Titus Timeout Podcast - Sizing VAV Boxes

[@ponkrapujari679]

god bless you Jenny

[@wile.2688]

Great info. Thanks!

[@thejibberjabberman]

Good session. Would Titus consider posting a podcast on how to size a air terminal for a space? Would be a good session to go hand in hand with this one?? Good session for beginning techs, and refresher for seasoned engineers.

[@stevelynch6394]

For those of you wondering why your numbers don't agree with this video, be advised that the inlet area (subsequently used to evaluate CFM) was incorrectly calculated at approx. 3:40 in the podcast. Inlet diameter was plugged in instead of radius using A=(pi*r^2)/144. To use diameter to calculate inlet area, A=(pi*d^2/4)/144 needs to be applied instead.

[@TitusHVAC]

Thanks for your comment. I went back and reviewed this podcast that was originally posted in 2013. I believe that your observation was made in error, because the presenter gave an example of an 8” diameter inlet and correctly plugged a 4” radius into the equation resulting in an inlet area of 0.349 sq ft, which is correct. While reviewing the podcast I found a number of other issues that need to be corrected or updated. For instance the presenter didn’t explain that the inlet sensor produces an amplified velocity pressure signal and our recommended minimum airflow rates are now based on a 0.01 in wg sensor - not 0.03 in wg. Also the presenter incorrectly stated that ECM efficiency decreases as motor speeds decrease. We will plan to pull this podcast and I’ll create one or two new podcasts to replace. - Randy Zimmerman, Chief Engineer

[@nramos137]

hello, I enjoyed your video, but using .03 as a starting point I dont come out with the same minimums as you did. Are you using the square of .03?

[@TitusHVAC]

Hello, it is the square root. The equation is shown in red around 2:30 in the video. It is V = 4005 x sqrt of Vp.

[@Jeff-Love-HVAC]

@@TitusHVAC i believe the question stems from using the data presented and the formulas presented and being unable to produce the same minimum cfm's as listed at 2:52 ... For the 4" diameter (r=2") opening, 4005 x sqrt of 0.03 = 694 fpm ---> 694 x pi x (2 squared) / 144 = ~60.5 cfm not 45 cfm. Also, as a check, the 694 fpm (V) matches the charted value for Vp of 0.03 (the relationship between V and Vp). The rest is basic math. What am I missing? Thanks!

[@TitusHVAC]

@@Jeff-Love-HVAC It’s a little complicated, because this podcast is quite old. At the time this podcast was created, we based our minimum recommended airflows on an inlet sensor signal of 0.03 in wg. Please note that this signal is an amplified velocity pressure signal produced by our AeroCross sensor. It is not velocity pressure. Velocity pressure has a K-factor of 4005 fpm, but the AeroCross is closer to 2500 fpm (varies slightly by size). A few years ago we revised our minimum recommended airflow down to an inlet sensor signal of 0.01 in wg. Our current minimum airflows are therefore based on this sensor signal.