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  1. #1
    Join Date
    Oct 2004
    Posts
    47

    How much return air duct needed?

    I am having a new home built on the Texas Oklahoma border. It has six inch walls and R-49 in the ceiling. After reading on this site, I decided a dual fuel system is the way to go (I have natural gas available). For some reason the HVAC sub did not like the idea. He wanted me to go total electric Heat Pump. He ended up installing a 5 ton ,Rheem 13 EER HP and a two stage, varible speed Rheem gas furnace at my request and he did it at what I considered to be a reasonable cost. I am not at the site very often due to the distance but the builder sends me photos. From looking at the photos, he has installed two what appears to be 12 inch return air ducts. I have attached a photo of one of them. They are both the same just different locations.My question is will this be enough for this system? I know what most of you think of flex duct but metal is way too expensive and he was wanting to gouge me even to use duct board. As I have a limited budget I agreed with his using the flex duct. I am now 67 so I figure it will see me out anyway. My only concern is the amount of return air. Thanks for any input.
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  2. #2
    Join Date
    Aug 2007
    Location
    Tucson, AZ
    Posts
    5
    It seems to me that for a 60,000 BTU/H system you will be needing around 1900 CFM airflow (max). As a 12" round duct is good for 600-800 CFM, two of them would probably be just adequate, but you would have fairly high velocity when the blower is at full speed and in my experience, this means noise. How important is the quietness of the system to you? Larger ducts (and grilles) mean reduced velocities and quieter operation. What is the design CFM the subcontractor is using for the system? Check out Manual D.

  3. #3
    Join Date
    May 2005
    Location
    Western Kansas
    Posts
    267
    600-800 for 12" is being generous by my design...I wouldn't want my name on it for a 5 ton system. New home designs make it hard to get proper return but a good installer will find a way to make it happen.

  4. #4
    Join Date
    Aug 2007
    Location
    Winter Haven, FL
    Posts
    4,380
    You can never have too much return. I like to see 150 sq inches of return grill per ton. 2- 12" flex ducts will be noisy, have him put another one in for you.

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    It depends on the rest of the duct system ,could be okay ,but very unlikely.

    Manual D as always is the way to find out.

    Other ducts could be undersized,ask to have the static tested when they start the system.

  6. #6
    Join Date
    Jan 2002
    Location
    Ft.Worth,Tx
    Posts
    4,584

    Loud~~~~~~~~~~air

    I would suggest that while it in rough in stage, have contractor change to 14" on both or maybe add another 12" in some other location.

    3) 12" 20x20

    2) 14" 20x25

    2)12" 20x20+ 1)14"20x20
    "Everyday above ground, is a good day".
    "But everyday that you have made a difference in someones life, may insure you stay above ground a little longer".<aircooled>

  7. #7
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    This is an area where I have heard very differing opinions from various pros, but none of them have endorsed two 12-inch flex returns as being enough for a 5-ton system. I am a homeowner in Texas but have bought ACCA Manual J, Manual S, and Manual D which are regarded as the gold standard of how to do things right. One of the rules in Manual D is for flex return you should keep air speed below 600 feet/min (I spoke with another pro who took obvious pride in his craftsmanship and he said he makes it below 400 fpm).

    To keep air speed down to 600 feet/min (Chapter 3 of Manual D), on a 5-ton system with 2000 cfm airflow you need one 24-inch return duct or two 18-inch. This is 450-500 square inches of duct cross section area. Possibly your system design will call for 350 cfm/ton and the total airflow will be lower. And it could very well be your returns are actually larger than the 12-inch you estimate, I certainly hope so(!). Ask your AC contractor what his design choices are.

    My own home had two 3-ton systems with a single 18-inch return for each, this was somewhat undersized per Manual D standards. One system had a tight-radius bend in the duct and its airflow capacity was diminished, the result was the system was operating against a "TESP" (Total External Static Pressure) of 0.75 inches water column, when the air handler was rated for a maximum of 0.50. This problem was addressed by adding return ducts so that now I have about 530 sqin, and TESP now measures about 0.55 for air conditioning.

    If it's not too late, let your AC contractor know you are interested in more than the bare minimum of return capacity. If he can deliver a system with 0.60 or less TESP then I believe you will be OK. With certain air handlers you can go higher, for example the Trane variable speed air handler is rated up to 0.90. To repeat, this is an amateur speaking not a pro, but one who has observed certain contractors have made bad choices in my own home (and some good choices too).

    Best of luck -- Pstu

    P.S. Is there going to be a 1-inch filter grille located 10-12 feet above ground level, or a central filter easier for somebody to reach?

  8. #8
    Join Date
    Oct 2004
    Posts
    47
    I thank all of you guys for your information. When you have this many pros say I need more return air, then I need more return air. I will talk to the HVAC sub tomorrow.

    pstu- You may be an amateur but you sound like you know what you are talking about. You may be right, maybe I am wrong and the returns are larger than 12 inches. I have attached a photo of the second one. I think the studs in that wall are 16 inch centers which makes me think they are 12 inch returns. If I am wrong and they are 14 or 16 inch returns, would this be acceptable or would I still need a third return. Thanks again to everyone.
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  9. #9
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    One 12-inch duct is 113 sqin, which suggests to me you would need four of them. One 14-inch is 154 sqin, 3 of them would get you into the right range. One 16-inch is 201 sqin, I feel that two of them would be cramped but if a pro wants to override that opinion let us listen to him... and ask him for that TESP measurement. Knowing your TESP is an objective measurement that can be discussed with any pro or on this board.

    You might consider asking for added returns in bedrooms, that certainly is good practice. If your AC contractor finds it difficult to redesign what is already installed, that often is the way to make both sides happy. This seems to be one of the few areas where "more is better". Lots of filter area helps too, a related but different topic.

    Best of luck -- Pstu

  10. #10
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,841
    Return air IMO is something you can never have too much of. Think of it like a checking account. As long as the income is more than the outgoing, you're all set. Same thing with air in an HVAC system. If you've got the proper amount of supply ducting and outlets, then the return should have more, IMO at least 25% more. Let the supply side do the limiting of the airflow in the system, never the return. Having said that, an ideal installation would have a return in each bedroom where the door will be closed at night to keep from pressurizing the room and forcing treated air out of the home. Likewise, the return in that bedroom must be balanced to take out only as much air as is pushed in so as to not take the room negative. Definitely increase those returns above 12-inch and add a third at 16-inches or more. Never too much return!!
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  11. #11
    Join Date
    Nov 2000
    Location
    Waco, Texas, USA
    Posts
    6,153
    Those look like two 16" returns to me. Definately NOT 12"

    By the way I love that dust ledge below your return
    "And remember my sentimental friend......that a heart is not judged by how much you love, but by how much you are loved by others" - Wizard of Oz.

  12. #12
    Join Date
    Nov 2000
    Location
    Waco, Texas, USA
    Posts
    6,153
    I have a few concerns:

    1. Why boast about wall thickness and R-values when you know nothing about the amount of air currents that will be flowing through that insulation? Example: I can wear a heavy coat in the winter but if its not zipped up the air currents will negate the insulation values of the coat. A light jacket zipped up tight will keep you warmer than a heavy coat flapping open, make sense?

    2. If your insulation is as good as you think then that must be one huge house to need a 5 ton. Old people don't need huge houses. Sumptin ain't right here. Maybe the house isn't 4500 sq.ft. and the unit is too big? Wow that would make the ducts the perfect size!
    "And remember my sentimental friend......that a heart is not judged by how much you love, but by how much you are loved by others" - Wizard of Oz.

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