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The K-Factor: Calibrating VAV Box Airflow


Why Do We Care?

By Lyn Gomes, P.E., CCP, LEED AP, CLCATT
Project Manager, Director, BCxA International Board

Airflow reported at the variable air volume (VAV) box is actually a measure of total pressure. It gets converted into an airflow by a multiplier, commonly called k-factor, that is generated during test and balance (TAB). K-factor is function of “duct area, geometry and dynamics of the pitot tube (flow measurement device),” according to www.hvacbrain.com. This multiplier can also be called flow multiplier or pickup gain and is often pre-programmed by manufacturers into the controller.

We care about k-factor for several reasons:

  • > Ensures the VAV box performs at the EOR’s capacities (i.e. minimum and maximum airflows), satisfying OA requirements and cooling/heating loads
  • > Maintains space pressurization (especially in the case of labs with volumetric airflow tracking)
  • > Evidence that balancing has taken place

Correlation of airflows to a pressure reading is usually done at a single airflow (often cooling maximum). This is called single-point calibration. Single point calibration is cheap and efficient for spaces that don’t require tight control or have an unbalanced return airflow (i.e., offices with plenum return systems). System function is not compromised significantly if the airflows at low levels are inaccurate. The k-factor can either be adjusted, or an additional calibration factor can be piled on top of it.

Because balancing is so important, and it is often not done (or done poorly), we check during VAV functional testing that k-factors have been entered into the control system. As noted earlier, many VAV controllers have pre-programmed k-factors, each for a different box size. However, installation can make them moot. For example, a k-factor for a 4” box installed at the end of a long duct run will be different than a box installed close to the main with a 90° elbow immediately before it. (For retrocommissioning projects, k-factors could be polled and validated that they are different.)

When checking, it is important to maintain perspective. For labs that maintain pressurization with an offset between the supply and exhaust airflows, single point calibration is usually not enough – you must check for multiple points of calibration because inaccuracies in actual vs computed flow can switch the space from positive to negative. Care should be taken during design review to require multiple point calibration (in the TAB spec) and a controller that will support multi-point calibration (in the controls spec). As the author’s experience at a municipal coroner facility demonstrated, calibration at a single point can lead to inaccuracies at low airflows. When the lab VAV boxes went to minimum, errors in calibration made the decomp autopsy room become positively pressurized, spreading noxious smells throughout the building! Unfortunately, the controller was incapable of multi-point calibration. The TAB tech had to spend quite a bit of time with a trial and error calibration at different airflows to find the best performance.

For further reading on the drawbacks of single point calibration, see Ron Simens’ (Facility Dynamics) excellent paper from the 2013 National Conference on Building Commissioning. The author observes, “inaccuracy of VAV airflow is one of the last items questioned until noise or temperature extremes in the zone force the issue to the forefront. Unless Cx professionals understand the causes and costs of this phenomenon and how to avoid the issue, other HVAC professionals are unlikely to expend the effort necessary to effectively address and resolve these problems.”

In summary, verifying k-factors during functional testing can provide an additional measure of quality that balancing has been done properly. Single point balancing is usually sufficient for commercial office spaces. For spaces with volumetric airflow tracking (e.g. labs), work needs to done during design – multiple point calibration needs to be specified in both the TAB and control equipment specifications. If it is not, be prepared to be onsite at the end of TAB in order to verify VAV box calibration in the operating range. A little bit of effort up front always prevents significant work later!

  1. Timothy Heinrich
    Reply

    I have been Balancing VAV’s since the pneumatic days when I was a Steamfitter & that was when the directions came in the box with the Controller. I basically Balanced using the Flow curve on the VAV stamp and with a Monometer across the High & Low Poly Tubing Tees provided. I would Zero the box, check the Duct Static, then resume to check High Flow, then Min Flow, back to High and check Min Flow again. That was done until I started working for Facilities in the Howard County Public School System, where I bought our Techs Dwyer Digital manometers at 0-4″wc range and here we have a Hood to set each Diffuser Properly. What I have learned since becoming a Construction HVAC Project manager, is that AABC, who certifies TAB contractors have never done Balancing the way I was taught at the Steamfitter Apprenticeship after finding so many issues I couldn’t understand. I read their new procedures and found an old scanned version of their old procedures and they are the same. VAV Balancing has only ever taken place at the diffusers with a Flow Hood & now with DDC Controllers, it is done the same way. I have been told over and over that the K-Factor is the Correction Factor, which means that the Air Coming out = the VP to CFM Calculation in the Controller, but that is very flawed. The reason is that if there were a 4″ cut in a Flex Duct, the TAB Contractor would never go into the ceiling to find it, but rather would simply add a K-Factor and go on his merry way. The best way is to Balance at the Flow Ring Hi & Low Tubes, since they are what controls the box, verify that the Hi & Low tubes are connected tightly & on the right ports (don’t assume), then Balance to the Flow Curve on the Sticker for the Size VAV you are Balancing. Once complete, measure your Diffusers which should all add up correctly unless there is a open flex or another issue. Make corrections and then you should find that there is no need to change the K-Factor. Above is the fool proof way to Perform VAV Balancing, yet every TAB Contractor I see needs a Lesson in Balancing, since they really no longer Test & Adjust anymore.
    Thanks for reading.

  2. Kevin Thurston
    Reply

    Fantastic article Lyn. I will be sure to pay close attention to the need for multi-point calibration on my jobs. We deal mostly with schools and commercial office buildings so this hasn’t been a significant issue. What I have noticed is that when an incorrect size of VAV box is installed, but approved by the design engineer after the fact, the balancer usually isn’t aware of it and calibrated using the wrong sized box.

  3. Ryan Lean
    Reply

    Thanks Lyn. You’d be happy to know that I googled VAV box K-factor to get some back-up for a discussion I’m about to have, and your article was the first to appear. Well written article. Appreciate Timothy Heinrich’s comments as well. Nothing replaces getting your head above a ceiling to ensure the installation isn’t compromised. We waste too much time in this industry putting band-aids on issues that shouldn’t have to be fixed.

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