*2.5. Rogue Zone*

ASHRAE Guideline 36 [24] defines high-performance control sequences for AHU–VAV systems. The "Trim and Respond" logic (see Sections 2.8 and 2.9) is adopted to reset the supply air temperature and static pressure setpoints at an AHU. The adjustment of these setpoints depends on the number of cooling "requests" generated by downstream zones that are served by the same AHU. For each time step, the change value of setpoint (SPchange) is determined by Equations (1) and (2) below:

$$\text{SP}\_{\text{changge}} = \text{SP}\_{\text{res}} \times (R - I) \tag{1}$$

$$R = \sum IM\_i \text{Request}\_i \tag{2}$$

where SPres is a unit respond amount (e.g., 0.06 inches for static pressure setpoint), *R* is the total number of cooling requests from the downstream zones, *I* is the defined number of ignored requests, *i* is the indicator of the downstream zone, *IM* is the importance multiplier that is used in the control sequence to decide if the cooling requests from the zone level should be used to control the upstream AHU, and *Request* is the cooling request from the zone. Therefore, if there is a rogue zone that continuously sends cooling requests whenever its schedule is on, due to the zone-level equipment problems, the parameter *R* will always include this request, and it keeps the setpoints in the control loop to its high end. Excluding rogue zones from the corresponding reset control strategies improves operation and increases energy savings. After the zone-level equipment problems that lead to the rogue zone are fixed, the rogue zone is no longer rogue, and all the control variables that are overwritten during the auto-correction process change back to their original value.

Two correction strategies were developed to eliminate the rogue zone impacts (i.e., to ignore the cooling request from the rogue zone). The first is to overwrite *I* in Equation (1). The auto-correction algorithm increases *I* by *n* for each currently identified rogue zone. The value of *n* is the same as the number of cooling requests determined in the control sequence of that rogue zone. The second is to

overwrite the *IM* of the rogue zone in Equation (2). When the FDD tool flags the rogue zone fault, the *IM* of the rogue zone is overwritten to be zero. Therefore, the cooling requests from the rogue zone can be removed.

#### *2.6. Improve Economizer High-Lockout Temperature Setpoint*

The previous five algorithms focused on correcting faults to restore the intended operation. This algorithm and the next three serve to provide more optimal control. They implement improved control setpoints or sequences when the FDD tool identifies the opportunity to do so.

After the opportunity to improve the economizer lockout temperature setpoint is identified, the setpoint is overwritten to the recommended value, following the flowchart in Figure 2. The recommended value can be determined based on the high-lockout limit recommended in the energy code [25]. For example, the recommended lockout setpoint is 23.9 ◦C, 21.1 ◦C and 18.3 ◦C, respectively, in the dry climate zone, the cold–humid climate zone, and the hot–humid climate zone.

#### *2.7. Improve Zone Temperature Setpoint Setback*

Similar to the algorithm in Section 2.6, this auto-correction algorithm overwrites the zone temperature cooling or heating setpoint during the occupied or unoccupied hours to the recommended values wherever there is an opportunity.

#### *2.8. Improve AHU Static Pressure Setpoint Reset*

The auto-correction algorithms for this and the next opportunities are most closely related to optimal controls. Both algorithms correct the fault "continuously" as it continuously adjusts the control variables to optimize the equipment operation (e.g., resets). They are relevant for AHUs without sophisticated reset strategies, such as no reset or simple resets based on return air temperature or outside air temperature.

The auto-correction algorithm uses the ASHRAE Guideline 36 [24] "Trim and Respond" logic for the static pressure setpoint. To optimize the operation of the AHU and minimize discomfort, the static pressure setpoint (SSP\_spt) is continually reset using the Trim and Respond logic between a minimum and maximum setpoint (SPmin and SPmax). When the supply air fan is o ff, the setpoint is the initial setpoint (SP0). The reset logic is active while the supply air fan is proven on, starting a delay timer (Td) after the initial device start command. When active, for every time step T, when the cooling request from the downstream zones ( *R*) is less than or equal to a defined number of ignored requests (*I*), the setpoint is trimmed by a trim amount (SPtrim), but no less than SPmin. If *R* is more than *I*, the setpoint changes by a respond amount, (i.e., SPres \* ( *R* − *I*)), but no more than the maximum response per time interval (SPres-max).

#### *2.9. Improve AHU SAT Setpoint Reset*

Similar to the algorithm to improve the static pressure setpoint reset, this auto-correction algorithm uses the ASHRAE Guideline 36 [24] "Trim and Respond" logic to reset the SAT setpoint continuously between a minimum and maximum setpoint. The control\_variable\_being\_overwritten is the SAT setpoint.

