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  1. #1
    Join Date
    Jul 2007
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    return/supply cfm ratio

    Is there a ratio or difference between return cfm and supply cfm on a package self contained unit. I have a 5 ton unit pulling 430 retrun cfm. i didnt check supply yet. I was gonna til I noticed as I was connecting thermocouples and gauges to get readings, compressor wasnt staying on. Call for cooling at unit was gone also. Turns out the accustat was like 4 ft underneath the frigging diffuser. What moron would put it there. Unless its really hot in there, its gonna short cycle everytime. Im goin back now to move it first, or atleast jump it to get system goin and check readings. But I really dont feel the air movement infront of that evap like I do on other similar units. Is there a ratio of supply/return cfm?
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  2. #2
    Join Date
    Jun 2006
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    Richmond, VA
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    It should be 1 to 1 if there is no fresh air vent or economizer.
    Give a man a fish, he will eat for a day. Teach a man to fish, he will eat for a lifetime.

    Give a man a capacitor, doesn't know what to do. Teach a man to install it, now he knows everything.

  3. #3
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    Jul 2007
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    New Jersey
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    Maybe I got a pinched flex supply line or something. But after i got it running, I noticed pressures were only 50/150 with 2* subcooling (txv) I started to cut water back (watercooled) and it rose up to 200. i shut it down until the plant gets up and running and the water isnt so cold. Its better to troubleshoot under normal conditions.
    “If I have seen further than others, it is by standing upon the shoulders of giants.” ~ Sir Isaac Newton


    I'm learning all the time, the tombstone will be my diploma!!!


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  4. #4
    Join Date
    Aug 2003
    Posts
    422
    Depends on if the unit is single fan or dual fan. (supply fan only or supply and return fans)
    Also, if dual fan, what minimum OA and minimum EA design is at full mechanical cooling.
    If single fan, what minimum OA is.

  5. #5
    Join Date
    Jan 2010
    Posts
    34
    That cfm on the return side is way low....sumthin is up

  6. #6
    Join Date
    Feb 2005
    Location
    In a van by the river
    Posts
    618
    It's only gonna blow out what it takes in. Weather it's coming from an outside air damper, return grill, unsealed ducts, etc it doesn't matter. If you're blowing 500 cfm, your "sucking" 500 cfm. You may not be getting all 500 at the return but you are getting it. So, I guess the answer to your question is definately a 1 to 1 ratio.
    ## + years in the field never made you a know-it-all This industry is far more diverse than you are

  7. #7
    Join Date
    Aug 2003
    Posts
    422
    In keeping with the general "rule of thumb" that 400 - 500 cfm expected per ton of cooling then, 430 cfm return would be low, or about 21% of the estimated 2,000 total cfm for a 5 ton unit (unless you are in an economizer cooling mode)
    Are you sure that the 430 cfm is actually return air and not minimum OA? (as 21% would seem more realistic for minimum outside air)

  8. #8
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    Jul 2007
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    New Jersey
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    Well 2 things...first, somebody told me i was using my anemometer wrong. It has a mode for CFM and another for Velocity(fpm). I know the vel. formula is vel. x area sq'. = CFM....but why would I have to do any other math involved when on CFM scale? Thats what they said... they said i have to take an average reading in CFM and multiply by area sq'????? This is the meter i have: Amprobe TMA10A.........the other point is, when this plenum was installed...the fan blows up and the two flex supplies comin directly off the top, arent centered in the plenum. The blowers in the middle and the flex lines are , 1 in the middle and the other all the way to left. Not sure how much that would effect it though.
    “If I have seen further than others, it is by standing upon the shoulders of giants.” ~ Sir Isaac Newton


    I'm learning all the time, the tombstone will be my diploma!!!


    Did you ever stop to think, and forget to start again?

  9. #9
    Join Date
    Jun 2006
    Location
    New Jersey
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    Quote Originally Posted by Tommy1010 View Post
    Well 2 things...first, somebody told me i was using my anemometer wrong. It has a mode for CFM and another for Velocity(fpm). I know the vel. formula is vel. x area sq'. = CFM....but why would I have to do any other math involved when on CFM scale? Thats what they said... they said i have to take an average reading in CFM and multiply by area sq'????? This is the meter i have: Amprobe TMA10A.........the other point is, when this plenum was installed...the fan blows up and the two flex supplies comin directly off the top, arent centered in the plenum. The blowers in the middle and the flex lines are , 1 in the middle and the other all the way to left. Not sure how much that would effect it though.
    You did measure the area of the duct and program it into the anemometer, before you started checking cfm......right. If you didn't and just switched it to cfm......the default is 1 sq/ft.

    For example, you have a 20x25 return. You have to program 3.59 into you anemometer. You then switch it to cfm and you would read 1487cfm instead of 430 cfm.

  10. #10
    Join Date
    Jul 2010
    Posts
    11

    Return too small??

    I have a 2250 sq ft home two story. I installed a York Affintiy modulating furnace and 5T 2 stage compressor. Very happy and it works great. Saving a lot of money on gas and electric, However, i notice the above foor 3-4 degree difference. When i calculated both retrun grills, one upstarts the other down stairs, i come out with 624 sq inches, or 4.3 sq ft. The return downstairs is a 14" duct and the upstairs is a 10".

    I have been told that my return size should be 1000 sq inches, or 18" on the duct size. The cooling difference from DS return (76) to DS supply (52) is 24 degrees. I think that is fairly good, but i am concerend withthe upstairs. The return and one supply line comes up between the two floors and runs along a soffet on the edge of the house. I am not sure if that soffet is large enough to add a larger return for upstairs. Also, there is an 3 ft crawl space but with the trusts, wires, pipes and ducting, it would be a bare to get to. I'll try to attached some pictures. I am just looking for some idea or thoughts.


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  11. #11
    Join Date
    Jul 2007
    Location
    Delaware
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    Quote Originally Posted by Tommy1010 View Post
    Is there a ratio or difference between return cfm and supply cfm on a package self contained unit. I have a 5 ton unit pulling 430 retrun cfm.
    You have bigger problems. You need to be pulling another 1500 cfms.
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