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  1. #1

    Novice with general new construction questions

    I am certainly a novice when it comes to HVAC systems and certainly don't know the proper terminology. I have a few basic questions about the location of the inside units, duct work, and returns. I live in central Texas and am building a 2 story 5200 square foot home with low E windows/doors and spray-in foam insulation throughout, including the attic. The units (3 3-ton units) I'm having are: Carrier Heat Pumps, 16.00+ SEER, high efficiency with 2-stage variable speed air handlers. All of the units will be in the attic but the location in the attic is my main question. In a well insulated home with the above units, what is the maximum distance a room being supplied by one of the units should be (maximum length of duct work) before you start losing significant efficiency (decreased air flow to the room with loss in coolness)? Basically, how important is it to have the main unit central to the rooms it's providing cool air to so the duct work is shorter versus the unit being farther away with longer duct work? I'm sure there are a lot of variables but maybe some general estimates or recommendations would help. Thanks

  2. #2
    Join Date
    Jan 2004
    Location
    PA
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    68,757
    Not as important as getting the equipment sized right. A spray foamed house shouldn't need 1 ton per 577 sq ft. get a load calc done by someone that is familiar with spray foamed houses.
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    How many times must one fix something before it is fixed?

  3. #3
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,744
    If it were my home, I would try to keep the units in the conditioned space. I would put the 1st floor system on the first floor (somewhere) utility room or build a room in the garage. There are building techniques that will allow you to run the ductwork in the floor trusses (for the first floor system). The second floor system(s) ductwork can go in the attic if need be. I just like to be able to see the air handler for easier service access and prevent it from sweating.

    I agree with B.T., 9 tons seems a little much. I know it gets warm in TX but I've seen new 2-story homes in IA with 2700 sq. ft. cooling with 3 tons without foam.

  4. #4
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
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    5,062
    Quote Originally Posted by George2 View Post
    If it were my home, I would try to keep the units in the conditioned space. I would put the 1st floor system on the first floor (somewhere) utility room or build a room in the garage. There are building techniques that will allow you to run the ductwork in the floor trusses (for the first floor system). The second floor system(s) ductwork can go in the attic if need be. I just like to be able to see the air handler for easier service access and prevent it from sweating.

    I agree with B.T., 9 tons seems a little much. I know it gets warm in TX but I've seen new 2-story homes in IA with 2700 sq. ft. cooling with 3 tons without foam.
    I agree wayyyyyyy over sized = short cycles, high humidity, extra cost etc.

    A proper design with manual D,J,and S is needed

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,428
    I would set Thermal Building Envelop Specs and building inspection to achieve < 5 tons Total capacity.

    http://www.sprayfoam.org/news/index....le_view&id=556
    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
    Nov 2004
    Location
    SW FL
    Posts
    6,428
    I come up with two 2-tons for a 5,200 Sq feet 2-story residence
    when using
    104'F Outside Air

    S.H.G.C. window < or = 0.35, U-value = 0.4
    R-28 ceiling spray foam (on roof deck .:. ducts in conditioned space),
    R-14 walls,
    semi-tight (ACH ~ 0.25) infiltration,
    ducts in conditioned space,
    6 people,
    2,400 BTU internal gain
    per ABRIDGED Manual J8 worksheet - ATTACHMENTS

    Duct in conditioned space saves about 14,000 BTU/Hr
    Tinted Window Heat Gain for 700 Square Feet (14% of floor area) is 25,000 BTU/Hr less than Single, Clear Windows.

    This assumed Window area may be more than what is going to be used in this residence.

    Maybe someone can suggest where this heat gain analysis could possibly
    be considered undersizing for a premier thermal building envelop.
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    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

  7. #7
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by George2 View Post
    If it were my home, I would try to keep the units in the conditioned space. I would put the 1st floor system on the first floor (somewhere) utility room or build a room in the garage. There are building techniques that will allow you to run the ductwork in the floor trusses (for the first floor system). The second floor system(s) ductwork can go in the attic if need be. I just like to be able to see the air handler for easier service access and prevent it from sweating.

    I agree with B.T., 9 tons seems a little much. I know it gets warm in TX but I've seen new 2-story homes in IA with 2700 sq. ft. cooling with 3 tons without foam.
    3200sqft with 4 tons on my home... and if I had a single system it would be 3 tons with at least 6 zones. My home is 88 years old and not foamed. Its' all in the little details. Larger home have more mass and don't see the peak heating and cooling loads in extreme weather. High temrpatures only last part of a day. Larger homes take a long time to heat up and cool off. On a 40F day, 1st stage on a 2 ton heat pump will hold temperature downstairs including some heat going to a full unfinished basement. On a 85F sunny afternoon, 1st stage runs only about 1/4 of the time downstairs to cool it.

    You build a house right, it will really amaze you.

    No reason you can't get under 5 tons, if not down to 4 tons. Thinks like a sealed attic and cool roof go a long way.

