Contractor wants to install 2nd air-return on single Central AC unit... please advise - Page 2
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  1. #14
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    Jan 2012
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    George...
    so you want to home run as many of the 16" supply ducts as possible to the unit itself then? This would require they made the box at the unit (which is another soft box) larger.

    I attached a diagram with new Red lines that are changes to the 16" supply lines. Let me know how this looks. When doing this, it dedicates a 16" supply to the problem room side of the house... this would supply more air without changing the ducts I assume (I say this because I don't think those ducts have much of any air flow at this point - not close to the size capacity)

    Is it still OK if one of the 16" supply lines comes off the end of the main box, or should they all come off the sides?

    Should I keep the existing 18" Return or change it to be 16"?
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  2. #15
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    Why not just add a new flex run to the trouble rooms?

    The proposed new return will not pull any significant air quantities since the air will take the path of least resistance.
    So use the new duct run to handle the trouble rooms after you disconnect from existing run.
    Seems like the most straight forward approach
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  3. #16
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    PS the best approach

    would be to install a real trunk duct and then reconnect the flex branches.

    i am partial to ductboard because it is sealed and insulated by it's nature. and costs less than insulated sheet metal
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  4. #17
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    Jun 2011
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    Houston area
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    Without getting too technical, add the return. As was already mentioned just make sure it's filtered....somewhere....in the return path. There is not one single thing you could do to that existing system that would help the Delta T anymore than adding the return. You've got a win-win deal,
    get it for "free" and get better performance.

    Bottom line, if you've got hot or coldspots the ONLY way to condition that air is to draw it through your HVAC system. This is accomplished by creating pressure differentials. Add the return.
    The picture in my avatar is of the Houston Ship Channel and was taken from my backyard. I like to sit outside and slap mosquitos while watching countless supertankers, barges and cargo ships of every shape and size carry all sorts of deadly toxins to and fro. It's really beautiful at times.....just don't eat the three eyed fish....

    `. .` .>(((>

    `... `. .` .>(((>

    .` .>(((>

    LMAOSHMSFOAIDMT

  5. #18
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    Jun 2011
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    Quote Originally Posted by JohnLongden View Post
    What's the current size of the existing return, and if he does put an additional return consider returns in the bedrooms attached as well. Are those rooms facing east, west etc...
    Right on brother, especially in the bedrooms if you can.

    It doesn't really matter the location, if the new, proposed or old return is on a west wall and the stagnant air is hot put a curtain or shade on the window. You will NEVER achieve proper air balance if stratification exists. And, any anomolies in the return path CFM's can be easily corrected by a good T&B tech.

    Sounds like your contractor is thinking in a good way.
    The picture in my avatar is of the Houston Ship Channel and was taken from my backyard. I like to sit outside and slap mosquitos while watching countless supertankers, barges and cargo ships of every shape and size carry all sorts of deadly toxins to and fro. It's really beautiful at times.....just don't eat the three eyed fish....

    `. .` .>(((>

    `... `. .` .>(((>

    .` .>(((>

    LMAOSHMSFOAIDMT

  6. #19
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    JohnLongden, this is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise here. Please apply to the AOPC today, thank you.

    You can find the rules for posting and qualifications here.




    Further infractions may result in loss of posting privileges.
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  7. #20
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    Dec 2010
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    Quote Originally Posted by genduct View Post
    The proposed new return will not pull any significant air quantities since the air will take the path of least resistance.
    So use the new duct run to handle the trouble rooms after you disconnect from existing run.
    Seems like the most straight forward approach
    Livin.....Genduct makes a good starting point. Run a new flex to the bedroom area and let the existing 16" feed the large 4 registers as is.
    This will save money and it should be all you need.

    Make sure they install balancing dampers in the 16" supplies (by the unit) so that you can balance the system.

  8. #21
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    Jan 2012
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    I think I'm almost clear on what I need to have them do. I have attached an updated diagram. Some answers to your open questions mixed in below, plus a few of my own.

    Here's what I get...

    - Break off the front rooms and bathroom to have its own 16" supply trunk
    - Leave the main rooms (open floor plan areas) with the 4 registers as is
    - Leave the master area as is
    - add balancing damper to all 16" trunks (not sure they'll do this but I'll ask)
    - add 16" return in small hall by problem rooms and bathroom
    - replace the soft junction box, at the unit, with a ductboard box (not sure they'll do this but I'll ask)

    Still unsure of...

    1) does the main junction, at the unit, need to have all the trunks coming off the sides? A while back, an AC repair person told me there should not be a trunk duct directly facing the fan/air direction - he said it helps pressurize the system. Again, not sure if you all think this is correct, or necessary in my case.

    2) do I need to downsize the current 18" return to 16" to match the new 16", two reasons to my questions...
    a) does 'path of least resistance' play here, and it will pull more air from the 18" return, thus not balancing the air flow in the house
    b) what if there is not enough on the return side of the unit for an 18 & 16 both?


    thx!
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  9. #22
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    I prefer to design all the supplies to come off the sides so the duct will pressurize, but with the volume dampers I don't think, for the added cost, it's a deal breaker.

    Regarding the RA......No, you don't need to make them both the same size. You basically end up with a "centralized" return system. If doors will be closed, then transfer grilles will need to be installed.

  10. #23
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    Jan 2012
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    Quote Originally Posted by genduct View Post
    The proposed new return will not pull any significant air quantities since the air will take the path of least resistance.
    So use the new duct run to handle the trouble rooms after you disconnect from existing run.
    Seems like the most straight forward approach
    Are you recommending I do not have them install a new 16" return?

    Quote Originally Posted by genduct View Post
    would be to install a real trunk duct and then reconnect the flex branches.
    Do you have a diagram example you can show me?

  11. #24
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    Philadelphia PA
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    That's right

    Are you recommending I do not have them install a new 16" return?
    I am suggesting you use that same flex (if you really have to) for a separate SUPPLY RUN to the "problem rooms"

    My PS suggested a REAL TRUNK DUCT to replace all those parallel runs of flex
    Installed correctly, flex starts out with 2x the resistance of real duct. Installed like it usually is the factor goes to 4 to 6 times the resistance. You can run a simple trunk and use the existing (now shorter) flex as branches
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  12. #25
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    Quote Originally Posted by genduct View Post
    I am suggesting you use that same flex (if you really have to) for a separate SUPPLY RUN to the "problem rooms"

    My PS suggested a REAL TRUNK DUCT to replace all those parallel runs of flex
    Installed correctly, flex starts out with 2x the resistance of real duct. Installed like it usually is the factor goes to 4 to 6 times the resistance. You can run a simple trunk and use the existing (now shorter) flex as branches
    I see... so you are suggesting they build a 20ish foot long box (trunk duct) - I suspect the contractor may not do this if the cost and time to build it is more than running one additional 16" flex duct. ... but I'll ask.

    Do you still suggest to run the new 16" Return?

  13. #26
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    Do you still suggest to run the new 16" Return?

    NOPE! you need to fix the supply side first! Then see if all well!
    You can't suck comfort into a space!

    If the trunk costs are added to the free install labor, your chances for sucess are greater
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

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