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  1. #1
    Join Date
    Mar 2011
    Posts
    6

    Should returns size in each room match supply

    I'm in a 4 BR house and am having issues with uneven cooling in the bedrooms. The bedrooms area has its own 2.5 ton unit w/the lower two levels having a 5 ton unit.
    I posted a few years ago about it and wound up having manual dampers installed which improved the situation slightly but didn't remedy it.
    I've had a few HVAC guys come in recently and I've basically been recommended to install returns in each rooms since my returns are currently undersized.
    The system is all flex and currently it's a 10"+8" return.
    They suggested the following
    MBR/Bath - 500sf - 12" Flex Supply (6"+6"+7"+7"+4") / 12" Flex Return
    BR2/Bath/Hall - (BR2=200sf) - 10" Flex Supply (7"+7"+4"+4") - 8" BR2 Return + 8" Hall Return
    BR3/BR4/Bath/Foyer (BR3=200sf/BR4=160sf) - 10" Flex Supply (7"+7"+7"+4"+6") - 8" BR3 Return + 8" BR4 Return

    The hall leads to stairs and an open 2 story Foyer and downstairs has a separate unit
    The bedroom doors will usually be closed which is why the per room returns were suggested.

    I had a few concerns that I wanted to run by you guys before giving them the go-ahead
    1) I looked up CFM for flex duct and it seems like BR2/BR3 with baths(7"+7"+4") will be getting 255 cfm but the 8" returns will suck 160cfm back. Should I ask them to use 9" returns for those two rooms instead?
    2) The Vaulted Foyer area accumulates a good chunk of heat rising up from downstairs. They did not mention a return in the Foyer area, should there be one?
    I'm only having them work on the upstairs unit, should I have them install an extra return from the top of the Foyer area to the downstairs unit since that's where most of the hot air originates?

    Also for reference this is a relatively new home (under 5 years old)

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,440
    Quote Originally Posted by mudgod View Post
    I'm in a 4 BR house and am having issues with uneven cooling in the bedrooms. The bedrooms area has its own 2.5 ton unit w/the lower two levels having a 5 ton unit.
    I posted a few years ago about it and wound up having manual dampers installed which improved the situation slightly but didn't remedy it.
    The system is all flex and currently it's a 10"+8" return.

    They suggested the following
    MBR/Bath - 500sf - 12" Flex Supply (6"+6"+7"+7"+4") / 12" Flex Return
    BR2/Bath/Hall - (BR2=200sf) - 10" Flex Supply (7"+7"+4"+4") - 8" BR2 Return + 8" Hall Return
    BR3/BR4/Bath/Foyer (BR3=200sf/BR4=160sf) - 10" Flex Supply (7"+7"+7"+4"+6") - 8" BR3 Return + 8" BR4 Return

    Should I ask them to use 9" returns for those two rooms instead?
    900 CFM for 2.5 tons which is more than sufficient for 1,060 square foot bedroom area.

    Four 9" and one 12" diameter flexible duct returns will work if the duct is stretched properly and not kinked.

    4 x 150 CFM + 1 x 300 CFM = 900 CFM total

    Many codes now require returns in every bedroom or room with a closed door.
    ACCA Manual D is used for sizing and arrangement of air distribution system.
    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
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,514
    Rooms further from the air handler will typically require larger ducts, closer rooms smaller ducts. Has this been accounted for in the ductwork layout?

  4. #4
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,035
    Where do you live that you need 7.5-Tons of cooling?
    How many sf is the rest of the home?

    Have you ever considered having a Home Energy Efficiency Audit performed?

    After the audit required retro-work is done, then have both a room by room Manual J & duct system blower performance Manual D performed.

  5. #5
    Join Date
    Mar 2011
    Posts
    6
    @ 54regcab
    I'm not sure, but the distances are also proportional to size (neglecting bathrooms/hallways) so not sure which is factored in more
    BR4=10-12ft run/ 160sf/7" flex (off of 10" trunk)
    BR2=20ft run/ 200sf/7"+6" flex (off of 10" trunk)
    BR3=20ft run/ 200sf/7"+6" flex (off of 10" trunk)
    MBR=30-35ft run / 420sf/7"+6"+6" flex (off of 12" trunk)

    @dan sw fl:
    I've had three contractors out so far. Two used a manual slide rule to calculate sizing (I believe a ductulator). One simply took measurements and left and I haven't heard back from him yet. None of them suggested a Manual D or a Manual J


    @Udarrel: Yes I have but I wasn't sure of what I should be looking for. From the companies I looked up they seemed to be offering widely divergent services with a good deal of them being tied into weatherization (things like 300 off weatherization if you get an energy audit). The place itself is relatively new and is well insulated with high efficiency double glazed windows, weatherstripping on all doors etc. I didn't want to waste money if it was going to suggest the obvious things which I've already taken care of. Is there a specific kind of audit that I should be looking for in relation to HVAC?
    Also the place is around 4500-4700 sf with the basement being around 1100 sf and the main / upper levels being around 1700 sf each (50*34 footprint)

