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how do you determine you have the right cfm on a ac airhandler? is there a simple way or a tool rather than all these complicated formulas i have seen.so you need proper air flow established b4 you adjust chge etc, what is a practical way of doing this. this has always been a guessing game for me!  thx


  • Techman
    Techman Member Posts: 2,144
    edited July 2012
    Simple Way?

    LOLOLOLOLOLOLOL!  With you my friend!
  • meplumber
    meplumber Member Posts: 678

    Balometer and a calculator. Nothing simple about it.
  • don_9
    don_9 Member Posts: 395

    As a tech in the field you use a manometer and measure total static pressure across the system then you check the manufacturer fan data and find the cfm for the total external static preesure you have measure.this will tell you how many cfm the system is putting out.

    Example.the system returm static is -.46 iwc The supply static pressure is +.38 =

    a total external static pressure of .84 iwc.So say we have a 2 ton system that require 800 cfm and we check the fan data from the manufacturer at .84 iwc that airhandler is only putting out 518 cfm with the blower speed set at high.As you can see the unit has low cfm.Remember most constant speed fan are only rated at .50 iwc.
  • Techman
    Techman Member Posts: 2,144

    Paul, is this a new install/startup? Or is this on a typical service call? Are you there to check air flow only or are you there to check how the "system" is running?
  • Paul_69
    Paul_69 Member Posts: 251
    cfm test

    just a general question on checking existing equipmentwith no apprent problems.i know a new innstall will have exact specs. on setting it up with charts etc.
  • TonyS
    TonyS Member Posts: 849
    Hot wire Anemometers

    were invented in the 80s to figure air mass for fuel injection. They are on almost every car today. The technology is well proven , dependable and has now been adopted with the math to give you direct readout instantly of any size duct you input. Not only does it tell you cfm in the trunks but also on any branch. They are under 3 hundred bucks and after you have one you will be telling the kids how (back in the day) you had to go to every room with a big bags. Try one you wont be sorry.
  • Techman
    Techman Member Posts: 2,144
    Simple CFM

    General service. Simply put your gauges on.The lo side temp of the P/T chart will KindaSorta let you know if there is a "problem". If the P/T  temp is 30* you have a problem. If the house is 80* and the P/T temp is 34* you have a problem. If the superheat @ the comp is low, you have a problem. At the supply registers,find the side of the register that has the most air flow and tape a thin strip of paper to the register and watch it flap around.You will see which rooms have an air flow problem.Then you can't complain about those dreaded formulas!.
  • don_9
    don_9 Member Posts: 395
    Not always the equipment

    Checking cfm is or should be priority in most cases being most distribution system are not size properly.And when you have outdoor temps with heat index that take your system beyond design load then its good to have the numbers to support you when you tell the customer their system is doing all it can do.

    We work on a 15 ton system this week that the customer complained he could not get it below 85 degree and airflow was a issue you can see where people has add extra ducts here and there along with fans install in the duct to get air to room that had none.

    The distribution system was design for a fifty ton unit.they replaced it with a 15 ton in 2004.The customer sweared it work good when they first put it in.So right off the back we knew he had airflow, but no velocity the air was not being push back to the return.

    We went up on the roof measure out pulley to get our rpm.then we measure the total static pressure.we nail down cfm thru the unit and of course no problem there.We measure our wetbulbs and then converted it to our enthalphy numbers.Ran the numbers and the system was putting out 177.862 btus.close enough.Sir your system efficency is at 92% and your problem is its over loaded and the ducts are to big for the fan drive you have in the equipment.Not to mention your outside air damper is close that is putting your building into a negative pressure.

    As always the reason for replacing with a 15 ton unit.Not enough money in the budget and the guy that came out told me that a 15 ton would work with the sq ft that we have.block building with no insulation and big single payne steel frame windows on a flat black roof.

    In short its not always a equipment problem and without checking airflow and running the numbers all you are doing is guessing.
  • Techman
    Techman Member Posts: 2,144
    edited July 2012

    I agree w/ Tony S. & meplumber & don. My suggestion get's you into the ballpark ,then their way gets you to homebase! A problem anywhere in the system  will usually  show itself at/on/in the compressor and it also ,usually, shows itself on the gauges. Recognizing that the gauge readings are "not what they should be" is one thing , then finding out/figuring out what IS wrong IS the fun part .Then being allowed to straighten out the goofup's IS even funner! That's what get's my professional rock's rolling!
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