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  1. #1
    Join Date
    Aug 2009
    Posts
    5

    Confused Sanity check - sizing

    Hi all

    I've been talking to some air condition installers and I cannot understand why they want to put in so heavy units.

    My house, which is not yet constructed, is a 4 floor building each about 600 sq feet.
    First floor has kitchen and garage. The garage is "outside"
    Second floor has a small office, bathroom and the living room, which is totally open to the hallway
    Third floor has two bedrooms and a bathroom.
    Fourth floor is identical to the third floor.
    Fifth floor only has an exit to the roof.
    The southern part of the roof is flat.
    The northern part is covered with a roof.
    There are only windows on the south side wall.
    The north side gets its light from a 10 feet x 8 feet skylight going down all floors.
    I live in Ho Chi Minh, Vietnam. Climate info

    I estimated that I needed about 75CFM fresh air per floor, which is done via a heat exchange unit on the fifth floor.

    Trane's first suggestion was 125.000 BTU for the whole house.
    Trane's second and rather more detailed suggestion wants a 24.000 BTU unit on each floor (96.000 BTU in total) and do 600CFM whole house ventilation with about 200CFM being exchanged with fresh air.

    Reetech wants to put a 24.000 BTU, a 32.000 BTU, 3 x 12.000 BTU, and 2 x 18.000 BTU units in, totaling 128.000 BTU.

    LG I just talked to on the phone, but they also quoted around 120.000 BTU.

    If I try to use the HVAC-calc residential 4.0 I end up at just 43.000 BTU for the whole house with no insulation and only 26.000 BTU with minimal insulation. I attached the reports to the post.

    Can someone help me determine what is the right answer?

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,998

    Thumbs up HVAC CALC - Quick Sanity Check

    Quote Originally Posted by davh View Post
    My house, which is not yet constructed, is a 4 floor building each about 600 sq feet.

    First floor has kitchen and garage. The garage is "outside"
    Second floor has a small office, bathroom and the living room, which is totally open to the hallway
    Third floor has two bedrooms and a bathroom.
    Fourth floor is identical to the third floor.
    Fifth floor only has an exit to the roof.

    The southern part of the roof is flat.
    The northern part is covered with a roof.
    There are only windows on the south side wall.
    The north side gets its light from a 10 feet x 8 feet skylight going down all floors.
    I live in Ho Chi Minh, Vietnam.

    I estimated that I needed about 75CFM fresh air per floor, which is done via a heat exchange unit on the fifth floor.

    and do 600CFM whole house ventilation with about 200CFM being exchanged with fresh air.

    If I try to use the HVAC-calc residential 4.0 I end up at just 43.000 BTU for the whole house with no insulation and only 26.000 BTU with minimal insulation. I attached the reports to the post.

    Can someone help me determine what is the right answer?

    90'F for summer temperature might be a little low for design.

    Two small A/C units
    WAG for moderate insulation:

    1.5 TON (18,000 BTU ) for 1st and 2ND Floors combined
    2.0 TON (24,000 BTU ) for 3rd and 4TH floors combined.

    You need to KNOW the actual construction.

    Outside Air is _ EXCESSIVE _ by nearly every measure.
    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

  3. #3
    Join Date
    Aug 2009
    Posts
    5
    Thanks Dan.

    What do you suggest for fresh air?
    About the 90deg, I did a double check at 105, and not really that much difference.

    I am not quite sure what you mean by "You need to KNOW the actual construction."
    The air con guys were all given the dwg cad drawing from the architect.
    I cannot attach it though dwg files are not allowed, nor is zip, and the PDF version is too big.

  4. #4
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,998

    Thumbs up Fresh Air max 100 CFM

    Quote Originally Posted by davh View Post
    Thanks Dan.

    What do you suggest for fresh air?
    About the 90 deg, I did a double check at 105, and not really that much difference.

    I am not quite sure what you mean by "You need to KNOW the actual construction."

    The air con guys were all given the dwg cad drawing from the architect.
    I cannot attach it though dwg files are not allowed, nor is zip, and the PDF version is too big.

    ASHRAE 62.2 provides the guidelines for fresh air in residences.

