View Full Version : VAV airflow
10-12-2007, 04:12 PM
I used Alerton Dual Duct VAV controllers to control 12 operation rooms. I made a custom application to use the hot deck airflow sensors for the return air CFM. (return boxes located on another floor) All is well until the air balancer shows up. He says the calibration factor is too high on these return boxes and he reads a different pressure at the pickup tubes on the return VAV and the controller airflow sensor. Personally I donít see a problem as long as I read proper CFM. Who cares what the calibration factor is? Pressure drop -- who cares as long as it is in proportion and doesnít affect accuracy of the CFM reading. What I need is some documentation that I can use showing the affects of tube length etc. I would really appreciate any help.
Are all balancers pot heads?
10-13-2007, 04:29 PM
If the calibration factor is too high, you could tweak the duct inlet size parameter to increase or decrease the measured flow as appropriate. Then the correction factor, for what it's worth, will be smaller. Personally, I've never understood how there can be pressure drop in a static pressure tube with essentially no flow. Pressure drop is a function of friction caused by the movement of air through the tube. No flow, no friction, no pressure drop, there may be some other factor that affects pressure in a long tube. Anyway, I've never seen any data and I'm with you: as long as the CFM are right, who cares?
10-13-2007, 07:41 PM
There are some specifications including LEED functional testing that could require that the automation system displayed values correlate with those findings of a certified air balance report, so your air flow issue can matter.
If the project you are on has specifications relating to system performance validation, check to see if you have yourself one of those projects where you are going to have to do some additional work to get your systems raw data to sync up with what the air balancer is reading.
Otherwise be prepared to clearly show that in at least three different parts of the flow curve, your CFM sensor readings jive with the air balance techs readings.
I always taught that our techs to get together with the air flow tech first to go over how our system was calibrated and how our system read CFM and then did a zone test or two so both techs were on the same page. Getting into a tiff with an air flow balance company is not good business in the controls or mechanical industry.
With LEED and other "system validation" performance specifications growing in popularity, it would be good to get a understanding and some procedures in place to help yourself with this issue. It will be showing up on more and more of the controls projects in the future.
I welcome it, because it is weeding out the "slam and go controls" contractors that don't care if the systems don't work anything better than just "OK".
Our industry brought this second checking upon ourself's, because so many big controls projects never realized the energy savings or quality control that the owners were promised.
Anyway, good luck.
10-15-2007, 03:26 PM
VAVs have hot wire mass airflow sensors
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