  6. #6
    Join Date
    Jun 2004
    Location
    Eugene, Oregon
    Posts
    1,018
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.
    Last edited by jpsmith1cm; 12-05-2012 at 09:30 PM. Reason: non AOP member

  7. #7
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,035
    Quote Originally Posted by mudgod View Post
    @ 54regcab
    I'm not sure, but the distances are also proportional to size (neglecting bathrooms/hallways) so not sure which is factored in more
    BR4=10-12ft run/ 160sf/7" flex (off of 10" trunk)
    BR2=20ft run/ 200sf/7"+6" flex (off of 10" trunk)
    BR3=20ft run/ 200sf/7"+6" flex (off of 10" trunk)
    MBR=30-35ft run / 420sf/7"+6"+6" flex (off of 12" trunk)

    @dan sw fl:
    I've had three contractors out so far. Two used a manual slide rule to calculate sizing (I believe a ductulator). One simply took measurements and left and I haven't heard back from him yet. None of them suggested a Manual D or a Manual J


    @Udarrel: Yes I have but I wasn't sure of what I should be looking for. From the companies I looked up they seemed to be offering widely divergent services with a good deal of them being tied into weatherization (things like 300 off weatherization if you get an energy audit). The place itself is relatively new and is well insulated with high efficiency double glazed windows, weatherstripping on all doors etc. I didn't want to waste money if it was going to suggest the obvious things which I've already taken care of. Is there a specific kind of audit that I should be looking for in relation to HVAC?
    Also the place is around 4500-4700 sf with the basement being around 1100 sf and the main / upper levels being around 1700 sf each (50*34 footprint)
    Since your home is rather new & of good construction all that is needed is the Manual J & the Manual D.

    You can have excessive Return air sizing. Be sure the filter areas are plenty large enough; best sized for 300-fpm velocity through a clean filter.

    I didn't have time to read everything. Where is/are the filter(s) located?

    Oversize Return ducts from rooms that need more airflow; low airflow room may also need another supply; also the RA filter if one in the rooms.

  8. #8
    Join Date
    Nov 2012
    Location
    Tahlequah OK
    Posts
    131
    Do a Manuel J load calculation. This will give you the amount of air required for each room. Have a qualified HVAC tech design your duct and get rid of the flex duct. Install metal duct and properly sized RA duct. This will solve your problem.

  9. #9
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,514
    Quote Originally Posted by udarrell View Post
    Since your home is rather new & of good construction all that is needed is the Manual J & the Manual D.

    You can have excessive Return air sizing. Be sure the filter areas are plenty large enough; best sized for 300-fpm velocity through a clean filter.

    I didn't have time to read everything. Where is/are the filter(s) located?

    Oversize Return ducts from rooms that need more airflow; low airflow room may also need another supply; also the RA filter if one in the rooms.
    I've never seen this happen. What are the symptoms of an oversized return? I suppose if somebody went nuts with it there would be a lot of duct surface area in unconditioned space...

  10. #10
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,035
    Quote Originally Posted by 54regcab View Post
    I've never seen this happen. What are the symptoms of an oversized return? I suppose if somebody went nuts with it there would be a lot of duct surface area in unconditioned space...
    Glad you noticed that 54regcab; I overlooked the typing omission!

    That should have been: You cannot have excessive Return air sizing. Be sure the filter areas are plenty large enough; best sized for 300-fpm velocity through a clean (cheap fiber glass filter).

    You can have a large deep box Return chamber directly under the furnace with super-sized filter grilles & no ductwork; the system will work efficiently, using good filters it will then have excellent filtration...

    Well, guess it's human to error...I can't even type anymore - having to use the eraser ends of pencils & hunt & pick due to real bad carpal tunnel in both hands.

  11. #11
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,514
    OK, that makes sense, no such thing as too much return
    Which rooms are too hot/cold?

  12. #12
    Join Date
    Nov 2012
    Location
    Charlotte, Michigan 48813
    Posts
    130
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.
    Last edited by jpsmith1cm; 12-06-2012 at 02:11 PM. Reason: non AOP member

  13. #13
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,035
    Quote Originally Posted by taxman View Post
    Just to clarify, are you saying that it should be about 300fpm, or that it shouldn't be over 300fpm and anything less is fine?
    Anything less than 300-fpm velocity is wonderful; the lower the velocity through an extra large effective sf area filter the better.

    Hart&Cooley Engineers say NOT to go over 400-fpm with a new clean filter; especially with a cheap throw-away filter; that only provides 100-fpm increase for debris loading of that particular fiber glass filter...

    The velocity should never exceed 500-fpm when using a cheap fiber glass throw-away filter; pleated filters can go higher but the 1" thick pleated filters have too much pressure drop at those higher velocities.

    Filter area sizing can gain or lose important efficiencies in important HVAC arenas; both filtering health & cleaner blower wheels & coils plus airflow efficiency areas.

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