State of Operation

The Outdoor Air Damper/Temperature chart lets you identify anomalous system-level operating states by plotting the observed and expected outdoor air damper position against outdoor air temperature. Per ASHRAE Guideline 36, healthy AHU operations should consist of four distinct operating states - mechanical heating, economizer, economizer with cooling, and mechanical cooling. Deviations from these states (i.e., operating in the incorrect state for the external conditions) can result from improper sequencing of events, inappropriate temperature or pressure limits, or inaccurate inputs from faulty sensors, and can lead to excessive energy use.

Identifying & correcting faults

Each of the four operating states - mechanical heating, economizer, economizer with cooling, and mechanical cooling - serves to deliver the appropriate conditioning requirements to the building while minimizing energy use. In each state, the mixing box damper position can be observed to reflect the active operating state at the time. Realize that any significant cluster of observed points which deviates from the expected points may indicate a state of operation fault.

If a state of operation fault is suspected, it is recommended that the state of operations sequencing logic is investigated. Consider the following example scenarios and the observed anomaly, expectations, and suggested corrective measures for each scenario.

Scenario 1: Economizer in heating state ❌

Observed: The outdoor air damper is partially or fully open at very low outdoor air temperatures. This indicates that the economizer state is operating in place of mechanical heating.

Expected: The outdoor air damper should be at its minimum position at very low outdoor air temperatures, indicating heating state operation.

Recommendations: Check the logic for economizer state availability. Check for overheating zones which may prompt economizer operation during or outside operating hours.

Scenario 2: Premature mechanical cooling ❌

Observed: The outdoor air damper is fully open AND at its minimum position at the same outdoor air temperatures. This indicates a conflict between the economizer and economizer with cooling state.

Expected: The outdoor air damper should be at its minimum position, indicating mechanical cooling state, at higher outdoor air temperatures than when the outdoor air damper is fully open.

Recommendations: A logical fault in the sequences of operation, such as inadequate dead zone allowance or a misplaced reference variable may inhibit the economizer with cooling state.

Health Index

The Health Index is calculated as the remainder of the mean absolute deviation of the observed and expected points from 100%. A higher Health Index indicates overall healthy state of operation settings.

For example, say there were three(3) observed positions at 38%@-10℃, 69%@5℃, and 20%@15℃, and the expected positions are 25@-10℃, 80%@5℃, and 20%@15℃. Then, the absolute deviation at their respective outdoor air temperatures would be |25%-38%|, |80%-69%|, and |20%-20%|, which are 13%, 11%, and 0%, respectively. Then, the mean (Average) of these differences shall be (13%+11%+0%) / 3, which is 8%. Then, the Health Index shall be 100%-8%, which is 92%.

Under the hood (For geeks)

The Outdoor Air Damper/Temperature chart was generated by plotting the observed relationship between outdoor air damper position and outdoor air temperature, and the expected relationship. The expected relationship was generated using the relationship between outdoor air ratio f_OA and mixing box damper position s_DmpMix, along with its estimated parameters (a, b, c), established in Mixing Box Damper (Equation C). Realize that the mixing box damper position s_DmpMix and outdoor air damper position s_DmpOA are synonymous. Rearranging to solve for outdoor air damper position s_DmpOA results in Equation A.

Low Health Index! Turbo

The Health Index here seems low... Let's fix that! See Identifying and correction faults for diagnostics and recommendations.

I'm on it!