At-A-Glance
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NA7.5.12 Automatic Fault Detection Diagnostics (FDD) for Air Handling Units and Zone Terminal Units Acceptance |
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Use Document NRCA-MCH-13-A |
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Purpose of the Test |
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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. Performing the FDD analysis within the distributed unit controllers is more practical because of the large volume of data. 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. |
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Instrumentation |
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FDD tests for air handling units and zone terminal units require no additional instrumentation for testing, since control algorithms are embedded in unit controllers. |
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Test Conditions |
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The air handling unit should be installed and the heating, cooling and economizer modes of operation tested. To perform the test, 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. |
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Estimated Time to Complete |
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Acceptance tests will take 1-2 hours for each air handler. Time for acceptance testing for terminal units depends on the number of boxes to be tested. |
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Acceptance Criteria |
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The system is able to detect common faults with air handling units, such as sensor failures, damper failures, actuator failures, or improper operating modes. |
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The system is able to detect and report common faults with zone terminal units, such as damper failure, actuator failure, or a control tuning issue. |
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Potential Problems and Cautions |
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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. |
A. Test Application
Newly Constructed and Additions/Alterations: Applies to any FDD system installed on an air handling unit or a zone terminal unit. A minimum of 5 percent of the terminal boxes (VAV box) shall be tested.
B. Functional Testing
Testing of each Air Handling Units with FDD controls shall include the following tests:
1. Sensor drift/failure: 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.).
Step 1: Disconnect outside air temperature sensor from unit controller.
Step 2: Verify the FDD system reports a fault.
Step 3: Connect OAT sensor to the unit controller.
Step 4: Verify that FDD indicates normal system operation.
2. Damper/actuator fault: This fault reports 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), by lowering the supply air temperature setpoint.
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 4: Verify that the control system does not report a fault.
Step 5: From the control system workstation, command the mixing box dampers to a minimum position (0 percent outdoor air) by raising the supply air temperature setpoint.
Step 6: Disconnect power to the actuator and verify that a fault is reported at the control workstation.
Step 7: Reconnect power to the actuator and command the dampers closed.
Step 8: Verify that the control system does not report a fault during normal operation.
3. Valve/actuator fault: This test covers faults such as an actuator failure, a valve stuck in an open or closed position, and/or valve leaks.
Step 1: From the control system workstation, command the heating coil valve to the full open position by temporarily setting the space heating setpoint higher than the current space temperature, if the system is not in heating mode.
Step 2: Disconnect power to the actuator and verify that a fault is reported.
Step 3: Reconnect power to the actuator and command the heating coil valve to full open.
Step 4: Verify that the control system does not report a fault.
Step 5: From the control system workstation, command the cooling coil valve to the full open position by temporarily setting the space cooling setpoint lower than the current space temperature, if the system is not in cooling mode.
Step 6: Disconnect power to the actuator and verify that a fault is reported.
Step 7: Reconnect power to the actuator and command the cooling coil valve to full open.
Step 8: Verify that the control system does not report a fault.
The following tests are designed to capture faults when the system is running in an improper mode of operation such as simultaneous heating, mechanical cooling, and/or economizing. (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 the control workstation reports a fault.
Step 2: From the control system workstation, override the cooling coil valve and verify that the control workstation reports a fault.
Step 3: From the control system workstation, override the mixing box dampers and verify the control workstation reports a fault.
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.
1. Sensor drift/failure:
Step 1: Disconnect the tubing to the differential pressure sensor of the VAV box.
Step 2: Verify the control system detects and reports the fault.
Step 3: Reconnect the sensor and verify proper sensor operation.
Step 4: Verify that the control system does not report a fault.
2. Damper/actuator fault – damper stuck open:
Step 1: Command the damper to be fully open. Override the space temperature setpoint below the current space temperature to force the system into maximum cooling. Another option is to command the VAV box to the maximum position through the control workstation.
Step 2: Disconnect the actuator to the damper.
Step 3: Adjust the cooling setpoint such 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 4: Reconnect the actuator and restore to normal operation.
3. Damper/actuator fault – damper stuck closed:
Step 1: Set the damper to the minimum position.
Step 2: Disconnect the actuator to the damper.
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.
4. Valve/actuator fault (For systems with hydronic reheat): 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.
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.
5. 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: Verify 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.
6. Disconnected inlet duct:
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.
Step 3: Reset the space temperature setpoint back to its original value.
7. Discharge air temperature sensor:
Step 1: Adjust zone setpoints to drive the box from dead band to full heating.
Step 2: Verify the supply air temperature resets to the maximum setpoint while the airflow maintains at the dead band flow rate.
Step 3: Verify that the airflow rate increases to the heating maximum flow rate to meet the heating load.