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  1. #1

    Confused

    I'm building a new 3400SF w/ additional 500SF bonus room 2-story home in NE Ohio. I have done the load calcs myself, both with an online version and with the HVAC-Calc (homeowner's edition) program I purchased online, and I am puzzled between what I've computed, what I'd expect, and what contractors are bidding.

    By the HVAC-Calc program, for the almost 4000SF home, I have come up with a total heat gain of 43,609 BTUH (3.5 ton) and a heat loss of 96,574 BTUH using "average infiltration", 95/5 outdoor temp specs w/ 94 grains of moisture, and 75/72 indoor temps with a 50% RH. (Cleveland, OH)

    I was planning on using spray-foam insulation on the 2x4 constructed walls, as well as the ceiling, so when I change from "average infiltration" to "tight, new construction", my heat gain drops to 38,207 BTUH (3 tons). Again, this is for about 4000 SF! As far as I can see, I've entered all the correct info for windows sizes, low-e glass, fireplace, square footage, directional info, and everything.

    I'm trying to find out if I've done something wrong, because of the 4 contractors I've had bid the job, 3 have insisted on 5 tons, and 2 have said that a 2-unit system (2.5/3 ton) is REQUIRED for the size of the house. One guy came in at 4 tons (he had the Whirlpool rep do the calculation), but I'm not so sure the other guys have even done the actual calculation (one mentioned the old "rule of thumb" of 1 ton per 800SF...). As a matter of fact, the ones who quoted 5 tons said they'd have me sign off on a release stating that they wouldn't be responsible if I chose to go with a lower tonnage unit.

    I was planning on having zone dampers for up/down with a single unit located in the basement. I just want to make sure that what I'm describing (3 to 3 1/2 tons) isn't too crazy sounding for the size of the house, and if I were to have to increase the size, if 4 tons would be a happy medium.

    As a side note, I'm wondering about anyone's experience with Heil A/C and furnaces, as well as the new-comer Whirlpool.

    Thanks,
    OH

  2. #2
    Join Date
    Nov 2000
    Location
    Waco, Texas, USA
    Posts
    6,153
    You have only done the LOAD. There have been no calculations done for the EQUIPMENT SELECTION.

    You might need to select a 5 ton unit to deliver 3.5 ton performance at a certain outdoor ambient temp. for example.....just an example.
    "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.

  3. #3
    Join Date
    Apr 2002
    Posts
    11,808
    perhaps contact customer support at hvac computers who sold you the software.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,097
    The program tells you how many btu's you need, not what size unit will deliver those btu's at your design conditions.


    What was teh sensible load, and the latent load the calc said you have.
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  5. #5
    Join Date
    Sep 2004
    Location
    Memphis
    Posts
    2,502
    I glanced at some Rheem specs & it'd take about 5 1/2 tons to handle 43,000 btu's. Be sure to add a safety factor into your calculation too.
    Life is like a jar of jalapenos. What you do today might burn your ass tomorrow.

  6. #6
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,179
    A friend of mine is a foamer. He did a remodel job and had an outside firm do the heat gain. I'll get their name for you, I think I have it at work. Anyway they called for less equipment than the local HVAC dealer thought they should have. So they went with what the dealer wanted to put in. Guess what? Having humidity issues due to the oversized equipment. I saw an e-mail from the HO to my friend saying "I should have listened to you".

    I don't know if 3 tons for that big house is right or not. Let me find this firm's name, might be worth checking with. They do charge for their service but guarantee your comfort and what your costs will be.

    Whirlpool is Armstrong equipment. Middle of the road in my opinion. Heil makes a nicer outdoor unit, probably similar on the furnaces.

  7. #7
    Join Date
    Aug 2006
    Location
    Rifle,Colorado
    Posts
    10
    I agree with BaldLoonie, with all the variables in todays construction when I was in the industry I had a outside firm do all my heat load calculations. I always had happy custamers so it was worth the cost.

  8. #8
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    What is the load breakdown (sensible and latent)? Manual S says you should size your AC to the sensible load and just make sure it will cover the latent load.

    I just took a quick look at some Lennox data. A 3.5 ton builder grade Lennox 13 SEER has a capacity of about 33,000 Btuh sensible and 8,000 Btuh latent at 400 CFM per ton at 95F outside, 75F inside, and 50% RH inside.

    Proper installation and charging is very important. Make sure your system has a TXV.

    [Edited by Panama on 08-09-2006 at 10:34 PM]

  9. #9
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295
    [QUOTE]Originally posted by ohioac123
    [B]I'm building a new 3400SF w/ additional 500SF bonus room 2-story home in NE Ohio.

    95/5 outdoor temp specs w/ 94 grains of moisture, and 75/72 indoor temps with a 50% RH. (Cleveland, OH)

    I was planning on using spray-foam insulation on the 2x4 constructed walls, as well as the ceiling, so when I change from "average infiltration" to "tight, new construction", my heat gain drops to 38,207 BTUH (3 tons). Again, this is for about 4000 SF!

