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  1. #1
    Join Date
    Dec 2019
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    Cool Finding Duct Designer?

    This is a bit of an odd situation and I am baffled at what I have found in the industry in my area. So perhaps I am the 'odd ball out" or the there is something going on with the contractors in my area?

    I have concluded that I need a professional (HVAC Tech or Engineer) to do a detailed evaluation of the ducts in my house and create a design or do the best optimization possible. However, I can not find anyone who is capable of doing it! That in spite of the fact that I am willing to PAY FOR IT to boot!

    Most of the companies that have come to look at and estimate replacing my 1955 lowboy Armstrong Oil Furnace (last tested at 67% efficiency ~15 years ago!), with a high efficiency gas unit (plus new condenser/compressor) have reported that return air flow is inadequate and they would increase the size of the main trunk and that is it. Then, if it doesn't work, we'll come back and fix it later. This gives the impression (rightly or wrongly) as though few contractors employ anyone with technical training in duct design and only have time to run out salesmen (no women in this industry?).

    I can't imagine that anyone would take their car to a shop where the mechanic says, well I'll fix this part and if it doesn't work, bring it back and I'll replace another part! No, a thoughtful mechanic might use an OBDII tool to discover the problem and fix it from there! I know there are air flow and pressure meter/gages available that could be used and contractors have hinted at their use AFTER a new system fails, but not before!

    I have run 8 contractors through the house and have quotes for HVAC systems and all say they will do "something" but no one has been specific about exactly what. (unfortunately that doesn't wash with me. My experience is a clearly defined scope of work is the best approach for project success!) I have called the Mechanical Engineering Chair at the local college and they don't have an HVAC program. I tried the technical college and the instructor there referred me to a guy, who referred me to another guy and so forth. That got a couple of dudes to come through to look at it but no one wants to take on the job. Or they just are not qualified and don't want to take the liability?

    One contractor proposed cutting a 24-in square hole in LR floor to increased return air flow, since that is the closest duct to the furnace. That looks like a band-aid approach similar to cutting a register into the return plenum. It will certainly get air flow and keep the coil from freezing, but it will not improve the cooling of the second floor! Why would I want to suck cold air off of the 1st floor floor instead of hot air off of the ceiling of the second floor?

    The suggestion to increase the return trunk size certainly makes sense initially, however, without proving there is enough return capacity, enough branch capacity, a larger trunk doesn't seem like the right fix. I guess that might encourage additional leakage as undersized branch system tries to fulfill the return air needs?

    So through all of this I have come to a bit of an education and am dangerous because I only know a few 'rules of thumb' and not enough of the engineering (like how/where static is likely to change & need to be considered) to know what I don't know.

    Here is what I do know:
    • 2-story over full unfinished basement house, concrete block with full brick face, furring strips nailed to block, plaster board on that, no insulation, built 1955, blower door test 3950 @ 50 pa, 2,100 ft2 nominal (42x25-ft exterior). Zip code 19810.
    • I have drawings of ducts in basement with measurements, measurements of rooms, measurements of return register grills, etc.
    • Original ducts (aluminum) were designed & installed when AC meant open the windows & run the whole house fan. Then at some point 'add-on' supply ducts (galvanized) were installed for AC in the ceiling of 2nd floor rooms. Old vertical coil housing remains (no coil) and now an A-coil sits in galvanized plenum)
    • Reported/Proposed A/C size is 3.5 to or 4.0 ton. Translates to air flow need of 1400 / 1600 CFM (400 cfm per ton of AC capacity - rule of thumb)
    • Return duct at furnace is 8 x 18 -in. Reportedly capable of 800 CFM so woefully inadequate.
    • - 4 panned joist bays serve as ducts to 4 return registers for 1st floor, reportedly 700 CFM each bay, so nominally enough flow (2,800 cfm) from those. However the bottleneck appears to be the "duct size" behind the baseboard registers - one concrete block removed so roughly 14 x 4 or 200 cfm each. That is assuming the grills which have only 30-in2 is adequate for 200 cfm flow (doesn't appear to be)
    • - Similar set up for the 2nd floor, however (other than register open area) the bottle neck appears to be that the returns drop down through 2x4 stud bays (14.5 x 3.5-in). Can't find exactly the CFM capabilities at 0.1 static but think it is somewhere between 160 cfm and 200 cfm each. For 4 bedrooms, there are just 3 return joist bays, so at max 600 cfm or at worst only 480 cfm. No surprise then that the 2nd floor is difficult to cool.
    • - So total return capabilities (register open space not considered) appears to be 800 cfm (1st floor) and 480 cfm (2nd floor) for a total of 1280 cfm at best. System appears it will be starved as a result of inadequate branch capacities. Therefore, increasing the main trunk appears to provides trunk capability but does not fix branch capabilities.


    I have identified 4 places where I believe additional return air flow could be provided with minimal interior damage. All would be in stud bays and 3 from the second floor and one from 1st floor. But how much of that is needed?

    Anyway, the bottom line is that I believe the best approach is a thorough review of the existing duct system and some calculations and then design or recommendations on retrofit of existing system. Once duct design is identified, then the job can be bid. Fix the problem and do it right, rather than just 'do something'.

    So how do I find someone to design this system? or what other 'education" or information do I need in order to make an intelligent decision here?

    -Frustrated Consumer

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    9,727
    Post Likes

    DESIGN PROCESS OUTLINE

    WHAT IS THE EXISTING A/C Equipment Set-up?

