VAV PERFORMANCE CASE STUDY THE PROBLEM The building experienced many complaints of stuffiness and over-heating. Three different air balancing companies calibrated and checked the VAV boxes and the building continued to have many complaints. Analysts of Pneumatic Systems Limited (APS) was called to investigate. APS found the static pressure in the supply duct dropped to 0.4”WG in the afternoon, which is very low from the design set point of 1.0” WG. The inlet vanes were fully open, but the system was requiring more CFM (cubic feet of air) than the fan’s capability. The VAV’s were pressure independent, with airflow controllers (see page 7.54), which should compensate for some drift in the supply air static pressure value, based on the instrument’s data sheet. The drifting static pressure values in the supply duct would cause the air balancers’ efforts to become out of calibration if the varying static pressure had an impact. We tested to see if the manufacturers’ data sheets were unconditionally true in the pressure independent claims. TEST PROCEDURE -1- We selected a VAV box and installed a o PSIG o 30 PSIG pressure gauge and 0” WG to .5” WG Magnehelic gauge allowing us to read the box’s velocity pressure and airflow controller’s branch pressure. -2- We checked the damper motor stroking range which was 3# to 13#. -3- We dropped the supply duct static pressure in steps, recorded the maximum velocity pressure, determined the relative CFM and recorded the branch pressure from the airflow controller as per the chart below. SYSTEM PRESSURE (“WG) MAXIMUMVELOCITY PRESSURE ( “WG) CUBIC FEET/MINUTE (CFM) AIRFLOW CONTROLLER BRANCH PRESSURE (PSIG) (ACTUATOR RANGE 3# to 13#) 1.60 .200 450 6.3 1.50 .190 440 6.0 1.25 .180 430 6.0 1.00 .150 400 5.0 0.75 .125 370 4.0 0.60 .070 290 3.0 0.50 .060 270 0.0 0.20 .020 187 0.0 8-146
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