location
    VA
    Posts
    1,790
    Can you post all of the general inputs. I would like to run them on Carmelsoft and HVAC Buddy Load Calc.
    "The only true wisdom is in knowing you know nothing" Socrates

  13. #13
    Quote Originally Posted by duckman06 View Post
    Yes i am in Northeast Louisiana. I would say 1 out of 50 homes that I go
    To have no insulation or only about 3 inchs of blown or worse batts.

    He will be insulating in the next few months but I never go by what is there now because most of the time people don't always say what they will due...

    (4) ton already installed and working great. It almost makes you wonder why not do rules of thumb? I hate to say that but look at this home and it is prime example. I just typing out loud.
    If you would like, I can run the load and see what I come up with. I use Wrightsoft. Just send me all the information about the house.

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