    What are the current energy uses ( kw-hr PER MONTH & Oil Gallons / month or season) for the last 24 months?

    < 4,800 Heating Degree Days for DE
    < 1,400 Cooling Degree Days

    _ _ _ ... … _ _ _

    Sealing the house And perhaps other enhancements
    should be pursued with the
    Goal of reducing the Q. Sensible Cooling to < 28,000 BTU/Hr.
    3 Tons * 360 CFM / Ton = 1,080 CFM Total

    TARGET: Reduce 3,950 CFM 50 to 2,700

    Or
    Alternate 1: Use a ~ 2-Ton multi-split Heat Pump for the ~ 1,050 Sq Ft upper level.
    _____ _ _ _ ~ 8 rooms .:. 4 heads

    _ Alternate 2: Use regular ( DX - direct expansion) Heat Pump located in the attic.

    MY Gmail address = racingdan11

    What is the current, measured total air flow?
    What is the current, measured E.S.P.?

    … XXX
    From POST # 1

    2-story over full unfinished basement house,
    concrete block with full brick face,

    furring strips nailed to block, plaster board on that,
    no insulation, built 1955,

    blower door test 3950 @ 50 pa,
    2,100 ft2 nominal (42x25-ft exterior).
    Zip code 19810.

    … XXX
    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
    Nov 2006
    Location
    Athens, Ohio
    Posts
    7,177
    Post Likes
    You can search for someone who has been trained in duct design in your area at this site.
    https://www.myhomecomfort.org/
    AOP Rules: Rules For Equipment Owners.

    Free online load calculator: http://www.loadcalc.net/


    There = not here. Their = possessive pronoun. They're = they are
    It's = contraction of it is. Its = the possessive form of it
    Too = also. To = expressing motion. Two = 2
    Then = after that, next. Than = indicates a comparison.
    Questions should end with a question mark "?" Statements end with a period "."

  4. #4
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    9,727
    Post Likes
    Quote Originally Posted by SeriousByr View Post
    This is a bit of an odd situation and I am baffled at what I have found in the industry in my area. So perhaps I am the 'odd ball out" or the there is something going on with the contractors in my area?

    I have concluded that
    I need a professional (HVAC Tech or Engineer) to
    do a detailed evaluation of the ducts in my house
    and create a design
    or do the best optimization possible.


    However, I can not find anyone who is capable of doing it!
    That in spite of the fact that I am willing to PAY FOR IT to boot!

    Most of the companies that have come to look at and estimate
    replacing my 1955 lowboy Armstrong Oil Furnace
    (last tested at 67% efficiency ~15 years ago!),
    with a high efficiency gas unit (plus new condenser/compressor) have reported that
    return air flow is inadequate and
    they would increase the size of the main trunk and that is it.

    I can't imagine that anyone would take their car to a shop where the mechanic says,
    well I'll fix this part and if it doesn't work, bring it back and I'll replace another part!

    No, a thoughtful mechanic might use an OBDII tool to discover the problem and fix it from there! I know there are air flow and pressure meter/gages available that could be used and contractors have hinted at their use AFTER a new system fails, but not before!

    I have run 8 contractors through the house and have quotes for HVAC systems and all say they will do "something" but no one has been specific about exactly what. (unfortunately that doesn't wash with me. My experience is a clearly defined scope of work is the best approach for project success!)

    So through all of this I have come to a bit of an education and am dangerous because I only know a few 'rules of thumb' and not enough of the engineering (like how/where static is likely to change & need to be considered) to know what I don't know.

    Here is what I do know:

    • 2-story over full unfinished basement house,
      concrete block with full brick face,
      furring strips nailed to block, plaster board on that,
      no insulation,
      built 1955,

      blower door test 3950 @ 50 pa,
      2,100 ft2 nominal (42x25-ft exterior).
      Zip code 19810.



    • I have drawings of ducts in basement
      with measurements,
      measurements of rooms,
      measurements of return register grills, etc.


    • Original ducts (aluminum) were designed & installed when AC meant open the windows & run the whole house fan. Then at some point 'add-on' supply ducts (galvanized) were installed for AC in the ceiling of 2nd floor rooms.


    I have identified 4 places where I believe
    additional return air flow could be provided with minimal interior damage.
    All would be in stud bays and 3 from the second floor and one from 1st floor.
    But how much of that is needed?

    Anyway, the bottom line is that I believe
    the best approach is a thorough review of the existing duct system
    and some calculations

    and then design or recommendations on retrofit of existing system.

    Once duct design is identified, then the job can be bid.


    Fix the problem and do it right, rather than just 'do something'.

    So how do I find someone to design this system?

    or what other 'education" or
    information do I need in order to make an intelligent decision here?
    1st: ACCA Manual J calc

    MY Gmail address: racingdan11

    https://www.energystar.gov/ia/home_i...ors/qispec.pdf
    Last edited by dan sw fl; 12-09-2019 at 05:31 AM.
    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
    Nov 2004
    Location
    SW FL
    Posts
    9,727
    Post Likes

    CONCEPTUAL MANUAL J

    2,100 SF
    2 STORY __ 42' X 25'
    300 SF Single Pane Windows

    1955 BUILT
    DE
    __ NEW CASTLE AP
    _______ DESIGN TEMP: 16'F

    _____ ______ _ _ _ 68,200 BTU/HR +/- 18%
    ______ ______ DT = 54'F
    _ HEAT LOSS RATE = 1,263 BTU / HR-'F

    Perhaps, ~ 1,200 CFM would be required.
    Attached Images Attached Images   
    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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