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  1. #1
    Join Date
    Jun 2006
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    96

    Return Air Vent Sizing

    I have a 3,200 sq. ft. single level home servced by 2 - 3 ton AC unts. The duct work and air handlers are in the attic. The first until services the LR/DR/Kitchen/Den, all fairly open areas. The stale air return is a 20" x 20" opening in the ceiling.

    The second unit services 3 bedrooms. Two of the bedrooms, each about 250 sq. ft. (9' ceilings) have 2 - 4" x 10" supply vents each. The master bedroom is 15' x 20', with a 7' x 15' dressing room (9' high ceilings). This room has 2 supply vents in the ceiling and one in the dressing room. The stale air return for this AC unit (20" x 20") is located in the main hallway. There are no returns in the bedrooms.

    Normally we keep the bedroom doors open, but when we close them, the airflow into the room is reduced. The airflow is quite strong and if we leave the doors open less than 4" open, the airflow will suck them closed.

    We are not interested in making large undercuts in the doors as this adds noise from the hallway (and ruins the carpet with dirt). I believe the air handlers are moving 1,200 cfm each.

    Question: What size returns would I need in each of the 3 bedrooms? Is there a way to make sure the returns don't add air movement noise from the air handlers in the bedrooms?

    Thanks


    kayjh

  2. #2
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
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    629
    A jumper duct may work. It's a return grill in bedroom attached with ductwork to outside the bedroom terminating in another return grill. Clear as mud?

  3. #3
    Join Date
    Jun 2006
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    96
    Quote Originally Posted by Daltex View Post
    A jumper duct may work. It's a return grill in bedroom attached with ductwork to outside the bedroom terminating in another return grill. Clear as mud?
    In my case, that would mean running a duct back to the main return plenum. The 20" x 20" return grill is placed about 4' from the air handler. The rest of the duct work is on the outfow side.

    The question is what size return grill and ductwork for each room and how noisy is it likely to be??

  4. #4
    Join Date
    Jan 2004
    Location
    PA
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    68,753
    As simple as your question is, the answer is even simpler.

    The returns each need to be as big as required to remove the same amount of air that is being supplied to each bedroom.

    With that info, the size of the register is easy to find. And if you can tell us teh length the duct has to run, if it will make any 90° bends, and teh type of take offs your contractor will use, we can tell you about what size duct he should use.

    So how much air in CFM is each supply delivering?
    How long will the duct be, and what kind of take offs, and how many 90° bends?


    PS: your system isn't moving 1200 CFM through that 20 x 20 filter grille.
    Last edited by beenthere; 07-30-2008 at 03:58 PM. Reason: added PS
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  5. #5
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    Jul 2008
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    If I understand correctly, a jumper duct is nothing more than two grills on opposite sides of the wall connected by a short piece of duct. The idea is to provide an easy path for air to flow out of a closed room that doesn't have its own RA grill/duct.

  6. #6
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    Jan 2004
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    PA
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    That is correct. And jumper grilles and ducts must be larger then if it was ducted to the air handler.
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  7. #7
    Join Date
    Jul 2008
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    My wife used to own a house that had a large jumper duct between the mechanical room/utility closet/pantry/storage room and the crawlspace, otherwise known as the return air plenum.

    It was a very old house. It was also a very small house.

  8. #8
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    1.Since they undersized the return grilles ,I'd be concerned the the rest or part of the ducts are undersized as well.

    2.If the ducts are undersized ,and is heats and cools well,I'd be concerned the the equipment(6 ton) is oversized.

  9. #9
    Join Date
    Jun 2006
    Posts
    96
    Thanks for the replies. I'd prefer not to use a "jumper duct" from the bedrooms to the hallway as noise from the rest of the house will travel easily into the bedrooms.

    As for the cfm air flow; I was advised by the hvac contractor that the minimum airflow to avoid ice up for my system would be 400 cfm/ton, which equates to 1,200 cfm (per unit). I didn't measure the return grill, but I do believe it is 20 x 20 (I'll check when I get home). As I mentioned, a tremendous amount of air flows back to the return, such that a solid core bedrom door will get sucked shut if left open less than about 4".

