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  1. #1
    Join Date
    Jan 2012
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    13

    Question Contractor wants to install 2nd air-return on single Central AC unit... please advise

    I'm new here so I apologize if this is not the correct forum.

    I have a contractor wanting to do work on Wed so this is urgent and your help is HIGHLY APPRECIATED!

    I hired a "green" company to make my central heating/air, and home in general, more energy efficient and increase the comfort level.

    After increasing attic insulation to R39 and duct sealing, I still have LARGE temperature differentials (average 4-5 degree delta from the rest of the house) in 2 of the rooms - the ones completely opposite side of the 2300sqft house from there the air intake resides.

    The contractor, free of charge, wants to install a new return in the small hall way by the two rooms.

    I want to ensure they are not going to make the system less efficient or cause other problems.

    The central air unit is 12 year old 5 Ton (I think) Carrier split system Prod 38CKC060-311-9; Model 38CKC060-310-9 ... the AC unit was just checked out by an authorized servicer and it is in perfect condition.

    The "problem" rooms are ~60-70 foot of duct work from the central unit (registers are in a Y config off the branch from that side of the attic), and the existing air-return has ~8-10 foot of duct connecting it to the unit.

    ... so you can see the length of the returns would have a major difference. I'm worried about this, and the sizing for the ducts themselves to ensure we are actually pulling air from the new air-return and not just wasting time or creating inefficiencies.

    Please see crude drawing as reference. I show the main registers, there are several small ones (4-6") in closets, entry, etc I did not put on there.

    Thank you in advance!

    Aaron
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  2. #2
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
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    2,702
    Adding the return, if done properly, will help the system. It may not help the room(s) in question.

    You may need to increase the size of the supplies to the room(s) as well. Try the return first (the price is right) and see what happens. It should help some.

  3. #3
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,997
    Hi Aaron,

    It is very difficult to give good advice when one can't see the actual situation!
    The local contractor ought to perform a manual D to see what is required to get the proper airflow to those two rooms.

    Where do you live, that 5-Tons of cooling was thought necessary? Here in SW WI we sometimes cool that size home (2300-sf) with 2, 2.5 or 3-Tons; it varies of course with the situation/conditions.

    A 5-Ton system requires at least 1750-CFM of airflow through the evaporator coil, that is difficult to get with only seven supply air outlets.

    1750-cfm / 7-SA registers = 250-CFM average airflow per register that can require considerable velocity depending on the size & type of ducting, etc.

    I hope the evaporator coil has a TXV metering device... as as that would help protect the compressor if there is low airflow through the coil.

    It takes a lot of filter area for 1750-cfm of airflow; how much do you have? Have him get as much filter area as possible. Use low pressure drop filters!

    I hope he sizes the new return grille area & ducting large enough. I would think it should help some with airflow to those two rooms.

    If dampers are not on the other branch runs, installing them might help with some balancing; but depending on the branch duct sizing there probably are not enough supply runs to get the velocity down where it should be for a 5-Ton system.
    The Square foot open area of the SA diffusers / into the CFM = fpm velocity.

    Why not put R-50 in the attic?

    It's very difficult to size things up from here; too many variables...

    However, Aaron could use more input here...
    Last edited by udarrell; 01-16-2012 at 12:14 PM. Reason: R-50 in attic...

  4. #4
    Join Date
    Jan 2012
    Posts
    13
    Thanks for the quick responses. Here's some answers to your questions / inputs... thanks for your continuing help... this is great!

    The contractor is not an AC expert. But for free, if we can get a better system than I have today it will be good.

    I live in Phoenix AZ - The 5 ton unit came with the house, the whole neighborhood uses the same one, changing is cost prohibitive. We will be adding more insulation too... probably get it to around R42 or so. It is free (I already paid them for the service) so I don't want to push it... the AC ducts are priority.


    Existing Return...
    It is a double filter grill that hold two 20x20" filters but the duct is only on one side (not centered) and seems to be 17.5" at the opening. Maybe an 18" flex duct?

    I found this guide= http://efficientcomfort.net/Rules_an...ct_Systems.pdf

    It says I should need much more return airflow (as you both also stated)... What makes more sense...

    1) change the existing 18" to a 16" and add the new return as a 16" and another 8" in the center of the house

    2) change the existing 18" to two 14" and add the new return as a 14" also.
    ... I ask since the new return near the problem rooms will be 7-8x the duct length as the existing return is now - want to make sure it pulls air as equally as possible, yes?

    3) other suggestions?

    Supply ducts...
    two somewhat open, large living spaces have 14" registers (4 total)
    four bedrooms have 12" registers (5 total)
    bathrooms, closets, etc have very small registers
    ... not sure of size of flex duct itself but I suspect they are approx 3" smaller than the registers?

    For the problem rooms, I'm thinking I'll have the contractor try to reduce the length of the supply ducts to those rooms... right now they are hanging and have some ups/downs, and he said they might be able to reduce the runs by cutting plywood barriers open to run through them to make the runs more direct, instead of using the existing openings of the barriers - which added length to the ducts.