    That is another option, place the unit in the attic, but spray foam the roof deck and use steel roofing and radiant barrier and insulation on top of the roof deck to create a cool roof using steel roofing. Make sure a vapor barrier is used on the roof deck.

    If possible, spend a little extra on a larger roof and create deepr overhangs. Many new homes only have barely 12" eaves. 2' should be a minimum, 3' even better. Ideally, thte upstairs is almost completely shaded for at least 8 hours a day in summer, but recieves more sun in winter.

  8. #8
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,428
    Design Condition: 96 Days over 90'F per this link
    http://www.climate-zone.com/climate/...as-fort-worth/

    I believe recent years in TEXAS areas have seen > 60 days > 100'F.

    ASHRAE 1% 92 to 102'F across th state.
    ASHRAE 0.4% is 100.3'F in Dallas & 104.2'F at Laughlin AFB
    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

  9. #9
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Even more reason to focus on things like cool roofs and radiant barriers. Its' really hot.

    If costs are a concern, use fewer windows, but make each one larger to keep the same window area. It will also propotionally reduce air leakage along with using more fixed windows like picture windows, transomes and sidelights. But keep them all low E. Operable windows are a big source of air leaks, and in a hot humid climate in a moderns home, there's limited days in winter where you'll likely use natural ventilation, And honestly, it doesn't save you that much because 1) you let humidity indoors in that you'll have to remove later when it heats up, and 2) AC systems are pretty cheap to operate in cooler conditions anyway. <65F a high effceincy, 2 ton, 2 stage unit on 1st stage uses less than 1000Watts.

  10. #10
    Thank you all for responding. Here's a little more information about the house, climate, and my concerns. The roof is going to be cement tiles (a mediterranean clay tile look). The temps where I live have stayed over 100 degrees for more than 100 days out of the year. My biggest concern is not adequately cooling the house. It sounds like the 9 tons may be a little much but I don't mind paying more for a comfortable home during our long summers. Is there any harm in having a "larger" system (9 ton total vs. 4-6)? Also, the house is already framed and the roof is going on now so there can be no alterations in placement of the units...they will all be in the foam insulated attic. However, one of my main question is how close to the rooms a unit is providing does the unit need to be or does it even matter (is having more/longer duct work worse)? Should they be in the attic area over the main portions of the house close to the rooms or over the attic in the garage with longer duct work traveling to the rooms? Thanks again

  11. #11
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,428
    Quote Originally Posted by loganparker View Post
    Thank you all for responding. Here's a little more information about the house, climate, and my concerns.

    The roof is going to be cement tiles (a mediterranean clay tile look).
    The temps where I live have stayed over 100 degrees for more than 100 days out of the year.

    My biggest concern is not adequately cooling the house. It sounds like the 9 tons may be a little much but I don't mind paying more for a comfortable home during our long summers.
    Is there any harm in having a "larger" system (9 ton total vs. 4-6)?

    Also, the house is already framed and the roof is going on now so there can be no alterations in placement of the units...they will all be in the foam insulated attic.

    However, one of my main question is how close to the rooms a unit is providing does the unit need to be or does it even matter (is having more/longer duct work worse)?
    Should they be in the attic area over the main portions of the house close to the rooms or over the attic in the garage with longer duct work traveling to the rooms?

    Thanks again
    My Biggest concern is not defining and adhering to specs for the Thermal Building Envelop.

    I cannot see Your Specific duct plans developed in accordance with ACCA Manuals J & D.

    Flexible duct Length SHALL BE LIMITED to 10 feet ( or it ain't gonna work too well).

    Of course, you may experiment and find out yourself.
    Then you can have an experienced mechanical contractor not beholden to the builder
    fix it for $$,$$$
    after a couple years of frustration.

    Now when & where did you say the layout and Manual D calculations would be
    provided and independently reviewed?

    Certainly with 9-tons
    one has about 30 times the chance of not doing the ductwork adequately
    than with one 2 ton and one 3 ton system.
    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
    Thanks Dan.

    Also, no basement in the house (most homes in this part of don't have a basement). So, since my home is being built by a builder who has his own subcontractor for the HVAC work, what would you suggest I ask to see from the subcontractor and/or builder to verify that what is being proposed is adequate/necessary. I've been told already that the recommendations are based on the house structure/components (# of windoes, door, insulation, etc.). Thanks

  13. #13
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,428
    Quote Originally Posted by loganparker View Post
    Thanks Dan.

    Also, no basement in the house (most homes in this part of don't have a basement). So, since my home is being built by a builder who has his own subcontractor for the HVAC work, what would you suggest I ask to see from the subcontractor and/or builder to verify that what is being proposed is adequate/necessary. I've been told already that the recommendations are based on the house structure/components (# of windoes, door, insulation, etc.). Thanks
    1. ACCA MANUAL J Calc
    2. ACCA Manual D Calc
    3. Duct Layout

    ALL SHEETS OF EACH.

    4. Architectural drawings
    5. Window specs
    6. Insulation spec
    Last edited by dan sw fl; 05-03-2013 at 03:52 PM. Reason: trust BUT VERIFY USUALLY
    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

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