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CFM estimate based on grill/filter sizes?

Pete_18Pete_18 Member Posts: 197
I have a split return & filters that are 14x20 and 14x25 (take off one inch in each direction for measuring the grill size). I need my returns to support 1000cfm (250cfm x 4 ton) to meet the spec my my AC. I realize there are many factors that go in to cfm and I need to find a competent AC company (if mine is not) to actually check the cfm, but I am curious if all else is perfect, if these grills are sized large enough where that is possible. I read somewhere that I could take the sq footage (14x20)+(14x25)=630 and divide by 200 (not sure if this is 250 in my case?). If I use / 200 calculation, I only get 3.15ton. Does this mean that my returns are undersized? Is there some other calculation I should use to determine if these two returns are large enough to support 1000cfm assuming other conditions are not problematic?

Comments

  • TechmanTechman Member Posts: 2,144
    You have a HiVelocity system, like Unico or SpacePak?
  • bob_46bob_46 Member Posts: 813
    With four tons of cooling you need 1600 CFM .
    bob
  • Steve MinnichSteve Minnich Member Posts: 2,541
    To size filters, you divide the cfm (1600 for 4 ton) by 300 fpm (recommended velocity) which gives you the recommended filter size in square feet.

    1600/300 = 5.33 sf. This is what you need.

    630/144 = 4.375 sf. This is what you have.

    You're a bit short.

    The size of your return duct is every bit as important.
    PHC News Columnist
    Minnich Hydronic Consulting & Design, LLC
    https://heatinghelp.com/find-a-contractor/detail/minnich-hydronic-consulting-and-design
  • Pete_18Pete_18 Member Posts: 197
    edited June 2015
    Unico is 250cfm/ton (so 1000 cfm is what it needs).

    They measured and I'm getting 980cfm, they didn't seem too worried about the 20cfm. Turns out that Unico may have nailed this problem, they recommended they double check the TXV to ensure it is a 4+ ton TXV, and turns out it is a CBBIZE-3 which only goes up to 3 1/2 ton. They are ordering a 4 ton TXV. Hopefully this finally nails my low ambient AFS trigger problem.
  • TechmanTechman Member Posts: 2,144
    What AFS problem?
  • Pete_18Pete_18 Member Posts: 197
    edited June 2015
    In low (high 50s, low 60s outside) ambient, AFS was triggering and shutting off the condenser and causing the house to not cool down properly. I had them put on a low ambient control (ICM326) and it made no difference. For example, 57F outside, house was 68, tried to bring the house down one degree and within 15m condenser shut off completely. Seems like Unico guy nailed this one because the TXV is not the right one for a 4 ton, only goes up to 3 1/2 ton. Hopefully I will finally have some good news on this nagging problem once they swap this.
  • Steve MinnichSteve Minnich Member Posts: 2,541
    I've never seen a high velocity system that doesn't cycle on the AFS anywhere near those temps.

    What are the duct sizes going to the split returns?
    PHC News Columnist
    Minnich Hydronic Consulting & Design, LLC
    https://heatinghelp.com/find-a-contractor/detail/minnich-hydronic-consulting-and-design
  • TechmanTechman Member Posts: 2,144
    edited June 2015
    I agree w/ Stephen Minnich.

    Now, I think you will be back here again ,after the TXV is changed along w/ the LLFD and have them put in a SightGlassMoistureIndicator. Then unless the TXV is really a "special" TXV then that little bit isn't going to matter to much. Take a 1.00 GPH oil burner nozzle @ 100psi, the nozzle delivers 1gph. Increase the press up to 150psi and you get a lot more oil ( 1.35 or 1.25 or so , I'll get my chart if you want me to)being pushed thur the nozzle. The TXV is the same . A 65*ambient ( KindaSorta 270psi)then a 95* ambient(KindaSorta 420psi)

    So, I think if you think you are haveing a AFS problem, you are actually having an AFS that is functioning properly.So, I think that you need an ODAir sensing control to bypass/parallel the contacts of the AFS at those lower ODA temps only. DO NOT BYPASS THE AFS UNDER REGULAR SYSTEM OPERATION, I think.

    I also think that the system will not require very much run time so there will be enough "off cycle defrost" time to defrost the frost that the AFS was trying to tell you about.I think.
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