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  1. #14
    Join Date
    Apr 2005
    Had a contractor I have been working with out today, I asked him about the 25x10 being too small for a 4 ton a/c. He took out a chart and said that it was adequate for 4.5 tons. Said running returns to both sides of the furnace was for 5 ton and up. I have heard that rule of thumb from other contrrators as well. Didn't mention what you wrote since I don't think it would have been well received. This is hard because I believe what you say. Did you get the 1600cfm by multiplying 4 tons by 400cfm per ton? Where did you get the info saying that I needed a 24x18 return? If I can get a published source I can show it to him without him getting too defensive.
    As far as having the returns on both sides I would be willing to pay for it but I will have trouble getting someone to do it. They will say I don't need it. Do you think that most people with a 3.5 to 4 ton A/C and a spaceguard filter are running high return static?

    [Edited by heetseeker on 06-08-2005 at 08:49 PM]

  2. #15
    Join Date
    Jul 2004
    West TN
    I was figuring up the duct size as if it had 1/2" duct liner.
    25 x 10 duct with 1/2" liner is 24 x 9 inside diameter.
    Using a chart out of the Manual D, that duct is equal to a
    15.6" diameter round pipe.

    Plugging that number into the chart for ducts with liner,
    it gives me 800 CFM using a relatively safe .06 Static.

    Since you don't have duct liner... my numbers are irrelavent now.

    Recalculating the size based on no liner....
    25 x 10 duct is equal to 16.8" Round
    16.8" round NO LINER will handle (uses a different chart)
    1200 CFM at .06 static (pretty safe number)
    1600 CFM at .09 static (which is not bad either)
    1800 CFM at .12 static (I'd have to do a full Manual D before I'd ok that)

    Moving on to the 24 x 8 Drop
    24 x 8 is equal to 14.6" round
    850 CFM @ .06
    1600 CFM @ .18 (Thats too high for me. But a full manual D
    may reveal that it is the correct size. I'd be reluctant to use it though personally.)

    Once you give me some info on the supply ducts...
    I can put together a "VERY GENERIC" manual D

    Typically 400 CFM per ton. So 4 ton would be 1600 CFM.

    Keep in mind...
    All this may still be irrelavant since you havent' had a load calculation yet.
    We're kinda skipping around.

    The best route would be load calc then duct calc

    One last thing....
    Why your guy's info differed from mine.
    Who's wrong? Both of us. Niether one of us did a full Manual D hehe.
    Why does his duct calculator say the 25 x 10 duct can handle 4.5 Tons of air? I've seen duct calculators that don't really agree with Manual D's charts. The charts in the Manual D book cover a variety of ducts; including, round galvanized, flex duct, duct board, oval duct, duct with liner... how does a duct calculator manufacture figure up which one to use? My guess is they get an average or something... but I may be totally wrong.

    Oh yeah... to answer you question about the Filter and High Static.
    Take a peice of the generic blue fiberglass filter and blow through it... its very easy to blow through.. Your cheeks don't hardly puff out at all(low static pressure).
    Take a peice of your filter and blow through it... its harder to blow through... resulting in your cheeks puffing out (higher static pressure)
    So... yes... your filter will result in a higher static pressure reading.

    Do you NEED two filters?
    Not really... but I'm a perfectionist... I like things to be better than just O.K.
    If it were mine, I'd prefer 2 filters... but that's just me.

    Same goes with coming in on both sides of the furnace.
    5 ton units require both sides. 4 ton units just one.
    (I prefer the side opposite the blower though)

    Why do I use a lower static pressure. i.e. the .06 I mentioned?
    Duct sized at .06 static results in slower air flow through duct resulting in less noise. However, given your situation with alot of panning... it wouldnt' matter if the air was going faster... the noise generated by the duct shouldnt' become a factor.

    Our duct systems are a tad oversized just for noise purposes. We do end up balancing the systems and adding dampers to slow things back down though.

  3. #16
    Join Date
    Apr 2005

    Its late and I haven't had time to answer your other questions yet. I hope to give you more info and pictures soon. I feel better about my static . If I get a 4 ton a/c I might drop the return static from .5 to .41 by just increasing the size of the drop. If I get a 3.5 ton I expect that the filter PD would drop further. Thanks

    [Edited by heetseeker on 06-09-2005 at 12:47 AM]

  4. #17
    Join Date
    Apr 2005
    E mailed you more info. Please let me know if you received it. Thanks

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