NA7.5.11 Fault Detection and Diagnostics (FDD) for Packaged Direct-Expansion (DX) Units Acceptance

Use Form NRCA-MCH-12-A

Purpose of the Test

The purpose of this test is to verify proper fault detection and reporting for automated fault detection and diagnostics systems for packaged DX units. Automated FDD systems ensure proper equipment operation by identifying and diagnosing common equipment problems such as temperature sensor faults, low airflow or faulty economizer operation.

Benefits of the Test

The test ensures that the FDD system can detect and report a number of common faults. FDD systems help to maintain equipment efficiency closer to rated conditions over the life of the equipment.

Instrumentation

The system test for refrigerant charge requires a calibrated refrigerant gauge with an accuracy of plus or minus 3%.

Test Conditions

Packaged unit and thermostat installation and programming must be complete.

HVAC system must be installed and ready for system operation, including completion of all start-up procedures, per manufacturer’s recommendations.

The system operating modes should already have been tested. If the system includes a field-installed air economizer, the economizer should already have been tested per procedures under forms NRCA-MCH-02-A.

Estimated Time to Complete

Construction inspection: 0.5 hour

Functional testing: 1 to 2 hours

FDD systems can have the capability to report alarms to a remote server, which are then accessible via a Web interface. It may be helpful to have two people conducting the test – one to perform testing on the unit and a second to verify reporting of the alarm to the remote interface.

Acceptance Criteria

The FDD system is able to detect a disconnected outside air temperature sensor and report the fault.

The FDD system is able to detect excess outside air and report the fault.

The FDD system is able to detect a stuck outdoor air economizer damper and report the fault.

The saturated discharge and saturated suction temperatures must be measured within 5°F of a calibrated refrigerant gauge.

Potential Problems and Cautions

Compared to the pressure sensors, the temperature sensors can have a longer response time to reach a steady-state condition. Therefore, the FDD algorithms may have trouble working properly during transitional states – for example, when the fan or compressor first turns on. The tester should be aware of the potential for false alarms that may occur during testing.