    As far as I can see, I've entered all the correct info for windows sizes, low-e glass, fireplace, square footage, directional info, and everything.

    I'm trying to find out if I've done something wrong, because of the 4 contractors I've had bid the job, 3 have insisted on 5 tons, and 2 have said that a 2-unit system (2.5/3 ton) is REQUIRED for the size of the house.

    I just want to make sure that what I'm describing (3 to 3 1/2 tons) isn't too crazy sounding for the size of the house, and if I were to have to increase the size, if 4 tons would be a happy medium.

    [QUOTE]

    You likely did not do much of anything wrong
    with the first of 4 steps, except ___ ...

    What are the losses due to the duct?

    Does the infiltration meet the I.A.Q. requirements?

    1. Load Analysis (manual J )
    2. Coil and Condenser Selection ( manual S )
    3. De-rating
    4. Fan Selection
    5. Duct layout and sizing ( Manual D )
    6. Diffuser layout and sizing

    http://www.accaconference.com/Mercha...duct_Code=29-D
    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

  10. #10
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    16,002
    Just call me a dumb Texan but what is a bonus room?
    __________________________________________________ _______________________
    “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

  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295
    Originally posted by mrbillpro
    Just call me a dumb Texan but what is a bonus room?
    A LONG WAIT line room

    Go to the Bonus room to pick up that extra check.
    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

  12. #12
    Join Date
    Jul 2006
    Posts
    169

    Lightbulb

    remeber this "variable speed blower and proper duct sizing. txv also" if ya dont spend it now, you will spend it later. 80% of units in az are oversized and 75% of duct is undersized. the local utility company did a study on it. check out aps or the az heat pump concil. or the national comfort institute. nci.

  13. #13
    OK, I just ran over my calcs again, and realized that I had calculated the wrong SF, since the plans changed. The actual conditioned SF of the house, including the bonus room, but not the basement, is 3,555 SF. This correct number was used in the heat load calcs, but my original statement that the house was 4000 SF was wrong.

    Here are the results including sensible gain:

    For spray foam insulation:
    Sensible Gain: 33,308 BTUH
    Latent Gain: 4,899 BTUH
    Total Gain: 38,207 BTUH (3 tons)
    Total Loss: 77,794 BTUH

    For standard "average" insulation/infiltration:
    Sensible Gain: 36,111 BTUH
    Latent Gain: 7,498 BTUH
    Total Gain: 43,609 BTUH (3.5 tons)
    Total Loss: 96,574 BTUH

    Calcs are based on basement, 1st, and 2nd floors being conditioned, so the ductwork isn't considered as a heat loss/gain factor.

    Basement: 1450 SF
    1st Floor: 1674 SF
    2nd Floor: 1881 SF
    1st & 2nd SF total: 3,555 SF

    For spray foam, here are the results (sorry that the columns don't line up):

    Component, SensibleGain, LatentGain, TotalHeatGain, TotalHeatLoss

    Windows 11,261 0 11,261 9,435
    Walls 7,878 0 7,878 34,144
    Infiltration 2,803 2,599 5,402 14,084
    People 3,000 2,300 5,300 0
    Ceilings 4,183 0 4,183 6,368
    Glassdoors 1,794 0 1,794 2,880
    Misc 1,200 0 1,200 0
    Doors 855 0 855 2,424
    Floors 334 0 334 3,764
    Skylights 0 0 0 0
    Fireplaces 0 0 0 4,695
    Duct 0 0 0 0

    Whole House 33,308 4,899 38,207 77,794
    (3 tons)

    When I use Average infiltration, everything else remains the same, except:
    Infiltration 5,606 5,198 10,804 32,864

    Whole House 36,111 7,498 43,609 96,574
    (3.5 tons)


    Actually, I had already called the software maker and asked him to review my specs. He did, and the only suggestion he had was to use "average" infiltration (even though I'm using foam). His reference manual DID state that there is no "safety factor" included in the calculation, but I would consider a jump from 3 to 5 tons too big, and possibly detrimental.


    I did do a CFM analysis, but I'm not sure how that works. It asks for the size air handler chosen for the house, and I'm not sure how to do that. I selected 1200, because I think that's what was recommended by the program, and it did it's thing, but I'm at a loss for what it means.

    The installer that I spoke with yesterday (Heil) said that he felt a 5 ton blower would be required to push the air to the whole house. I'm a little confused with the terminology, though, since I thought blowers were based on CFM and not tonnage (BTUH). The house is 74'x33', with the "bonus room" above the garage.

    Does this shed any light on my situation?
    Thanks for all the replies!



    By the way, from what I'm aware, "Bonus Room" is a realtor-coined term used to describe a room in a house that can't really be considered a bedroom (maybe floor joists are spread too far apart for a conventional load, or the ceiling is angled, etc.), and that previously couldn't legally be considered as a bedroom of the finished home. It may or may not be considered part of the finished square footage, but a realtor certainly wants to point out that somone *might* be able to use the room. It may have been previously known as "storage" or "additional room" or even "useable heated attic space". Maybe it's an Ohio thing.

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