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  1. #1
    Join Date
    Feb 2008
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    Dallas & Longview, TX
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    629

    4 Tons for 1371 sq.' to much?

    This 4 ton unit is for the upstairs. I currently have a 4 ton unit that does cycle on 100* days but not much. There is not enough insulation in the attic and the ductwork is in the attic. The the insulation will be increased after new ducting placed. The current guestament of the ducting is around 1000 CFM but will be redesigned to handle 1400 CFM.

    I had 3 companies come out and give bids and all 3 said 4 tons. The only one that did a manual J said 3.5 Tons after insulation improvements. I bought the HVAC-Calc and used accurate inputs but did include upgrading the insulation in the attic and came out with 43,672 (39,548 sensible) BTU's heat gain.

    This is the question: The window load is 22,933 with no shading internal or external except for the South facing windows that are shaded by a large overhanging porch. I never leave all the window shades open in the summer when the sun is shinning directly in so is it necessary to use the recommended "no internal shading" for the windows? It would lower the gain alot to label them shaded. The total gain would then be 35,983 (sensible 31,859).

    If the average system has 70% sensible capacity the I'd still be at 4 tons with the shades closed.

    This area is bedrooms and bathrooms. The hottest part of the day no one will be up there.

    I am looking into the posibility of installing a 4 Ton zoned system that ties into the downstairs (currently a 4 ton package unit w/ only electric heat $$$). Using the unit to cool the upstairs at night by setting the downstairs stat up 10* then setting the upstairs stat back 10* to cool the downstairs during the day.

    The house was built in 1953 w/o AC or heat so it's not easy to do anything but the complexity of extra ducting to zone it may not be feasable.

    Thoughts?

  2. #2
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    Jan 2008
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    4 tons should cool that square footage with the windows open.
    Seriously, that seems a bit high. Mabe that's what's needed, but my old, so-so insulated, 1300sqft house, is cooled with 2.5 tons. And 2 tons would most likely do the job here.
    "Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler

  3. #3
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    5,012
    I can't help it, I am still laughing.

    First, do something that is cost effective about the windows, there are numerous approaches that will change those load factors.

    Second, the new foam insulation is safe & very effective, it also is effective reducing air infiltration.

    Third, radiant barriers work.
    After you have done everything, - do manuals' J, S, & D.

    The 1000-cfm on a 4-ton with possible other duct problems, the delivered BTUH may have been 1.5 or less than 2-ton.(?)

    It is difficult for me to believe that yo would need more than a 2.5-ton system. However, you make the decisions with your local contractor.

    I cool perfectly even in 104-F Heat Index a 1937 farm home near 900sf first floor with a little Half-Ton 6,000-BTUH, all electric home except for oil heating that cost me a fortune. Electric bill 40 to $45 a month the year round.
    - udarrell

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,366
    How open is the first floor to the second floor.

    More attic insulation and RB may allow you to drop to a 3 ton unit.
    What ID temp are you using for design.

    Some people for get, that at 100 degree outdoor temp, a 4 ton unit loses capacity.
    Plus, if your trying to maintain 70° indoor temp, a 4 ton unit will lose between 11,000 and 14,000 BTUs of sensible capacity.
    So at 100° OD temp, and 70° ID temp, a 4 ton unit may only have 26000 BTU's sensible capacity.

    What infiltration rate did you use.
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  5. #5
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    14,915
    I'm really skeptical of the 3.5 and 4 ton loads for that space...
    If, for whatever reason, it really does work out to that big of a load, you really need to do some work on the house to reduce that load.

    My whole house is about the size of your upstairs, built in 1966, has single pane aluminum windows, and only about 3" of insulation in the attic, but has decent window shading.

    The 3 ton system that is currently in the house cycles off regularly when it is 100+ outside, and cools the house from my 85º set back temp down to 78º in about 3 hours on a 104º day.

    My house actually needs a 2 ton system and some more insulation.
    If more government is the answer, then it's a really stupid question.

  6. #6
    Join Date
    Feb 2008
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    Dallas & Longview, TX
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    Quote Originally Posted by 2old2rock View Post
    4 tons should cool that square footage with the windows open.
    Seriously, that seems a bit high. Mabe that's what's needed, but my old, so-so insulated, 1300sqft house, is cooled with 2.5 tons. And 2 tons would most likely do the job here.
    I appreciate the reply but don't feel comfortable using rule of thumb guidance.

    Quote Originally Posted by udarrell View Post
    I can't help it, I am still laughing.

    First, do something that is cost effective about the windows, there are numerous approaches that will change those load factors.

    Second, the new foam insulation is safe & very effective, it also is effective reducing air infiltration.

    Third, radiant barriers work.
    After you have done everything, - do manuals' J, S, & D.

