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  1. #1
    Join Date
    Jul 2006
    Posts
    71
    Hi,

    This is a great forum. I have learned so much that I am actually EXCITED to spend $30K on an heating and AC system!

    Here is my situation. I have calculated my heating and cooling requirements using HVAC-Calc inputting very conservative values. The program spits out that I just need a 4 ton system for cooling. The local HVAC contrators who have NOT DONE a manual j calculation think I am crazy. "5 ton minimum" "I would add two 3 ton system" " 1 ton per 800 sqft" etc.

    So I am I wrong to assume that a 4 ton system will work?

    Here are additional details:
    rancher w/ basement
    r4 insulation on outside, r13 in walls
    west side partially shaded
    aluminium radiant barrier in ceiling under rafters (over 5,000 sqft of the stuff!!)
    latest andersen windows, only two facing north, most facing west.



  2. #2
    Join Date
    Sep 2002
    Posts
    3,708
    Yes but you left out the most important thing with your equation.

    What do you yourself deem comfortable?Like when its a 100 degree day would you like to maintain 75 degree 50% humidity.

    Just looking at the wall insulation value i would say 4 tons seen alittle lite for that many square feet.
    Opps their i go using sq feet again to answer a question on the size of the unit.

    Someone, smack me!


  3. #3
    Join Date
    Jul 2006
    Posts
    71
    Sorry here is more info:

    Summer indoor temperature 73 degrees
    total wall insulation r18 (including exterior polyiso)
    Summer gains in moisture = 100
    Summer temperature design point = 100 degree (like the temperature outside today!)
    indoor relative humidity = 50%
    5,375 total, 4,000 finished, 2700 each floor

    To be on the safe side should I just spend the extra $$ on a 5 ton system?



  4. #4
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,189
    It's really a 2700 SF ranch with walkout. Forget about the 2700 SF basement unless it's all west glass on the w/o.

    4 tons should be fine. The radiant barrier is the kicker. Your largest heat gain is being greatly reduced using the radiant barrier.

    I would recommend a zoning system on that large of a home. It will greatly increase the comfort.

    I've recently completed construction of a 1600 SF w/o plus 600SF bonus above the garage. 4 zones (3 on the ranch; 1 on the bonus; 1 on the basement). I don't have radiant in the attic. Cools great. I designed at OD 90°F / ID 75°F / 50% RH.

    Just ran a quick load with radiant and outdoor design of 100°F. ...

    2.56 tons for my home.

    I'd say go for it.
    Perhaps you should have read the instructions before calling.

  5. #5
    Join Date
    Oct 2003
    Location
    Washington
    Posts
    7,405
    If you did the load calc properly, stand by it.

    You and your furnace will both be glad you did!

  6. #6
    Join Date
    Jul 2006
    Posts
    71
    Concerning the radiant barrier it only cost $500 ($.10/sqft) for the whole house. Without any insulation and a BLACK roof you walk into the house and you are immediately cooled. My roofers and framers could not believe it.

    The HVAC people shrugged it off - "that's the first time I've seen it" or "what book did you read?" or just plain ignored it.

    HVAC Calc does not include an option for a radiant barrier so my manual j is very conservative.

    Looks like it stays a 4ton until someone SHOWS me otherwise.








  7. #7
    Join Date
    Nov 2000
    Location
    Waco, Texas, USA
    Posts
    6,153
    Maybe your load calc came out at 4 ton but what size equipment will you need to reach that goal? Keep in mind that 4 ton equipment will not deliver 4 ton results! Esp. when it gets super hot outside.
    "And remember my sentimental friend......that a heart is not judged by how much you love, but by how much you are loved by others" - Wizard of Oz.

  8. #8
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,008
    I am from Houston Tx. and if you were here I would say "you are nuts"
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  9. #9
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    Where are you? I'd say go for the 4 ton.

    Is just one side of walk out basement exposed to the weather?

  10. #10
    Join Date
    Jul 2006
    Posts
    71
    I am from fredericksburg, va. still damn hot here too!

  11. #11
    Join Date
    Jul 2006
    Posts
    71
    The basement side walls are partially underground with a full walk out condition in the rear of the house.

  12. #12
    Join Date
    May 2004
    Location
    Fayetteville TN
    Posts
    656
    I live in southern-middle Tennessee. I have 3 ton (capacity controlled TXV installed) on 5200 ft of conditiioned space (1 1/2 story, brick-nothing special). This unit has maintained 73db@48% for the 4 years I've had it. Now I'll admit that it is not as factory designed it but the equipment does not determine the load. If the load calc says 4 ton and your equipments meets the structure demand in both sensible & latent capacities, go with the load calc. (double check that you did,t miss anything in the calc)
    If You don't have time to do it right, when will you have the time to do it over?

    "Perception is Reality" Look & Act like a Professional

  13. #13
    Join Date
    May 2006
    Posts
    230
    Originally posted by downtown
    The local HVAC contrators who have NOT DONE a manual j calculation think I am crazy. "5 ton minimum" "I would add two 3 ton system" " 1 ton per 800 sqft" etc.
    So, ask them to show you their load calculations. Go over their input data to see if it is right. That should clear up any confusion. If they haven't, or won't, do a load calculation then you know enough to cross them off your short list of HVAC suppliers.

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