VAV box reheat is active, damper is open to maximum ... - BetterBricks

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How to Find the Problem(s) by Inspection Inspection Step 1 Inspect the direct-digital-control (DDC) system to see if the AHU is in cooling or heating mode. If it is in heating mode, then the zone reheat is providing the additional heat that the AHU can’t provide. This can occur during morning warm-up. Inspection Step 2 If the damper control has been overridden in the DDC programming, determine why before correcting the problem. Some reasons for override may be: • The space has a high minimum make-up-air requirement such as for a fume or exhaust hood. • The damper was overridden during the cooling season to meet a temporary need for space temperature and was not subsequently reset. • Tenant complaints about poor air circulation, or “dead-air” conditions. The first condition has no simple solution and may actually be part of the original design. The second and third conditions require a review of the space-cooling loads to ensure that proper primary air is available under all conditions. The diffuser distribution should also be checked to make sure the supply air is not short-circuiting to the return-air plenum. Inspection Step 3 Inspect the damper operator to make sure it is operating properly. Cycle the damper using the DDC system while observing its response locally. If there is no change in position, the damper operator may have failed. A DDC technician can determine whether this is the case. Make sure there are no obvious air leaks at the flex connection to the VAV box. Inspection Step 4 The AHU may have a pressure-control problem that is starving the VAV box of primary air, forcing the damper to be wide open just to meet minimum airflow requirements. Inspect the AHU static pressure (SP) setpoint (in inches of water, typically denoted as "wg) and variable-speed-drive (VSD) speed to see if the static pressure design conditions are being met. This condition can also be caused by excessive duct leakage or a closed fire damper upstream of the VAV box. Inspection Step 5 In conjunction with Inspection Step 4, the AHU static pressure may be too high and the VAV box damper actuator does not have enough torque to overcome it. This typically occurs in VAV boxes close to the air handler where the duct static pressure is the highest. How to Confirm the Problem(s) by Trend Logging Trend log the following: • AHU supply-air temperature (SAT) • AHU static pressure (SP) • VAV box SAT, if available • Damper position • Heating coil percent capacity (feedback and not output from the DDC system) Graph the heating coil percent with respect to the other four points. Look for correlations in the trends. Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpTlsMstrPrb22.doc • 2
If the damper position is constant when you would expect variation, then the motor operator or DDC-box controller (including the pressure transducer and its tubing) may be the problem. If the damper position is constant, the duct static pressure is not satisfied, and the AHU VSD is at 100%, then the system capacity is inadequate, or there may be a large duct leak somewhere in the system. (Contractors have been known to remove terminal units and leave the medium-pressure ducting wide open for added ventilation during renovation projects.) If the AHU SP is being maintained and the VSD is cycling as needed to maintain the setpoint, there may be a closed fire damper upstream of the VAV box. Inspect any other VAV boxes being fed by the same branch duct (downstream of the last fire damper) to see if they exhibit similar problems. Example of Normal Operation If your graph looks similar to the one below then your system is operating as it should and the problem is resolved. The damper is at minimum position while reheat is active. When the thermostat calls for cooling, the reheat goes to zero and the damper opens as needed to satisfy the space requirements. Note that the fan static pressure is maintained throughout the day. Normal VAV-box operation Examples of Abnormal Operation VAV boxes are notorious for having many different modes of failure. Below are two examples of what a trend log of abnormal operation might look like, but your own trend-log graph will probably have its own unique features. If your graph looks similar to one below then your system is not operating as it should and the problem is unresolved. The reheat is energized at 50% once the damper rises above minimum setpoint. Airflow from the AHU to the pressure-sensor location is not a problem since it can maintain static pressure. Inspect the static pressure at the VAV box locally to make sure it has adequate pressure. There may be a partially Produced by BetterBricks, the commercial initiative of the Northwest Energy Efficiency Alliance BOpTlsMstrPrb22.doc • 3
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VAV box reheat is active, damper is open to maximum ... - BetterBricks

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