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  1. #1
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    Effects of a small return plenum?

    Title says it all. Ive been told that the return plenum size has nothing to do with the restriction of the return and that its more based on the pipes that are connected to it. Told to me by a 20 year exp. Installer that has never had anyone say to him to change the retuen plenum and make it bigger.

    Im very interested if plenum sizes do matter.

    By the way, how does one correctly size the pathetic excuses of mixing boxes

  2. #2
    Join Date
    Aug 2016
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    Ask any woman size matters. the return has to be large enough for the system. There really isn't to big for a return but there is defiantly to small. The blower can only move the air allowed by the duct both in and out. One of the tell tell signs and complaints is noise, small returns are loud and whislely (is that a word?) They starve the evap stealing efficiency. They limit air output so some rooms get air and others don't. Customers complain it's not working yet the numbers add up so you end up head scratching. I could go on and on. short answer it matters

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  4. #3
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    ^^^^^^^ this.

    One other issue is it needs to be big enough to accommodate fittings designed for good air flow.

  5. #4
    Join Date
    Nov 2012
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    San Pedro, California, United States
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    Just cause you are using 20" returns, doesn't mean you can undersize your plenum..
    How are you gonna shove the air the return ducts are grabbing in a tiny plenum, just no bueno

  6. #5
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    Sep 2014
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    NW Arkansas
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    Units are rated with an open return plenum... So basically no return. Once you add one you are starting to restrict air more than design. That is why we stay at/under .5 static

    Only time a return is to big is if it costs more than you can get to build or it is in a 150'F attic and the air is picking up a ton of heat (but better insulation will fix that problem) .

    Sent from my SAMSUNG-SM-G891A using Tapatalk

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  8. #6
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    Thank you for all the replies. I knew it did matter.

  9. #7
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    Aug 2011
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    A ruff ruff rule of thumb. Filter Grill needs to be 200 square inches per ton of AC. Example a 20 by 20 is 400 square inches barely enough for a 2-ton. Also some return grilles are not all created equal some are more restricted than others

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  11. #8
    Join Date
    Jun 2007
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    Quote Originally Posted by doug1111 View Post
    A ruff ruff rule of thumb. Filter Grill needs to be 200 square inches per ton of AC. Example a 20 by 20 is 400 square inches barely enough for a 2-ton. Also some return grilles are not all created equal some are more restricted than others
    Or 20x20=400 x 2 = 800 cfm. Double the square inches and that is the amount of cfm it can handle Same thing you said, just another way. I always like to bump it up the next size on returns.
    I like DIY'ers. They pay better to fix.

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  13. #9
    Join Date
    Sep 2002
    Location
    Virginia
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    just came from a home with 2 ton AC with Natural Gas furnace , and the furnace sits on the plenum. All they had was a 12x12 grill cut into the side of plenum box with 14 x 20 filter taped to outside of grill The thing was tripping the high limit so much it finally burned the limit up because it was acting as a thermostat.

    Customer was asking me why I want to make return bigger " its been working fine for 5 years "

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  15. #10
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    Sep 2007
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    edmonds wa
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    I hope you told him that is why you only got 5 years out of it.
    UA Local 32

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  17. #11
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    Get yourself a magnahelic, U-tube manometer, or one of them newfangled electronic gizmos that measure air pressure. Then, whenever you have the time, measure the static pressure on the return and supply at the unit. Sort of opens up a new world if you haven't been doing it.

    For example, last winter one of our techs was having a problems with a heat pump tripping on high head when in heat mode. He couldn't figure out what was going on. I measured a total of 1.1 or 1.2" water column, and the nomenclature plate said maximum 0.6" wc. Restricted airflow due to ducting.

    I also measured 25*F subcooling. Reduced it by half and the unit stayed running. As far as I know it is still running like that. And, also as far as I know, it made it through the summer just fine.


