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  1. #1
    Join Date
    Aug 2005
    Posts
    3

    Confused

    With a 5T AC and 92K gas furnace, how many sq inches of return air grating/duct is recommended? I have about 272 now and it seems really low. Putting in a new system and want to get this right.

    Thanks so much, Greg

  2. #2
    Join Date
    Sep 2004
    Posts
    3,157
    see manual D

  3. #3
    Join Date
    May 2005
    Location
    Western Kansas
    Posts
    267
    You're right gkk...it's too small...the entire duct system needs to be looked at by a professional.

  4. #4
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989
    Originally posted by gkk2001
    With a 5T AC and 92K gas furnace, how many sq inches of return air grating/duct is recommended? I have about 272 now and it seems really low. Putting in a new system and want to get this right. Thanks so much, Greg
    Use manual D!
    The linked page provides some rough estimates and some general information; however, use the manual D for proper sizing.

    http://www.udarrell.com/proper_cfm_b...g_systems.html

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    The link above gives you some numbers to consider,in most cases they are minimal,you may need larger ducts.


    A manual D ,is exactly what is needed ofr the ducts,when calling for estimates,be specific,your also need a man. J ,to be sure you really neeed that large of a system.If you have upgraded windows,doors ,or insulation,since it was installed ,you likley need a smaler system,could have ben oversized originally.

    It's probably oversized for A/C as the ducts being too small,reduces capacity,so if it's colled well it's most likely running like a 4 ton with the undersized ducts.


    So ask for both,if they can't/won't keep dialing,don't waste their time and yours.

    A five ton furnace will need the return the the bottom or both sides ,not just one side of the furnace.

  6. #6
    Join Date
    Jan 2001
    Posts
    7,814
    For a standard metal return air grill...Its 200 square inches of face area per ton.

  7. #7
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295

    Exclamation Get it Right

    Originally posted by gkk2001
    With a 5T AC and 92K gas furnace, how many sq inches of return air grating/duct is recommended? I have about 272 now and it seems really low.
    Greg,

    Is 5_ton the appropriate size?

    __~5 Square feet (700 sq. inch) of filter area
    and 3+ square feet ( 450 sq. inch) of duct.

    Send drawing and house specs to racingdan11 at comcast.net
    for a full evaluation.

    [Edited by dan sw fl on 08-08-2005 at 12:51 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

  8. #8
    Join Date
    Sep 2004
    Posts
    3,157
    Send drawing and house specs to racingdan11 at comcast.net
    for a full evaluation.


    DD: is this a free service that you are offering to our readers? If so can we referr others to the same link?

  9. #9
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295

    Depends on WORK SCOPE

    Originally posted by ct2
    Send drawing and house specs to racingdan11 at comcast.net for a full evaluation.

    DD: is this a free service that you are offering to our readers? If so, can we refer others to the same link?
    Informed recommendation based on my considerable experience and my Quick Load ( "Ball Park" ) Calc is free as I frequently offer here.


    ACCA Manual J and D Designs

    Formal, Written evaluation with equipment selection and detailed, scaled duct layout for new construction or major renovations is obviously subject to an appropriate fee depending on the actual work scope to be determined for each case.

    I will assist individuals as best I can
    based on their Budget, Defined Needs,
    and the SPECIFIC building envelope information provided through completion of my
    Building Load Analysis Parameters &
    Comfort Specialist Summary Form.
    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

  10. #10
    minimum of 144 sq.in per ton. 5 x 144 = 720

  11. #11
    Join Date
    Jul 2003
    Location
    Philadelphia Pa.
    Posts
    461
    I have 10 return grills in my house they are of different sizes depending on the size of the room and one large room has 2 grills. Should I be adding the sizes of each grill together and then plug the total into this formula?

  12. #12
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,295

    Exclamation Big Enough

    Originally posted by jerrod6
    I have 10 return grills in my house they are of different sizes depending on the size of the room and one large room has 2 grills. Should I be adding the sizes of each grill together and then plug the total into this formula?
    ADD.

    I.E. 10 * 12"x12" = 1440 Sq. Inch
    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

  13. #13
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Bottlenecks

    All the people who recommend Manual D already know this but... filter area is only one part of the need. If you have marginal area for the filter, it will load up quicker and the dirty filter will raise ESP (External Static Pressure), perhaps enough to reduce airflow and cause other malfunction. I believe the disapproval we often hear from pros about 3M filters being too restrictive, applies especially in this case. On the other hand if you have lots of filter area in your system, I believe you can prove that ESP is not adversely affected (I watch ESP and sometimes use Arm & Hammer 8 MERV filters). Unfortunately the number of houses with too little filter area seems to outnumber those with enough.

    The short form of that message is you are not guaranteed to be in safe territory by checking filter area. You also need to have some concern about the pipe area behind that filter. One rule within Manual D is that you want to measure the air speed at your normal CFM and keep it to a low level, perhaps 500 FPM (feet/minute). It is nearly impossible to have too much return area, so you won't need to worry about that.

    With such a number of returns in your system, I would like to think the calculations of filter area and pipe area, will indicate you are OK. Running thru a Manual D analysis will be the most professional way to check this. I believe measuring your system ESP will also signal whether it is OK. And calculating FPM in your ducts is still simple enough, perhaps you can do this even though not many people can do a Manual D.

    Hope this helps -- P.Student

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