EDC Building | Fault Detection & Diagnostics | Mode of Operation
Mode of Operation
The Daily Fan Switches chart lets you monitor the proportion of time that the AHU supply fan switches ON/OFF based on how many times they are switched ON/OFF in a single day. Ideally, the fan switches only twice per day, once ON and once OFF - this indicates a proper functioning system-level schedule. Switching LESS than twice per day or MORE than twice per day may indicate a mode of operation fault and may result in excess energy use, particularly during after-hours. The Time of Switch chart lets you monitor when the fan switches ON/OFF relative to the total amount of ON/OFF switches recorded.
Ideally, the AHU fan switches only twice per day, once ON in the morning and once OFF in the evening. If the fan switches LESS than twice per day or MORE than twice per day for
a significant portion of time (i.e., a large portion of the chart is red or orange), there is likely a mode of operation fault.
If a mode of operation fault is suspected, there are two likely scenarios: If a significant portion of the chart indicates that the fan switches LESS than twice per day,
it is likely that the AHU is operating continuously, resulting in excess energy use during after-hours - ensure that the weekly AHU schedule is functioning correctly, is one exists, and that no override requests are in place.
If a significant portion of the chart indicates that the fan switches MORE than twice per day, it is likely that the AHU is night-cycling, resulting in excess energy use during after-hours - investigate zones for chronic overheating during unoccupied hours and adjust wake-up temperature setpoints accordingly.
| LESS than twice per day | TWICE per day | MORE than twice per day |
|---|---|---|
| Observed: AHU fan never or only once switches ON/OFF in a single day.
Recommendations: Check for overlooked override requests or lack of weekly AHU schedule.
|
Observed: AHU fan switches ON/OFF twice in a single day.
Recommendations: None. |
Observed: AHU fan switches ON/OFF three or more (3+) times in a single day.
Recommendations: Check for chronic overheating in zones and consider increasing unoccupied-hours zone overheating temperature setpoint. |
The Health Index is calculated as the fraction of time that the AHU fan switches only twice per day. A higher Health Index indicates overall healthy mode of operation settings.
For example, if the AHU fan switches twice per day for 75% of the time, then the Health Index shall be 75%.
The Daily Fan Switches chart was generated by identifying and counting the number of fan state changes. Due to the non-binary nature of the data (i.e., fan state represented as a variable rather than ON or OFF),
a change in fan state was heuristically identified as a sudden increase or decrease in the fan state - in other words, if the fan state increased or decreased by 50% or more in the span of one timestep, a change in fan state (i.e., fan is switched ON/OFF)
is classified. The presented data consist of the cumulative number of days where a fan state change occurs X times in the span of a single day in proportion to the reference period of the data (i.e., total number of days analyzed). X is categorized as one
(1) through ten-plus (10+) number of fan state change occurences in a single day.
For example, for data of a reference period of ten days, if the fan state changes twice per day for seven of the ten days, then the presented data shall represent two fan state changes for 70% of the time.
Low Health Index!
Turbo
The Health Index here seems low... Let's fix that! See Identifying and correction faults for diagnostics and recommendations.