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K-factor and airflow measurement

user-6356169 | Posted in Mechanicals on

Can anyone shed light on how to determine your “K-factor” at a given point in a duct? Reason I ask …

We’re installing two Fantech Iris Dampers, on main trunks from ERV (exhaust from and fresh air to the house.) These Dampers, very helpfully recommended by John Semmelhack here on GBA, have integral pressure taps that enable you to measure differential pressure with a manometer. The idea was to be able to regulate and balance airflow by adjusting these dampers. As I look at the manual for these dampers, I see that in order to calculate airflow from this pressure measurement, you need to plug in a value for “K-factor”. But how do you determine your K-factor? These dampers will be placed in short sections of straight 6 or 8 inch round galvanized duct (at specified minimum distances away from elbows.) Is that enough info to get a ballpark figure for K-factor?

My HVAC installer will need to plug some figure in here, and I’d like to double check on his calculations. But also in the future, we might want to adjust airflow ourselves. I knew we’d have to purchase a manometer, but what about this K-factor?

I tried to find an answer to this on some of the online HVAC forums, but everything seemed to be related to commercial HVAC controls where your K-factor would be provided by the manufacturer.

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  1. GBA Editor
    Martin Holladay | | #1

    As far as I can tell, "The source of the K factor for this type of measurement is the manufacturer of the diffuser or flow station. These manufacturers specify the K factor that must be used when making flow measurements using the pressure taps. Several K factors are usually supplied, depending on the pressure and flow rate measurement units that are being used."

    The quote comes from this document: Use of K-Factors with VelociCalc Plus Air Velocity Meter.

    More information from a different source -- a web forum (

    Velocity = ((2* vp)/density)^0.5

    Volumetric Flow = Velocity * Area

    so ... Volumetric Flow = K * (vp/density)^0.5

    K is representative of the duct area, geometry, and dynamics of the pitot tube
    Is not a linear relationship but for Air flow in a duct it kind of is linear over the ranges we are interested in for say, a VAV.

    Different manufacturers of VAVs have different ways of calculating vol.flow from VP ... some have Area and a separate K for the pitot/duct...some simply have a single factor to encapsulate both. Some VAVs use vol.flow as the controlled variable, others simply use Velocity as a controlled variable and then present Vol.flow at the front end by multiplying by duct area.

    so, K depends on a few things including how the VAV software is written.

  2. user-6356169 | | #2

    Thanks Martin, I think I was looking at some of the same HVAC forums. I'm sorry to have troubled you because I think I found my answer. But it was actually your summary "K is representative of the duct area, geometry, and dynamics of the pitot tube" ... that got me thinking in the right direction. I guessed that the pitot tube must be integrated into this damper, and since the dampers are for different size duct wouldn't each size damper have it's own K-factor? Actually it's not mentioned in the manual for the damper but when you look at the damper itself, as shown here ...

    ... you see that the K factor is determined off a scale on the damper itself. There's a range there that's determined by how far you open up the damper.

    Again, sorry for the trouble, but maybe it'll help the next person ...

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