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Thread: Return Air

  1. #1
    Join Date
    Jun 2011
    Posts
    2

    Default Return Air

    Can someone tell me how much return air is needed for a 5 ton Air conditioner sytem? I currently have a 20 x 30 filter grill. The chase that runs up into the attic is 27.5 x 19.5.. The blower is set on 2000 cfm.

    Can I add another return filter grill on the other side of the wall next to the current return air grill, or should I see about adding another 20 x 30 filter grill in another part of the house? If so any tips on mounting a return box in the ceiling?

  2. #2
    Join Date
    May 2011
    Posts
    251

    Default Re: Return Air

    5 tons needs 2000CFM. You don't want to exceed 500fpm on a return duct. So you'll therefore need 4 square feet area of return air duct.. or 576 square inches of free air grill area. I think most grills only have about 80% free area. The rest is obstructed by the louvers. IF it's a filter grill, you don't want face velocity to be over 400fpm, so in your case, you'll want another return grill.... or if the equipment is older, you could verify if you actually need a 5 ton unit. A 20x30" is adequate for a 4 ton unit and a 120k BUT furnace.

    The next problem however is hat the chase is a little small to. Again, better sized for 4 tons. If it's not a newer vairable speed furnace or air handler, you're probably not getting 2000 CFM because the static pressure is too high.

    But pt answer the question, yes you could just add another filter grill on the opposite wall of the same chase.

    But the equstion you might ask, is do I need 5 tons. IF hte unit is more than 15 years old, it might be worth having a load calculation done. OR ask your self thsi question, on the hotest day of the year, does the unit just barely keep up or eve fall behind just a little? IF not, it's probably oversized.
    Last edited by motoguy128; 06-12-2011 at 04:10 PM.

  3. #3
    Join Date
    Jun 2011
    Posts
    2

    Default Re: Return Air

    Thanks very Much for your help

    I'll see about adding another return on the opposite wall. One contractor wanted to put a new filter grill on top of the old one? I need 5 tons. This house is 2699 sq ft and on the hottest day it will barely keep up. I'm 120 miles south of Houston Tx where it gets quite hot in the summer..

    I was considering a new system and the girl that came out did a load calc and I actually need a little more than 5 tons..lol

  4. #4
    Join Date
    Jul 2009
    Location
    Northeast
    Posts
    661

    Default Re: Return Air

    If the equipment is set up to operate at 400 cfm per ton, as it should be, then reducing cfm 50 to 75 cfm per ton under high humidity conditions will have the effect of taking longer to satisfy the thermostat and taking more moisture from the air due to a colder evaporator surface temperature

    A 30 x30 return grill is good for 2850 CFM's but the return duct also has to be sized correctly, not just the grill.
    Last edited by Sten; 06-12-2011 at 05:20 PM. Reason: more info

  5. #5
    Join Date
    Nov 2011
    Location
    boston,ma
    Posts
    114

    Default Re: Return Air

    I think I have the same problem with not enough return air flow. I had a carrier infinity 59k btu 3-stage ics furnace and 2-ton ac installed last year and kept having the change filter message pop up everytime we use a,c or high heat is activated. All brand new ductwork and My only return duct is 20x6 runs 15 feet to common hallway and the grill size is 20x8. The suppy is 14x8 runs 10 feet and goes to 7 six inch flex ducts with 4x10 otlets and grills.The house is only 1,080 sq. feet.The original installer has left the state, and now the new contractor i hired has found a few issues, he wants me to add another return off the return plenum 8"pipe up inside wall to second floor hallway, where there are 2 bedrooms to a new 8x14 grill and says to only use a merv 8 filter not the carrier merv 11 that i bought 4 off the original installer. he says the staic pressure .89 is to high on high speed and i will burn out the esc blower motor, he has also turned the cfm's on the infinity controller down to 800, it was at 950. He says its ok for now but I should do the return duct before the next cooling season, does any of this sound correct? the original installer states he does theses every week and has not had any issues for the last 15 years and my 1 year install warranty is up, I paid a lot for this system i cannot believe it would not be installed cooectly. any advise would be much appreciated. Thanks!

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