Airflow monitoring in VAV systems

VAV systems need a way to measure outdoor airflow under varying load conditions. Typically that is done with an airflow station, directly measuring outdoor airflow. If you have an alternative approach and you can demonstrate it provides the same result it should be acceptable. Pre-LEED, projects often only monitored damper position to maintain a fixed percentage of outdoor air. The problem with that approach is that if the VAV system was operating at 50% load, you'd also end up reducing ventilation by 50%. So you need to make sure your approach adjusts and is accurate at part load.

Hi Paula, The HVAC engineer of a project i'm working on wants to specify a pressure difference meter for the AHU. Did this work for your project in the v4 version?   Thanks ! 

Hi guys, gona bring this topic back up because it is still a problem for projects in Europe. We have a VAV system with 100% fresh air, no recirculation. Our whole approach was to measure the pressure difference. The pressure difference between the intake duct and the supply duct should give an accurate result on how much pressure is lost through the whole AHU. Considering the pressure loss of all components it should be sufficently precise. Can someone confirm/reject my thoughtprocess?  BR Chris

Hi Christoph We proceed exactly that way on our projects (in Switzerland with 100% outside air) and so far it's been accepted. I write a narrative explaining how to convert the Delta P on the fan to the airflow difference (and how to set up the alarm using pressure values to match the LEED requirements +/15% on airflow) I havent had any problem with it. Best regards, Anne  

Hi Anne, thanks a lot for your fast response and confirmation. This makes a lot of things easier.  BR Chris