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Automated Fault Detection and Diagnostics (AFDD) for Roof Top Units

Challenge: Most roof top unit (RTU) economizers are known to fail leading to significant energy impact.

Objective: Detect economizer and ventilation failures as they occur and notify building operator to correct them.

Implementation: Rather than running the AFDD agent continuously, the AFDD processes will run on schedule or on demand to identify problems.

Air-side automated fault detection and diagnostic (AFDD) agent for RTUs

AFDD has a set of algorithms that monitor the performance of a RTU and detect faults with outdoor-air control and economizer operations using sensors that are commonly installed for control purposes. The algorithms utilize rules derived from engineering principles of proper and improper RTU operations. Seven algorithms are included in the air-side AFDD agent for detecting and diagnosing faults with RTUs:

  • compare discharge-air temperature with mixed-air temperature for consistency
  • check if outdoor-air damper is modulating
  • detect sensor faults (outside-air, mixed-air and return-air temperature sensors)
  • detect if the RTU is not economizing when it should
  • detect if the RTU is economizing when it should not
  • detect if the RTU is using excess outdoor air
  • detect if the RTU is bringing in insufficient ventilation air.

These algorithms support mandatory RTU fault detection and diagnostic requirements for new RTUs in 2014 California Title 24 Building Code.

The intent of these algorithms is to provide actionable information to building owners and operations staff. In today's market environment, no one has time for nuisance interruptions, so the algorithms can be tailored to minimize false alarms. On the other hand, if RTU systems and their controls are starting to fail, having an indicator (a.k.a. “check engine light”) of a real problem is always helpful – especially if it allows operations and maintenance staff to be proactive, rather than reactive.

AFDD Diagram

Inputs - The inputs for the air-side AFDD agent include:

  • outdoor-air temperature
  • return-air temperature
  • mixed-air temperature
  • discharge-air temperature
  • outdoor-air damper position
  • heating command
  • cooling command

Outputs - The outputs from the air-side AFDD agent include error code and energy impact.

Configuration Parameters - Configuration parameters: The configuration parameters for the air-side AFDD agent include:

  • economizer type
  • minimum damper position
  • design supply air flow
  • rated cooling efficiency
  • blended electricity rate (energy and peak charges) and cost impact threshold

Research Areas


Additional Information