    Quote Originally Posted by pmck94 View Post
    Title says it all. Ive been told that the return plenum size has nothing to do with the restriction of the return and that its more based on the pipes that are connected to it. Told to me by a 20 year exp. Installer that has never had anyone say to him to change the retuen plenum and make it bigger.

    Im very interested if plenum sizes do matter.

    By the way, how does one correctly size the pathetic excuses of mixing boxes

  18. Likes rollandsaxton liked this post
  19. #12
    Join Date
    Feb 2013
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    Quote Originally Posted by BBeerme View Post
    Get yourself a magnahelic, U-tube manometer, or one of them newfangled electronic gizmos that measure air pressure. Then, whenever you have the time, measure the static pressure on the return and supply at the unit. Sort of opens up a new world if you haven't been doing it.

    For example, last winter one of our techs was having a problems with a heat pump tripping on high head when in heat mode. He couldn't figure out what was going on. I measured a total of 1.1 or 1.2" water column, and the nomenclature plate said maximum 0.6" wc. Restricted airflow due to ducting.

    I also measured 25*F subcooling. Reduced it by half and the unit stayed running. As far as I know it is still running like that. And, also as far as I know, it made it through the summer just fine.
    I bought myself a fieldpiece dual port manometer with 2 dwyer static pressure probes and on all my jobs i go to i take the TESP, coil pressure drop, filter pressure drop, and the restrictions of the return and supply. Almost, if not all systems i have come to are too restrictive for the system.

    Sometimes the salesmen at our company decide to upsize the furnace and or tonnage and dont bother touching the ducting because the customer says they feel the unit isnt keeping up. With the new system check ups, poor system wont be lasting very long nor will the performance or efficiency be there even though the customer states they have lower electric bills because of it.

    Due to high restriction ive already had to replace capacitors for the blower motors and the units arent even 2 years old yet...

    The worst part is, when doing the temp rise of the furnace as well as the static pressure i cant even adjust the fan speed because it will overshoot the rated temp rise and pretty much trip the high limit.

    Although ive been taking a consensus and customers agree that if the salesman originally added on a ductwork modification to help alleviate the issue and definitely increase performance then they wouldnt be in this situation even if the the cost of it was a bit more.

  20. #13
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    May 2014
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    Sometimes the cost is a lot more. Situations vary. But I'm glad you are checking static pressures. When I first got in the trade, very little concern was given to return and supply static pressures. For the most part, you just had your rule of thumbs that you knew what would work.

    It's a bit different these days. There are a couple of installs that I'm pretty sure were done by the company where I work now, and have low airflow due to ducting. I brought it up to the owners and they weren't too concerned. But those installs are plagued with problems. I do all commercial, but the excuses are always the same where ever you go. No money, time constrains, whatever . . .

    Anyway, I kept on bringing up how the returns are too small for the newer higher efficiency units. And finally I heard one of the owners get on board with it saying to the crew in one of our meetings that we need to pay more attention to the ducting on our installs. So I'm glad they're on board with it now.


    Quote Originally Posted by pmck94 View Post
    I bought myself a fieldpiece dual port manometer with 2 dwyer static pressure probes and on all my jobs i go to i take the TESP, coil pressure drop, filter pressure drop, and the restrictions of the return and supply. Almost, if not all systems i have come to are too restrictive for the system.

    Sometimes the salesmen at our company decide to upsize the furnace and or tonnage and dont bother touching the ducting because the customer says they feel the unit isnt keeping up. With the new system check ups, poor system wont be lasting very long nor will the performance or efficiency be there even though the customer states they have lower electric bills because of it.

    Due to high restriction ive already had to replace capacitors for the blower motors and the units arent even 2 years old yet...

    The worst part is, when doing the temp rise of the furnace as well as the static pressure i cant even adjust the fan speed because it will overshoot the rated temp rise and pretty much trip the high limit.

    Although ive been taking a consensus and customers agree that if the salesman originally added on a ductwork modification to help alleviate the issue and definitely increase performance then they wouldnt be in this situation even if the the cost of it was a bit more.

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