13.78 4. NA7.5.12 FDD for Air Handling Units and Zone Terminal Units Acceptance

NA7.5.12 Automatic Fault Detection Diagnostics (FDD) for Air Handling Units and Zone Terminal Units Acceptance

Use Form NRCA-MCH-13-A

Purpose of the Test

Fault detection and diagnostics can also be used to detect common faults with air handling units and zone terminal units. Many FDD tools are standalone software products that process trend data offline. Maintenance problems with built-up air handlers and variable air volume boxes are often not detected by energy management systems because the required data and analytical tools are not available. Because of the large volume of data requiring analysis it is more practical to perform the FDD analysis within the distributed unit controllers. The acceptance tests are designed to verify that the system detects common faults in air handling units and terminal units. FDD systems for air handling units and zone terminal units require DDC controls to the zone level. Successful completion of this test provides a compliance credit when using the performance approach. An FDD system that does not pass this test may still be installed, but no compliance credit will be given.

Benefits of the Test

The test will ensure that the FDD controls are able to detect and report common faults with air handling units and VAV boxes. Fan power consumption will be reduced due to proper operation of the air handler, as well as VAV boxes that are responding correctly to zone demand requirements. Cooling energy will be reduced due to proper operation of the VAV boxes since a VAV box that is providing too much air to a zone will end up overcooling the zone. This results in wasted energy on the heating side, since the reheat coil will then need to be activated.

Instrumentation

FDD tests for air handling units and zone terminal units require no additional instrumentation for testing, since control algorithms are embedded in unit controllers.

Test Conditions

The air handling unit should be installed and the heating, cooling and economizer modes of operation tested. To perform the test, it may be necessary to use the building automation system (BAS) to manipulate system operation to achieve the desired control. BAS programming for the operation of the chillers, boilers, air handling units, and pumps must be complete. All equipment startup procedures must have been completed per manufacturer’s instructions. All control sensors must be installed and control loops tuned. Document the initial conditions before any overrides to the building automation system.

Estimated Time to Complete

Acceptance tests will take 1-2 hours for each air handler. It may be helpful to have two persons performing this test. Time for acceptance testing for terminal units depends on the number of boxes to be tested.

Acceptance Criteria

The system is able to detect common faults with air handling units, such as a sensor failure, a failed damper or actuator or an improper operating mode.

The system is able to detect and report common faults with zone terminal units, such as a failed damper or actuator or a control tuning issue.

Potential Problems and Cautions

Difficulties could be encountered with manipulating the control system if not familiar with the programming language. Therefore, a controls contractor should be on-site to assist with the testing.

• The threshold for a sensor drift fault should be given in percentage of full range, or in units for each type of sensor (temperature, differential pressure / airflow rate, etc.) This tests the sensor fault by disconnecting the sensor.

Damper/actuator fault: this includes a failed actuator, or a damper stuck in an open closed or fixed position.

Step 1: From the control system workstation, command the mixing box dampers to full open (100 percent outdoor air). This may be done by lowering the supply air temperature setpoint at the control workstation.

Step 2: Disconnect power to the actuator and verify that a fault is reported at the control workstation.

Step 3: Reconnect power to the actuator and command the mixing box dampers to full open by maintaining the supply air temperature setpoint.

Step 5: From the control system workstation, command the mixing box dampers to a minimum position (0 percent outdoor air). This may be done by raising the supply air temperature setpoint at the control workstation.

Step 6: Disconnect power to the actuator and verify that a fault is reported at the control workstation.

Step 8: Verify that the control system does not report a fault during normal operation.

• Valve/actuator fault: this test covers faults such as actuator failure, a valve stuck in an open or closed position and valve leaks.

Step 1: From the control system workstation, command the heating coil valve to the full open position. This may be done by temporarily setting the space heating setpoint higher than the current space temperature, if the system is not in heating mode.

Step 3: Reconnect power to the actuator and command the heating coil valve to full open.

Step 5: From the control system workstation, command the cooling coil valve to the full open position. This may be done by temporarily setting the space cooling setpoint lower than the current space temperature, if the system is not in cooling mode.

Step 7: Reconnect power to the actuator and command the cooling coil valve to full open.

Inappropriate simultaneous heating, mechanical cooling, and/or economizing: these tests are designed to capture faults when the system is running in an improper mode of operation. (For systems with integrated economizers, economizer and cooling operation can be simultaneously enabled.)

Step 1: From the control system workstation, override the heating coil valve and verify that a fault is reported at the control workstation.

Step 2: From the control system workstation, override the cooling coil valve and verify that a fault is reported at the control workstation.

Step 3: From the control system workstation, override the mixing box dampers and verify that a fault is reported at the control workstation.

Testing shall be performed on one of each type of terminal unit (VAV box) in the project. A minimum of 5 percent of the terminal units shall be tested.

Step 1: Disconnect the tubing to the differential pressure sensor of the VAV box.

Step 1: Command the damper to be fully open. This may be done in a variety of ways, depending on the capabilities of the building automation system. Override the space temperature setpoint to be below the current space temperature to force the system into maximum cooling. Or, command the VAV box to the maximum position through the control workstation.

Step 3: Adjust the cooling setpoint so that the room temperature is below the cooling setpoint to command the damper to the minimum position. Verify that the control system reports a fault.

Step 3: Set the cooling setpoint below the room temperature to simulate a call for cooling. Verify that the control system reports a fault.

Step 4: Reconnect the actuator and restore all setpoints to their original values to resume normal operation.

This fault could be caused by actuator failure or a valve stuck in an open or closed position. This test is only applicable to systems with hydronic reheat.

Step 1: Command the reheat coil valve to (full) open by setting the heating setpoint temperature above the space temperature setpoint. Wait for the controls to respond to the command to open the reheat coil valve open.

Step 2: Disconnect power to the actuator. Set the heating setpoint temperature to be lower than the current space temperature, to command the valve closed. Verify that the fault is reported at the control workstation.

Step 3: Reconnect the actuator and restore all setpoints to their original values to resume normal operation.

• Feedback loop tuning fault: this test is designed to capture a fault that might occur from excessive hunting or sluggish control.

Step 1: Set the integral coefficient of the box controller (reset action) used for airflow control to a value 50 times the current value. Reduce the space temperature setpoint to be 3°F below the current space temperature to simulate a call for cooling.

Step 2: The damper cycles continuously over a period of several minutes. (The cycling period time depends on the type of controller used but is typically on the order of a few minutes.) Verify that the control system detects and reports the fault.

Step 3: Reset the integral coefficient of the controller to its original value and reset the space setpoint to its original value to restore normal operation.

Step 1: From the control system workstation, command the damper to a minimum position (full closed) by raising the space temperature setpoint.

Step 2: Then disconnect power to the actuator and verify that a fault is reported at the control workstation.