Results 1 to 13 of 13
  1. #1
    Join Date
    Feb 2005
    Posts
    39

    Question

    I have a customer who can not get below 78 on a 88 degree day. The customer was givin a copy of their Manual J. The company who installed their AC unit (new home-17 months) did not include some items for heat gain. The did not include any heat gain for lighting, people, duct loss and included blinds in all windows. This is a 1800 sq ft ranch with daylight basement. They have a 3 ton unit and the man J
    I'm not a man J expert but I think their 3 ton unit is undersized. Their Man J had 32,000 total cooling heat gain.
    I don't think I can help them, only give advice of contacting the builder.

    Can anyone tell me about how much heat gain was left out?
    I'm thinking about 6000-9000 btu????
    If I'm correct, they need a 3.5-4 ton unit.


  2. #2
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,752
    With the limited info you gave, you could be right.
    Or it could be a bad duct design, undercharged unit, or overcharged.

    600 sq ft to the ton for a 17 month old house is not out of line.

    Need alot more info, or, do a load calc yourself.
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  3. #3
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579


    There is a false assumption in our business that just because a Manual J calculation was done, that it is correct. Even if a computer software program was used the old adage GIGO (Garbage In, Garbage Out) applies.

    ACCA, the folks who set the load calculation standards have several load calculation classes available. More people need to take those classes or take a class from someone who has attended ACCA's instructor class on load calculation using Manual's J and D.

    There is a good chance that the contractor just performed what he thinks is a Manual J calculation to satisfy the unknowing customer or made an awful mistake out of ignorance and miscalculated. And, you seem to have found their mistake.


  4. #4
    Don't sound out of line for most houses in my area. But, a 'load calc' (by you) will give you piece of mind you're looking for.

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,428

    Exclamation Location Location Location

    Sounds ~ right. Location: Texas or Maine?

    Seems more likely to be an installation set-up/operational problem.

    Location Location Location
    applies to HVAC & Real Estate.

    Send me a drawing and I'll tell you Size for Sure.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
    Join Date
    Feb 2005
    Posts
    39
    Western Ky.

  7. #7

    Re: Location Location Location

    Originally posted by dan sw fl
    Sounds ~ right. Location: Texas or Maine?

    Seems more likely to be an installation set-up/operational problem.

    Location Location Location
    applies to HVAC & Real Estate.

    Send me a drawing and I'll tell you Size for Sure.
    Location... he's only talking 10 degree delta. And it won't keep up...

    Now that's a problem, no matter where it's located!

  8. #8
    Join Date
    Sep 2002
    Posts
    4,970
    Originally posted by NormChris


    There is a false assumption in our business that just because a Manual J calculation was done, that it is correct. Even if a computer software program was used the old adage GIGO (Garbage In, Garbage Out) applies.
    I totally agree with you Norm. It supprises me when a person gets done figuring out the heat gain or loss, then goes to decide if there should be any kind of fudge factor added. Now no one will really give you a staight answer on that one. I have heard everything from adding nothing to adding 25 - 30 % as a fudge factor. It just amazes me that a lot of times a unit will be over sized for that 5- days that it might hit -20 bellow. Now does that make sense,but dam if it does you will be up a creek with the owner. Guess you could add the 30 % and claim its in case you add on, ya like thats right. Year ago when I started out a distributor did our heat and cooling caculations as a free service for buying from them. It didnt take that long to realize their j manuel had a fudge factor plus that added a lot just to cover their butt. Not like you didnt blame them but it was ending up so over sized we would drop down to the next size of both furnaces and air conditioners to keep from being over sized .....now doesnt that make sense....NOT It was nice having it done but with the computor programs I started doing it myself then to be wondering what it really should be. YEP sure wish it wasnt a guessing game on adding a fudge factor or not.

  9. #9
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    Sounds like this unit needs to be checked over, doesn't sound like 3 ton is too far out of size for this situation. Get your installer to check it all out before you go questioning the size of the unit. Delta T of only 10 degrees is not going to give you a whole lot of recovery. Was the unit used at all during construction? Make sure it wasn't or you are certainly going to have some restriction in the evaporator coil.
    "Go big or Go Home"

  10. #10
    Join Date
    Feb 2004
    Location
    southern illinois
    Posts
    5,536
    i'm with everybody else,...it sounds close.....check for return leaks,too high blower speed,and like B.A. said if it was used during construction that is bad.

  11. #11
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,031
    Unless you live in a terribly humid climate, at an 88 degree ambient a 3-ton unit will normally get 15 to 20 degrees delta.

    Where I live we have 2-ton units on 2400-sq.ft. homes that do great. We also have 96-F and 112 heat indexes here.

    In my opinion your A/C has perhaps numerous problems that need to be located and remedied. Do some research.

    http://www.udarrell.com/air-conditio...fficiency.html


  12. #12
    Join Date
    Feb 2004
    Location
    southern illinois
    Posts
    5,536
    with all due respect,....i'm curious are these homes that you are getting 1200 sq. ft. per ton out of..built in-ground or what?

  13. #13
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,031
    No, they are not built into the ground. This is in SW WI, the homes have basements; some are two story homes. We have very cold winters here: ouitdoor winter design at Madison, WI is -11, summer design is 88-DB, 73-WB, around 48% RH.

    We have had a number of days in the mid-90's and it can tap 100-F with heat indexes above 116-F. MilW had 116-index a while back, high humidity plus 96-F.

    Homes should have R-38 in the attics here however older homes seldom have anywhere near that.

    I own a home on a farm, 2-story with a deep basement, that was built in May of 1937. It is shaded some from the west later in the afternoon and has storm windows, but a lot of air infiltration from various sources. I cool the entire first floor area with a little 6,000-Btu/hr, 9.7 EER Whirlpool window unit. It does a perfect job in the hotest weather for around 920-sq.ft.; three rooms and an open hall stairwell that is not cooled from upstairs during the daytime! Hot July electric bill with electric hotwater heater was just a hair over $45.00. Here is how I do it:

    http://www.udarrell.com/aircondition..._charting.html


    The little half ton A/C is rated for up to only 220-sq.ft. area! I can prove every fact I have stated here! I am a retired long time HVAC contractor and in the field tech.



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