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Converting Knots to CFM

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Rodney Summers
Rodney Summers Member Posts: 748
in A-C, Heat Pumps & Refrigeration
I want to measure CFM from each of our poorly installed high velocity air conditioning system's 2" dia. supply outlets and then compare the aggregate CFM to the air handler's rated CFM. The purpose is to detect leakage / inadequate airflow.

I purchased a TurboMeter, an electronic wind speed indicator made by Davis Instruments. It can read wind speed in knots which is equal to FPM x 100.

How do I convert knots to CFM? I know that the TurboMeter is what high velocity AC firms use for this purpose. Please note that I am a homeowner and not a contractor.

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  • bob_50
    bob_50 Member Posts: 306
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    Knots to CFM

    How about furlongs per fortnight. Cross sectional area of the duct in square feet times average velocity in feet per minute=CFM.
  • Empire_2
    Empire_2 Member Posts: 2,343
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    Knots to CFM

    For all round outlets multiply knots by 2 to obtain cfm. For the slotted outlet multiply reading by 4 then add 4 to get CFM. Example: round outlets with knots reading 16.3 has 32.6 CFM. Normal range is between 20 and 40 CFM. Slotted outlets with 7.1 knots has 32.4 CFM. Normal range is between 15 and 35 CFM.

    There is a meter similar to what you have that will automatically convert this for you. Hope this helped. One additional thought,..before you even check these check the external static pressure in the supply plenum,...should be around 1.5" of water.

    Mike T.
  • don_163
    don_163 Member Posts: 67
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    Wow

    Did I read that right 1 to 5 inches?No wonder they only need one return.
  • Empire_2
    Empire_2 Member Posts: 2,343
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    Sorry don,......;-)

    Was early in the morning and eyes and fingers weren't working yet....;-) I corrected it just for you... Actually it's 1.8" max and no less than 1.0". Recommended is 1.5" wc

    Mike T.
  • don_163
    don_163 Member Posts: 67
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    i knew that.

    Just messing with ya.Good Morning!

  • Empire_2
    Empire_2 Member Posts: 2,343
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    :--)

  • S Ebels
    S Ebels Member Posts: 2,322
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    Sounds like

    Sounds like you have a hi velocity A/C system ala Unico or Space Pak. If so, your manual will provide you with info on setting the outlet shutter to acheive correct CFM based on amperage draw. PROVIDED of course that your installer used the correct number of outlets per ton of cooling. Find your manual and look it up. It'll tell you how many outlets you need along with how many additional you need to compensate for long runs.

    I was in a new house that we had done the heating in checking out the boiler. I noticed the condensing unit cycling on/off frequently and asked the HO about it. (The A/C was installed by the builder's regular F/A guy) The HO stated that it had cycled like that since new and he thought that was normal. In the next breath, he said the A/C couldn't keep the house cool. I was curious, always wondering how our competition does their work, and asked if I could look in the attic.

    Jump to the good part here. The Unico AHU/coil assembly was on the very west end of the house which is approx. 75 ft from end to end. The installer ran about 25 feet of 9x9 Unico duct off the air handler and terminated it. From there he had used the required number of outlets per ton (5) but failed to compensate for the ultra long lengths of 2" supply needed to get to the far end of the house. He had the 20 outlets recommended by Unico for a 4T A/C but most were so long that he actual airflow had to be less than half what it should have been. When you go over a certain length (20'?) you have to add additional supplies to compensate. By my calculation at the time, the guy should have had 34 outlets to achieve the right airflow. Could be this is what's wrong with yours. I told the HO about this but never heard how it turned out.
  • William Faust
    William Faust Member Posts: 168
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    1.0 to 1.8\" WC

    Thanks to those who explained that knots x outlet area in s.f. = CFM.

    What does a WC figure actually represent in layman's terms? Pressure? What would be the reasons for an unacceptable reading if correct dia. tubing was used for line sets?
  • bob_50
    bob_50 Member Posts: 306
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    W.C.

    WC is inches of water column. It's an accurate way to measure small pressures. 27.72 inches of water column equals one pound per square inch. The 1.0 to 1.8 inches WC is a pressure inside the duct not the line set.
  • Empire_2
    Empire_2 Member Posts: 2,343
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    S Ebels...

    I have seen that to where the trunk line is really short. Especially with unico, they would rather you run more trunk line as close to termination as possible. 10' @ 40 cfm per outlet and 10% reduction for every 5' over 10'.

    My .02

    Mike T.
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