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  1. #1
    Join Date
    Oct 2007
    Location
    Southern California
    Posts
    27

    Question Design Temp vs. Actual

    I just did my Manual J.

    HVAC-Calc gave me 91 degrees as the summer Design Temp (I'm in So Cal) but I'm thinking I need to use maybe 100 to make sure I have enough capacity when it gets that hot - which it does maybe 5-10 times a year.

    Based on 100 degrees it gives me a heat gain of about 30K - a 2.5 ton. Most of the contractors have been quoting 3.5-4 ton.

    Am I thinking about this correctly using 100 degrees?

    Should I be looking at a 3 ton just to be on the safe side? I understand you don't want to oversize the AC but would 3 ton be oversized in this situation?

    Thanks for any help!

  2. #2
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    I am a homeowner in Texas, but have been through the similar thought process. Is there any possibility of having a 2nd AC on standby for those hot days, maybe a window unit? For comfort with a standard single stage AC, you want long runtimes. That is good for humidity too, but you may define it as not a concern in your climate. You tell me.

    Hvac-Calc may be tricking you with the size question. It's not nearly as simple a number as Hvac-Calc would have you think. You need to separate that into sensible and latent loads, and usually sensible is the limiting factor. The AC hardware must be de-rated to reflect your design temperature, and your indoor conditions make a difference too. It is all covered in ACCA Manual S, which is available for about the same cost as your Hvac-Calc license. Some equipment manufacturers such as Trane provide detailed performance data which reduces the need for Manual S.

    What I would do is: show my calcs to a friendly AC contractor, who can talk/work with you about how to translate into the right nameplate capacity. This may be easier said than done. You probably will be able to tell by common sense, if the other guy is padding the estimate using unrealistic inputs into the model. In Texas I hear of AC pros consciously manipulating input data in order to arrive at the answer they want.

    Let me confess: I know very well the thought process that you are going through, that it is better to err on the side of oversizing rather than undersizing. I myself jazzed up the recommended outdoor temperature from the recommended 94F, up to 99F -- just had a "sleep at night" problem about that. That is why I first asked if some kind of standby AC could be employed in your house. In my house that takes the form of 2-stage AC. But in hindsight, all my info tells me there is no flaw in the ACCA methods.

    It is likely that going through the Manual S process, you will already be at the larger of the two sizes you mentioned. In that event 3.0 tons will *not* be oversized. My opinion is 100F is overkill to design for, when 91F is the official recommendation. Perhaps volunteer help from the board's professionals would answer your sizing question. If not, I would consider it highly appropriate to pay one of them to do the sizing math for you, that is something feasible over long distance.

    Best of luck -- Pstu

  3. #3
    Join Date
    Oct 2007
    Location
    Southern California
    Posts
    27
    First off, thanks for the lengthy reply!

    Actually we are in a dry climate so the latent heat gain is minor - in this case 460 BTUH, almost nothing. If I bump up the humidity to the next nearest city it only goes to 1000. So the number I am coming up with is pretty much the sensible heat load.

    Another factor here is we haven't turned on our AC in 20 years! As long as the temp is under 95 we're pretty comfortable. Which means we are probably only going to turn it on the 5-10 times per year when it gets that hot. And we would probably set the t-stat to 78. Even running the calc at 75 and the outdoor temp at 100 and the higher humidity I still only get a heat gain of 32,500 - 2.5 ton. When I run it with the design temp I only get 25K - 2 ton.

    So to answer your question about the 2nd AC - probably doesn't make sense. It's just not cost effective given the amount we are likely to use/need it. But I can see why it would be important to you in Houston!

    BTW - I like your van Gogh quote.

  4. #4
    Join Date
    Nov 2007
    Posts
    43
    ....a sensible gain of 32,500BTUH is in 3 1/2 Ton territory.

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,296
    Quote Originally Posted by RCW View Post
    ... I am coming up with is pretty much the sensible heat load.

    Another factor here is we haven't turned on our AC in 20 years! As long as the temp is under 95 we're pretty comfortable.

    Which means we are probably only going to turn it on the 5-10 times per year when it gets that hot.

    And we would probably set the t-stat to 78.
    If you are going to use your A/C
    < twice / decade ...


    or < 10 / year for that matter










    get a couple $120 window units.
    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
    Dec 2006
    Location
    Arkansas
    Posts
    292
    Quote Originally Posted by dan sw fl View Post
    get a couple $120 window units.

    How about a bucket of ice and a cheap fan?

  7. #7
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,296

    Thumbs up

    Quote Originally Posted by dcmcm5 View Post
    How about a bucket of ice and a cheap fan?
    Works for me!
    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

  8. #8
    Join Date
    Oct 2007
    Location
    Southern California
    Posts
    27
    Well, that would be OK with me except if I ever sell the place they are going to expect AC. Hmmm, but maybe I could sell it with the bucket and fan and tell them where to get the ice...

    I think I'll probably split the difference and go with the 3 ton.

  9. #9
    Join Date
    Oct 2007
    Location
    Richmond, VA
    Posts
    103
    Add it when your ready to sell.

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