Checking airflow
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  1. #1
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    Checking airflow

    What is the best way to check airflow for cfm? I'm considering an anemometer but not sure yet. I've read that you can check airflow with wet buld and dry bulb temps but not sure how that works.

  2. #2
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    You can place the unit on heat and let it run about five minutes or so to reach a "steady state". Then use the formula CFM= BTU/1.08xTD
    If it is electric heat, you can take voltage reading, measure resistance through the heat strips and use voltage/resistance to come up with amp draw. voltage times amp draw gives you watts. Watts x 3.41 gives you btuh. If it is gas, you need to know the effiecency of the furnace to get more accurate, but if it is a 100,000 btuh input 80% furnace, then you can just use 80,000 btu as the output rating and should get you in the ballpark. If you have the Mfg. info on the coil, you can measure pressure drop across the coil and get real close to by comparing the pressure drop to the literature on the coil. Increase pressure drop equals increase airflow. Hope this helps.
    Bad information is worse than no information at all.

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  3. #3
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    How well does the pressure drop across the coil test work on older systems? I suspect the coil needs to be cleaned before an accurate coil pressure test can be ran?

  4. #4
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    I would be curious to see more possibilities on this. Since Energy Star has become the 2 worst words in the English language this is what I have used to determine airflow...

    First, I always use a transition between all components. Furnace, coil, media guard, etc...and set my blower speed according to manufacturer performance chart to deliver as close to 350 CFM/ton at .5 ESP as I can get. When system is up and running I compare actual ESP to chart to see what I'm getting and allow a max on gas fired furnace ESP of .6 and air handlers ESP of .8 Anything above I start checking at boxes and returns. I also like to see the static on the return to be 1/3 of the ESP.

  5. #5
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    Quote Originally Posted by 54regcab View Post
    How well does the pressure drop across the coil test work on older systems? I suspect the coil needs to be cleaned before an accurate coil pressure test can be ran?
    Yes, a coil would need to be relatively clean to get an accurate reading.
    Bad information is worse than no information at all.

    There are three kinds of people in this world. Those who can count and those who can't!

  6. #6
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    For using pressure drop, to know actual airflow, you need to have a performance chart for something. Either the blower on the unit, or pressure drop chart for the coil both wet and dry.

    Without either one of those, all TESP testing will tell you is .5 or less = good, .5 or more, maybe not good.

    Temp rise is a simple test that gives you a real # to look at. (see surenuff)

    But when/if you use the test, make sure the blower is set to cooling speed, otherwise you will be seeing the airflow in heat mode and not for cooling.

    Performance charts are great, but on service calls, they don't exist.

    A anemometer would be a good tool to have, especially fixing hot/cold areas caused by lacking flow to a particular area.

  7. #7
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    Heat rise is good for ele heat fan coils except in the summer. If its 95 outside and you are troubleshooting, last I want to do is heat the customers already hot house up more. I have a electronic anemometer that measures FPM and then use a chart for R/A type. Its not perfect, but better than not checking it.

    Like anything, its gets easier the more you do it. I remember not checking SH on electrical problem no cools, but now its second nature and shows a dirty coil quickly.l

    I also like using product data and ESP when I can get it,, but usually don't have it.

    As far as wet/dry blub, you got me there, sensible and latent capacities, yes, CFM? not that I know of. Of course a large TD will show you low airflow is a problem.

  8. #8
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    When the ac is operating a change in enthalpy of 6.67 btu/lb across the evaporator coil indicates 400 cfm/ton.


    Sent from my iPad using Tapatalk HD

  9. #9
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    Quote Originally Posted by heatingman View Post
    For using pressure drop, to know actual airflow, you need to have a performance chart for something. Either the blower on the unit, or pressure drop chart for the coil both wet and dry.

    Without either one of those, all TESP testing will tell you is .5 or less = good, .5 or more, maybe not good.

    Temp rise is a simple test that gives you a real # to look at. (see surenuff)

    But when/if you use the test, make sure the blower is set to cooling speed, otherwise you will be seeing the airflow in heat mode and not for cooling.

    Performance charts are great, but on service calls, they don't exist.

    A anemometer would be a good tool to have, especially fixing hot/cold areas caused by lacking flow to a particular area.
    That is sorta the reason I said he would need the literatue on that specific coil.
    Bad information is worse than no information at all.

    There are three kinds of people in this world. Those who can count and those who can't!

  10. #10
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    Quote Originally Posted by Senior1 View Post
    Heat rise is good for ele heat fan coils except in the summer. If its 95 outside and you are troubleshooting, last I want to do is heat the customers already hot house up more. I have a electronic anemometer that measures FPM and then use a chart for R/A type. Its not perfect, but better than not checking it.

    Like anything, its gets easier the more you do it. I remember not checking SH on electrical problem no cools, but now its second nature and shows a dirty coil quickly.l

    I also like using product data and ESP when I can get it,, but usually don't have it.

    As far as wet/dry blub, you got me there, sensible and latent capacities, yes, CFM? not that I know of. Of course a large TD will show you low airflow is a problem.
    He was asking for ways to check without anemometer, as he has not purchased one yet.
    Bad information is worse than no information at all.

    There are three kinds of people in this world. Those who can count and those who can't!

  11. #11
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    Quote Originally Posted by talbot3 View Post
    When the ac is operating a change in enthalpy of 6.67 btu/lb across the evaporator coil indicates 400 cfm/ton.


    Sent from my iPad using Tapatalk HD
    Good answer!
    Bad information is worse than no information at all.

    There are three kinds of people in this world. Those who can count and those who can't!

  12. #12
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    Always here

  13. #13
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    Quote Originally Posted by talbot3 View Post
    When the ac is operating a change in enthalpy of 6.67 btu/lb across the evaporator coil indicates 400 cfm/ton.


    Sent from my iPad using Tapatalk HD
    I would like to understand this a little bit better, would you mind explaining this, i guess i havent ever fully understood enthalpy, and how would you go about figuring the 6.67 btu/lb across the evap

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