    The 1000-cfm on a 4-ton with possible other duct problems, the delivered BTUH may have been 1.5 or less than 2-ton.(?)

    It is difficult for me to believe that yo would need more than a 2.5-ton system. However, you make the decisions with your local contractor.

    I cool perfectly even in 104-F Heat Index a 1937 farm home near 900sf first floor with a little Half-Ton 6,000-BTUH, all electric home except for oil heating that cost me a fortune. Electric bill 40 to $45 a month the year round.
    - udarrell
    Your site lead me to my quest for getting my home properly conditioned so I start with thanking you for that.

    Regarding the windows- There are 348.5 sq. feet of windows. What cost effective way would there be to reduce the load other than closing the shades?
    The insulation- foaming the upstairs walls may be affordable but that only covers 25% of the house. The attic is a strange construction and although I will get bids on both foam, RB and more venting, I will be increasing the R value and that is reflected in my input in the Manual J calculation and it is only a heat gain of 1,060 BTU's.
    I'll post pics of the attic as some of you have probably seen this type construction.


    Quote Originally Posted by beenthere View Post
    How open is the first floor to the second floor.

    More attic insulation and RB may allow you to drop to a 3 ton unit.
    What ID temp are you using for design.

    Some people for get, that at 100 degree outdoor temp, a 4 ton unit loses capacity.
    Plus, if your trying to maintain 70° indoor temp, a 4 ton unit will lose between 11,000 and 14,000 BTUs of sensible capacity.
    So at 100° OD temp, and 70° ID temp, a 4 ton unit may only have 26000 BTU's sensible capacity.

    What infiltration rate did you use.
    ID design temp is 75* even though we drop it to 70* when sleeping. The current system has the return at the top of the stairs and the stat right above it so setting it to 76* drops the room temps to 70* due to the heat from the 78* downstairs rising. The opening isn't larger than the stairs so I'm guessing 6' x 8'. There is no supply in the landing at the top, only the return and stat but we keep the doors open. The new ducting will have returns in the rooms away from the landing to minimize the temp. difference between the rooms and the landing.

    Quote Originally Posted by mark beiser View Post
    I'm really skeptical of the 3.5 and 4 ton loads for that space...
    If, for whatever reason, it really does work out to that big of a load, you really need to do some work on the house to reduce that load.

    My whole house is about the size of your upstairs, built in 1966, has single pane aluminum windows, and only about 3" of insulation in the attic, but has decent window shading.

    The 3 ton system that is currently in the house cycles off regularly when it is 100+ outside, and cools the house from my 85º set back temp down to 78º in about 3 hours on a 104º day.

    My house actually needs a 2 ton system and some more insulation.
    I'm skeptical also but don't know what to do. If I go with the manual J figures and it's oversized then I loose. If assume it's wrong without know why and am wrong- I lose. I thought you guys would see the delima I see. I thought I would get a load calc. of 2.5-3 tons and get it done. When my own calcs. matched the pros it just seemed strange. Recent threads have me thinking of going with 4 tons and zoning it to the lower level that is also 4 tons. This would reduce my total capacity from 8 to 4 tons on this part of the house and this is the part we are in 95% of the time.

    Thanks to all and anymore advice is greatly appreciated!

  7. #7
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    This is the manual J calc. with blinds closed. When sleeping the blinds will be closed but during the hottest part of the day only the ones with sun shining directly in will be closed.

    If there is another report that would help, let me know and thanks again.
    Attached Images Attached Images

  8. #8
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    Did you deduct the window sq ft from the wall sq ft.
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  9. #9
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    Quote Originally Posted by beenthere View Post
    Did you deduct the window sq ft from the wall sq ft.
    No I didn't. With that much window sq footage it will probably make a noticable difference. I'll post back after redoing it.

  10. #10
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    Feb 2008
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    Dallas & Longview, TX
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    New calculation with window sq footage subtracted from wall area:

    With all blinds closed:
    Total gain: 33,501
    Sensible: 29,377
    Latent: 4,124

    With some blinds open like it is now during the day:
    Total gain: 40,027
    Sensible: 35,903
    Latent: 4,124

    This doesn't even include the heat rising from the downstairs.

  11. #11
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    But you see how one detail can change the load by a fair amount.

    I would double check everything again.

    Also, did you say your current 4 ton cycles on 100 degree days.
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  12. #12
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    Yes it does cycle on 100* days. Set point raised to 78* during the day and approx. 30min. on and 10 off. This is with a leaky supply plenum - duct board taped together. It keeps the bedrooms @ 75* at this setting.

  13. #13
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    That indicates that the current 4 ton is over sized.
    If the supply plenum is leaking, then your pulling in unconditioned air from somewhere to make up for it.

    Plus, if your duct system is as undersized as you indicated, your current 4 ton, isn't running at 4 tons of capacity.
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