OBSERVATIONS -1- The thermostat was demanding maximum cooling at all times through the test period. -2- The box CFM dropped as the supply static pressure dropped. -3- At .60” WG the damper motor, with a range of 3# to 13#, was still theoretically under control of the airflow controller, which was producing a branch pressure of 3 PSIG. -4- From 1.60” WG to 0.60” WG the maximum controlled airflow dropped from 450 CFM to 290 CFM as the controlling branch pressure of the airflow controller dropped from 6.3 PSIG to 3.0 PSIG. CONCLUSIONS -1- In this test we witnessed a 35.5% drop in maximum air flow when the supply duct static dropped from 1.60” WG to 0.60” WG. -2- The drop of 35.5% was when the airflow controller had continual control over the throttling range of the VAV box damper motor. -3- The airflow controllers are proportional only control and the observations are exactly what a proportional controller should do. Proportional controls require an error from set point for the branch pressure to change. From a maximum air flow of 450CFM to a maximum air flow of 290 CFM the controls were exactly on calibration. -4- Air balancers should be very sensitive to the supply duct static pressure, assuring it is at normal set point while adjusting VAV boxes. -5- The term “pressure independent” creates the illusion that the supply duct static can vary without impacting the VAVs’ performance. This is a false understanding. 8-147
RkJQdWJsaXNoZXIy ODY1ODQy