  5. #5
    Join Date
    Jan 2012
    Posts
    13
    Quote Originally Posted by Isaac3384 View Post
    Since I didn't see any sizes of duct for the supply duct leading to the rooms with issues, I'm just throwing a guess out there. You may, in addition to the return, need to up size the trunk line. Friction loss plays a major roll in a lack of airflow.
    I'm not sure of the duct size for the supply to those two rooms, the register is 12" square so I suspect the duct is 8" - but it branches so I need to check what the truck is before it branches.

    Also... for the return sizing - I've attached a visual layout of the main home areas so you can get an idea of the potential placement of the returns and sizing needed for each.

    Black Boxes: existing supply registers (large and small accordingly), placement in main areas are accurate
    Red 4-point Star: where the existing 16-18" return is today
    Blue 5-point Star: where contractor wants to put a 16" return
    Green Diamonds: based on the linked PDF in my last post, should I install a small 8" return in one of the diamond spots?

    ... thoughts?

    thx!
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  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,350
    Aaron,

    I understand your concern that you don't want to spend time chasing after rainbows if that can be helped. That said, I echo what Udarrel said; if nobody has ever made an effort to know how much air is supposed to be supplied to the rooms that have a marked temperature difference from the rest of the house, there's no way to know how much air needs to be returned from those same rooms!

    Rooms that are abnormally warmer or colder than the rest of the house are, from an HVAC standpoint, rooms that aren't getting enough cooled or heated air to adequately offset the amount of heat being gained to or lost from the rooms through the building shell.

    So, to answer your original question, will adding return air vents in these problematic areas help? A little...maybe. But if not enough heated or cooled air is getting to these rooms in the first place, nothing else matters.

    Now I have a question for you: if you leave the doors open to these rooms that are too hot or too cold, does the temperature difference go away or become less?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  7. #7
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    Lancaster PA
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    Isaac3384, 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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  8. #8
    Join Date
    Jan 2012
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    13
    I have more solid info for you and updated the diagram to show supply ducts (in dark Blue) and the problem rooms (though not to scale). You'll notice the problem rooms are at the end of the run, and split several times - while also, the size pf the ducts are reduced.

    I verified...

    Return is 18" approx
    - 10' long and is a C curve on fan side since it is near edge of house and must wrap around the end of the AC unit

    Supply Trunks are 16"
    - one feeds the main part of the house; comes off end of the plenum, inline with the air from the fan.
    - one feeds only master, master bath + closet, laundry room; comes off side of plenum
    --- which is why the master gets blasted with air until blocked 75% of the register.

    Please let me know what changes you think would help, without tearing it all out since I don't think they will do that.
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  9. #9
    Join Date
    Dec 2010
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    The Quad-Cities area (midwest).
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    The register size has nothing to do with the supply duct size. You'll need to remove the registers and measure. Make sure they haven't used a reducer as well.

    I think adding another 20"x25" filter grille with a 16' flex run wouldn't hurt. You're a little short now.

    When I look at both of your lay-outs, I think your main (supply) run is too small to feed all the supplies at the far end of the house. We'll know more when you tell us the sizes of supplies "feeds".

    Also, like you mentioned, what is the supply size that splits and feeds the two bedrooms?

  10. #10
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
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    Livin.....Thank you for the new drawing. I've never designed ductwork to run like that. Maybe someone here can say it'll work okay that way.

    That said, we need to know all the sizes as it goes out to each register. In other words, you said it starts as a 16" main feed, then it reduces...........where?

    Is it all flex duct?

  11. #11
    Join Date
    Jan 2012
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    13
    Sorry, I did forget to mention... All bedrooms (except the master) reduce down to 9". So the 3 bedrooms rooms being fed at the far/branched end go from 16" trunk that makes a Y, then the 16" Y makes another Y to reduce down to 9" feeds for 3 bedrooms and 4 or 5" feed for that end's bathroom.

    It is all flex duct, except where you see the blue box, that is a box with 'soft' sides... don't know how to describe it.

    While adding that additional 16" return, would you suggest changing the 18" to a 16" and adding an 8-9" return somewhere in the middle of the open floor plan space? Or just keep it to adding the 16"? The issue I see is that 18" return is tight right now, and that side of the AC Unit really does not have any more space to add anything to.

    @shophound - when the doors are opened the room's temp differences are less. but, even with the door open, it still ranges from 3-5 degrees F temp difference from the main home areas.

  12. #12
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    Livin.....It would be a lot easier if I were there to explain this, but I'll give it my best.

    The 16" flex is only putting out about 1,000 cfm. I don't like that much flex. But it's there and we'll deal with it.

    I would take the 16" off the "soft" box that feeds the liv./din. rms and run it to the air handler

  13. #13
    Join Date
    Dec 2010
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    Sorry.....I hit the wrong button......to continue......and maybe pick up one additional supply off that feed.
    Then the 16" feeds the far end of the house. You'll need to enlarge the supplies to the problem areas.
    Regarding the additional return, it doesn't matter where it goes. As long as you; A: Leave the bedroom doors open or; B: Put in transfer (return air) grilles.

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