    The bedroom a/c ductwork is a 16" solid round (insulated) pipe with 6" solid extensions to the ceiling vent. The runs are no more than 10' from the main plenum with the final 2' being flexible insulated pipe.

    The master bedroom would require about 15' of duct and 1 - 90 and 2 - 45's

    The first bedroom would require 20' of duct and 1 - 90 and 2 - 45's

    The third bedroom (right next to the intake) would require 1 - 90 and about 6' of duct.

    I'm not sure what you mean by the kind of take off the contractor will use. I'm assuming he would just connect the return ducts to in side of the intake plenum in the attic? Or does he need to build a "connector box" for he 3 return ducts to connect to and then connect that to the main return duct on the intake side of the air handler?

    I also have concerns that the system is over sized. I seem to get a fair bit of cycling with the system and it is quite drafty in the house when the system is running. I would have preferred 2 x 2.5 cfm which would have allowed for a longer run time with lower fan speeds (1,000 cfm), although I'm not sure if the main ductwork would have needed to be sized differently.

    The hvac contractor is a bit of a pain to work with and is slow to return to correct problems (with the new heating system as well).

    I may have to hire an engineering firm to sort through some of my issues, tender out the reqquired work and have another contractor make the changes - sending the bill to the original contractor. What a pain!

  10. #10
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    Your contractor is wrong. If 400 CFM was the minimum, then when your air filter got slightly dirty your system would freeze up.
    With that small return, its very doubtfull you have 400 CFM per ton now.

    The return duct for any of those bedrooms will be atleast the same size as the supply duct to them. Some will require a larger duct.

    If you use return filter grilles, they will be about 12 x 12.
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  11. #11
    Join Date
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    Quote Originally Posted by kayjh View Post
    Thanks for the replies. I'd prefer not to use a "jumper duct" from the bedrooms to the hallway as noise from the rest of the house will travel easily into the bedrooms.

    As for the cfm air flow; I was advised by the hvac contractor that the minimum airflow to avoid ice up for my system would be 400 cfm/ton, which equates to 1,200 cfm (per unit). I didn't measure the return grill, but I do believe it is 20 x 20 (I'll check when I get home). As I mentioned, a tremendous amount of air flows back to the return, such that a solid core bedrom door will get sucked shut if left open less than about 4".

    REPLY--We use 350 cfm per ton,everyday here in steamy Florida.He's mistaken,testing for the correct cfm to be set is critical.


    The bedroom a/c ductwork is a 16" solid round (insulated) pipe with 6" solid extensions to the ceiling vent. The runs are no more than 10' from the main plenum with the final 2' being flexible insulated pipe.

    The master bedroom would require about 15' of duct and 1 - 90 and 2 - 45's

    The first bedroom would require 20' of duct and 1 - 90 and 2 - 45's

    The third bedroom (right next to the intake) would require 1 - 90 and about 6' of duct.

    I'm not sure what you mean by the kind of take off the contractor will use. I'm assuming he would just connect the return ducts to in side of the intake plenum in the attic? Or does he need to build a "connector box" for he 3 return ducts to connect to and then connect that to the main return duct on the intake side of the air handler?

    REPLY---Connector box for each one to connect is required.Connecting to the existing box and grille,will not draw enough air from the new returns.It's actually code in Florida.



    I also have concerns that the system is over sized. I seem to get a fair bit of cycling with the system and it is quite drafty in the house when the system is running. I would have preferred 2 x 2.5 cfm which would have allowed for a longer run time with lower fan speeds (1,000 cfm), although I'm not sure if the main ductwork would have needed to be sized differently.

    REPLY--350 cfm per ton,so 1050 cfms.

    The hvac contractor is a bit of a pain to work with and is slow to return to correct problems (with the new heating system as well).

    I may have to hire an engineering firm to sort through some of my issues, tender out the reqquired work and have another contractor make the changes - sending the bill to the original contractor. What a pain!
    See REPLY above.

    You shouldn't need an engineer,just a well qualified HVAC Contractor.

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