    I believe that 80 CFM is sufficient for 2,300 sq. foot 2-bedroom residence.
    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

  5. #5
    Join Date
    Aug 2009
    Posts
    5
    Thanks again Dan

    I found out that the ASHREA weather database is totally wrong for HCMC.
    It should be
    Winter: DB 34 deg celcius, WB 29.5 deg celcius (72% RH)
    Summer: DB 32 deg celcius, WB 29.2 deg celcius (81% RH)

    With this I get around 65.000 BTU, with 120CFM ventilation.
    or 70.000 BTU with 135 CFM ventilations.

    It is a 4 bedroom house.

    I did get the load calculations from Trane, but he is also using the bad weather data, where the RH is only 53%. And the calculation didn't seem to include any information about the materials that the house was constructed from, so don't quite trust them.

  6. #6
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,998

    Question Inconsistency leads to unbelieve ! !! !!!

    Quote Originally Posted by davh View Post
    Thanks again Dan

    I found out that the ASHREA weather database is totally wrong for HCMC.
    It should be
    Winter: DB 34 deg celcius, WB 29.5 deg celcius (72% RH)
    Summer: DB 32 deg celcius, WB 29.2 deg celcius (81% RH)

    With this I get around 65.000 BTU, with 120CFM ventilation.
    or 70.000 BTU with 135 CFM ventilations.

    It is a 4 bedroom house.

    I did get the load calculations from Trane, but he is also using the bad weather data, where the RH is only 53%. And the calculation didn't seem to include any information about the materials that the house was constructed from, so don't quite trust them.
    26,000 first post, 65,000 BTU here .. at 90'F plus you 'double checked' it at 105'F and found very little difference.

    How could AnyOne trust Your calc, if you can have such VAST DifferenceS !! !!!?.

    My WAG would be 43,000 BTUh Sensible _ ? 7,000 __ ? Latent which is not close to either of your numbers!
    BUT just might the the best target for a cooling load at 93'F.



    Also, you use more than 50% over the 80 CFM outside air.
    Do you Know your house is a big leaker?
    Last edited by dan sw fl; 09-19-2009 at 02:02 PM.
    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
    Aug 2009
    Posts
    5
    Sorry.

    The sensible + latent is still around the original number, but it adjusts the recommended up to 70k to get to 75% sensible load.

    It doesn't seem like the various load calculation programs like extreme humidity much, there is a ton difference between 79% and 81% in recommended load, and around 0.05 ton difference in the actual sensible + latent.

  8. #8
    Join Date
    Sep 2005
    Location
    Texas
    Posts
    2,793
    you will want to use a whole house dehumidifier rather than counting on the a/c equipment to do it. do you have your house plans on PDF?

  9. #9
    Join Date
    May 2000
    Location
    Monmouth Junction-NJ-USA
    Posts
    5,967
    I'm not going to run a load calc on the dwelling but you have provided some pretty high wb temps there. RH is s a big factor there.
    If you really know how it works, you have an execellent chance of fixin' er up!

    Tomorrow is promised to no one...

  10. #10
    Join Date
    Aug 2009
    Posts
    5
    Yeah.
    This one has Tranes suggestion on it.
    It is in vietnamese and metric units though.
    sheet Model = top floor/roof
    sheet 2 = floor 3
    sheet 3 = floor 1
    sheet 4 = floor 2
    sheet 5 = floor 4
    Attached Images Attached Images
    Last edited by davh; 09-19-2009 at 02:09 PM. Reason: spelling

  11. #11
    Join Date
    Sep 2005
    Location
    Texas
    Posts
    2,793
    this design has ductless mini-splits written all over it...I would look at Daikin.Inverter technology is coming fast.

  12. #12
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,998

    Thumbs up Perfect fit for mini-split

    Quote Originally Posted by air2spare View Post
    this design has ductless mini-splits written all over it...I would look at Daikin. Inverter technology is coming fast.
    Air2spare is the first to state the obvious for this Unique residence.

    Sanyo, Mitsubishi and others offer mini-splits that will handle this 5-story residence.

    Inverter technoolgy is anything but new.

    http://www.mrslim.com/Products/Categ...ctCategoryID=5

    Mr Slim and City-Multi should be applied by a professional designer to determine the best fit.
    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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