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Transfer Grille Sizing

I installed a Spacepak ESP-2430V (2.5 ton, 550 SCFM) high velocity air conditioning air handler in my basement.  The main return is in the basement and the unit serves the basement and the first floor.  12 out of 15 total outlets or 80% of the outlets are on the first floor.  To get enough return air flow from the first floor to the basement we are going to install a transfer grille in the stairwell.  The main return grille is 14.25" x 25" (356 sq. in.), the main return plenum is a 16" x 12" oval (151 sq. in.) and the Spacepak return inlet is 15" round (177 sq. in.).

How many "free area inches" should be in the transfer grille (which will pass about 80% of the return air)?

Thank you in advance.


  • Techman
    Techman Member Posts: 2,144
    Transfer grill

    The basement has only 3 supply outlets ,but, it also has 100% of the return air?! O.K.  Each ton of cooling  is equal to about 250-350 cfm of air.So you are looking at 625-875 cfm total, so, 80% of that is ???.Why can you not get a 2nd return from the 1st floor? 2 returns is what is recommended.I would size the transfer grill for 100% [ if possible]to prevent "back pressure" from building up on the 1st floor.  
  • JohnG_3
    JohnG_3 Member Posts: 57
    Transfer Grille Sizing

    Thanks.  At this point it is not feasible to add a second return from the first floor.  So the transfer grille will have to suffice and if that doesn't work I can always leave the basement door open.

    I ended up choosing a transfer grille with a free area of 80% of the size of the main return grille.
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