Rev Up Your Vav

Engineered Systems, Jan, 2000 by Steven Angle

Heating. Next, set the box to full heating by raising the thermostat or providing a cold source near it. Observe whether airflow reduces smoothly to the correct heating setpoint, and if the reheat valve opens to 100%. Note the supply air's exact temperature increase.

Cooling. Finally, set the box to full cooling by lowering the thermostat or placing a warm source near the thermostat. Observe whether the reheat valve closes. The supply air should increase to the correct cooling setpoint, and the supply air temperature should decrease to match system primary supply alr temperature.

Other steps to confirm proper performance include verifying that the control valve opens upon the failure of control power, and verifying high- and low-temperature alarms by raising or lowering a room's temperature as appropriate.

DOCUMENTATION VERIFICATION

Verify that the following are complete if applicable:

* Shop drawings;

* Submittals;

* Training;

* O&M manual;

* Balance report; and

* Record drawings.

THE RESULTS

Table 1 shows the number of deficiencies discovered during the commissioning process. Several of these deficiencies are worth discussing in more detail.

Nine constant-volume boxes using a pneumatic reheat valve and thermostat were found to be installed without the main supply air connected. The reheat valve on these boxes had failed to full heating.

Fifty-two VAV boxes had problems with the DDC controller and its programming. In some cases, the tuning constants were set "fast" as an aid to the balancer during balancing, then not returned to the correct settings when balancing was completed. In other cases, the minimum or maximum airflows were not programmed correctly, sometimes because the parameter was changed by the engineer during construction but the information didn't get communicated. Several had software problems that caused the program to hang up and not control properly.

Twenty-three VAV boxes had problems with the reheat control valve assembly, including several instances where the manual isolation valves were found closed. Control valve problems included poor installation of the pneumatic lines, causing pressure on the control valve body and preventing proper operation. In a few cases, control valve actuators were not yet connected to the valve; this was not easily noticed because they were located above the finished ceiling.

Twenty-five VAV boxes could not achieve the maximum airflow recorded on the balancer's report. This was sometimes due to an incorrect static pressure setpoint for the system main air handler, because it had not been communicated to the DDG contractor. Elsewhere, problems were due to dampers that were binding and couldn't reach the full open position. In one case, this problem was noted on the balance report, but it had not yet been corrected by the mechanical contractor.

Eight thermostats were in poor locations. In almost all cases, they were installed according to the design drawings but ended up too close to coffee machines or copiers that generated heat; or, they were located directly in the throw area of a nearby diffuser. Frequently, the owner had located equipment in locations that were different than planned, and the thermostat had to be relocated.