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Thread: load calc

  1. #1
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    Jul 2007
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    load calc

    I have been researching options on a new system, including duct work, for about 6 months. I have(so far) gotten 3 quotes. I didn't ask and no one offered a Manual J calc. All have said I need to go with the same size, 3 tons.
    I have replaced old single pane aluminum sliders with double pane, low-e, single hung vinyl windows. I know the windows made at least some difference-the old ones produced a lot of condensate, interior in winter and exterior in summer. New windows have been dry.
    The AC would run short times in summer, but kept it cool. I have learned that it is not necessarily a good thing to run short times due to humidty, but not being an HVAC guy I did not know this and figured I was doing well since the unit did not run a long time. Now I learn that long run times during the hot muggy season is good.
    I figured when I ran the calc it would suggest a 2 or 2.5 ton unit. Well, the calc says 3 tons. And it is close enough that when I change my indoor design temp down 2 degrees(from 73 to 71) in summer it goes to 3.5 tons! I am going over my #s again, but don't think I missed anything. Any comments on this issue are appreciated. BTW, I am planning on a heat pump with VS air handler. And, if affordable, a 2 stage outdoor unit.

  2. #2
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    What did you to calc.,HVACCALC??

    FYI:

    At 73 indoors,Manual S and manufacturers data will tell you how to and that you must derate the sensible btus of the equipment.

    Derate is 835 btus sensible for each degrre below 80 indoors ,per 1000 cfms.

    Mfrs. data is at 80 indoor return air.

    My guess is ,if you have a 3 ton now and it did well,your calc is somehow off.

    Call around and find a Pro,that can and will do a Manual J.

  3. #3
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    Jan 2004
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    Lancaster PA
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    Sounds like you added safety to the numbers you entered into the calc.
    What infiltration did you use.
    What was your OD design temp you used. Did you make it higher then your areas normal high.
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    How many times must one fix something before it is fixed?

  4. #4
    Join Date
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    oregon
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    I would highly recommend a two stage VS system. But I will also recommend to call someone ( pro) who does MAN J every job. They are out there. Load calcs are only as good as the info they are fed. People who do load calcs every day and are well versed in manual J may be more accurate on their calcs due to their experiance.Before you choose your contractor ask for refrences for people who have systems like the one your are considering. Their feed back will give you a good idea of the contractors ability. Call more then one reference. Load calcs are probably one of the most important part of the bidding proccess. It gives the bidder a foundation to start from.
    Genius = The guy who can do anything...except make a living!

  5. #5
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    Jul 2007
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    coastal georgia
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    dash- Yes, I used HVAC Calc.
    As to the finding a Pro issue-I have asked around(there are 5 hvac techs where i work) and they all tell me they know of no one in our area that does them for residential. Also, I have a family member that is a tech and his outfit does not do them(he also said i shouldn't use them, but that is another story). I asked one of the techs at work who used to do all commercial work who he thought might be good. He said his former company always recommended a certain company not because they thought they were good, but because they did not compete in the commercial business.
    There are 20 contractors with the large ads in the yellow pages, and about 50 yellow page entry's total and not one says anything about running calcs. I haven't given up yet, 'tho.
    beenthere-I used default design conditions for my area and avg infiltration(but was leaning toward poor). The normal OD temps are 93 and 27 at 116 grains of moisture. Indoor conditions at default are 73 summer, 70 winter, at 55% RH. To be honest, I wanted to add some safety into my calc but decided to go with default for an honest calculation.

  6. #6
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    What is the sensible, and the latent load btu.
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  7. #7
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    The sensible is 35481
    The latent is 2814(690 for people and 2124 for infiltration)
    As suggested by the program, I did not add any shading. However, we keep the drapes closed at least 95% of the time and the house is rarely in direct sun due to lots of trees.
    I have decided on a VS fan for sure, but don't know if I can afford the 2 stage unit. How much, in percentage terms(no pricing) is 2 stage vs single stage?

  8. #8
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    Adding safety is NOT safe! ASHARE, ACCA, IGSHPA, CONSUMERS REPORTS, ENREGYSTAR, EPA, ect, ALL recommend and even state that you should not use a contractor if they do not do a load calc. Most contractors who don't do them don't know how to do them. How do you know the first system was sized correctly? And if you have improved the homes weatherization you may need a different sized system. Load calcs are very important! Ask your contractors when you call them about load calcs, that way they will know you want one before they come out.
    Genius = The guy who can do anything...except make a living!

  9. #9
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    geodude- I agree completely and suspect my original system was not sized correctly due to the short AC run times. The added weatherizing I have done made me think the load calc would definitely show less than 3 tons and I was surprised. But, I admit there may be (probably is) some operator error in my calc. Finding a good contractor is the trick, of course. And if I find one that does a calc that does not mean he has a good sheet metal guy or install team. I am heading toward analysis paralysis as I do on most big projects, but the cost for this system is significant $ for us.
    Thanks for the replys so far!

  10. #10
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    Call everyone in the phone book,ask questions,since you know what info. needs to be gathered,you'll have some idea if they are doing the M-J correctly.

    IMHO. Chances are if they can do a M-J,and many can't ,their design and install will be fine.

  11. #11
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    Jan 2007
    Location
    Minneapolis, MN
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    Try here for a contractor to talk with, if they are members they probably believe in the design process and do load calcs. http://www.acca.org/contractors/

  12. #12
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    Location
    Massachusetts
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    Unfortunately 7echo, you're swimming in a pool where the real pro's gather and we all know the importance of Manual "J", Manual "D" and Manual "S", not just for the numbers they produce but because of what it says about the company you're dealing with. I can tell you with a straight face that our Comfort Advisors do a complete Manual 'J' for EVERY SINGLE JOB. How else can one proceed with a Manual 'D' analysis of your distribution system to determine if you have proper ducting, proper airflow in and out of rooms? You're quite right, you're making a substantial investment and that should not be taken lightly. Nor should the money be invested in a company that thinks it's Okay to GUESS. You've obviously been asking everyone you can find if they do load analysis and they all say "No", which tells me they're all uneducated techs who've gone into business with little or no training beyond fixing the mechanical breakdowns. Keep searching. There are true professionals out there who's companies employe professional people. Call everyone in your local phone book and if that doesn't get the results you need, get neighboring phone books and keep calling. Check out www.acca.org and also go to any manufacturer's website and start calling the dealers in your area. The brand of equipment bears little forecast to the end result so don't let brand figure in unless you've got multiple companies offering the load, design and equipment selection by the book.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  13. #13
    Join Date
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    Location
    oregon
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    Hey 7 echo! Check out the thread " houston help need analysis done" Skippedover posted a nice check list of questions to ask your contractors. Keep looking and don't get discouraged. Quality contractors who care about doing the best they can for you ARE out there! Ask for referrals to people who have the system you are considering, and Check them! You are going to invest hard earned money in your next system, research your contractor and use the list posted by skippedover.
    Oh and by the way, are you a pilot?
    Genius = The guy who can do anything...except make a living!

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