## **3. Results: Preliminary Testing**

Three commercial FDD providers participating in this research selected a subset of the algorithms that were created by the authors and integrated them into their development product environments for field testing. The partners chose the relevant algorithms for a variety of reasons, including: the expected ease of implementation, the reduction of operational cost, savings potential, and the ability to solve problems common to their customers. The implementation process varied depending on the platform, but generally consisted of the following phases: (1) confirm/add two-way communication functionality between the FDD and the BAS, (2) build an auto-correction interface to communicate with the building operator, (3) translate the algorithms into the FDD programming environment, (4) modify the BAS programming of the specific building to integrate the new control actions sent by the FDD tool, and (5) commission and test the new system. Further details are presented in Lin et al. [26]. This section illustrates the test results of two auto-correction algorithms: "Rogue zone" and "Improve AHU supply air temperature setpoint reset" for one implementation partner. Section 4 summarizes the challenges that were faced by three partners during the implementation process, as well as the solutions that were used by one or more project partners to mitigate them.

In the preliminary testing, the two routines were deployed in a commercial FDD product (SkySpark® by SkyFoundry) and tested on two AHUs in a building in Berkeley, California, US. between March 3 and April 5, 2020. The goal of this preliminary test was to determine whether the enhanced FDD solutions were able to correct faults without adverse operational effects.

#### *3.1. Description of the Testing Site and Equipment*

Table 2 summarizes the test site and equipment information. AHU01 and AHU02 are structurally identical. Figure 5 shows the BAS graphics (i.e., native dashboard) for one of the two AHUs.


**Table 2.** Test site information.

**Figure 5.** BAS graphics for the AHU02 at the test site. AHU01 has a similar structure.

Both AHU01 and AHU02 were controlled by a control sequence implemented in the native BAS control language and hosted on local controllers. Each AHU was controlled independently. The supply air temperature cooling and heating setpoint was reset based on the algorithm highlighted below in plain English:

	- ◦ Calculate the average cooling demand output for all the zones served by the AHU (cooling demand output is the output calculated by the PI[D] loop based on the proportional, integral [and derivative] component of the difference between zone cooling setpoint and zone temperature).;
	- ◦ Constrain the results between min = 3% and max = 12%;
	- ◦ Linearly map the average output to a calculated cooling setpoint between 18.3 ◦C and 12.8 ◦C. The value of 3% average cooling output is mapped to 18.3 ◦C, 12% is mapped to 12.8 ◦C, and all the values in between are calculated linearly;
	- ◦ The heating setpoint is fixed to 12.8 ◦C, except for when the cooling setpoint reaches 12.8 ◦C. In that case, the heating setpoint becomes 12.2 ◦C.

The existing SAT control strategy is relatively efficient, compared to common practice in the industry (fixed setpoint or resets based on outdoor temperature or return temperature alone and no deadband). However, the current strategy presents two limitations: (1) it responds to outlier zones or rogue zones, although minimally, as the reset is based on an average cooling demand outputs from all the zones; and (2) its calibration parameters (e.g., min and max average zone feedback of 3% and 12%, respectively) were established via trial and error and personal judgement. Given the limited capabilities of the BAS zone controllers (i.e., field devices in Figure 1), the reset strategy was entirely calculated within the AHU controllers.

The FDD tool connected to the BAS is a commercial product managed by a consultant and the facility manager of the site. The tool allows for custom programming and bi-directional communication to the BAS via the BACnet network. In contrast to the BAS, the FDD tool coding language is a modern scripting language with the ability to use high-level functions that allow the portability of the code between the buildings and equipment. The two auto-correction algorithms were coded using this platform and tested on the two AHUs. In the FDD tool, a zone was identified as a rogue zone when one or more disqualifying conditions were detected for that zone and the zone was sending a request to the AHU. The zone requests are calculated based on zone PID loop output >95%. Disqualifying conditions for cooling requests include:


#### *3.2. Auto-Correction Code in the FDD Tool*

## 3.2.1. Code for "Rogue Zones"

The code adopts the first correction strategy in Section 2.5 and overwrites the number of ignored cooling requests from the identified rogue zones. The number of requests and ignored requests are calculated as in Equations (3)–(5):

$$R' = \max(R - I\_{\text{total}}, 0) \tag{3}$$

$$I\_{\text{total}} = I\_{\text{def}fault} + I\_{\text{roχnc\\_zones}} \tag{4}$$

$$I\_{\text{rough\\_zone}} = \sum\_{i} I\_{\text{rough\\_zone\\_i}} \tag{5}$$

where *R* is the number of net cooling requests from the downstream zones of an AHU; *R* is the number of total cooling requests from the downstream zones; *Ide f ault* is the default number of ignored cooling requests (set by the user); *Irouge*\_*zones* is the number of ignored cooling requests from the all rogue zones; *Itotal* is the sum of the previous two variables; and *Irouge*\_*zone*\_*<sup>i</sup>* is the number of cooling requests from the *ith* identified rogue zone. *R* is calculated by subtracting the sum of all the rogue zones ignored based on the conditions described above (*Irogue*\_*zones*) and a default minimum of the ignored zones (*Ide f ault*) from all the requests ( *R*). If the equation leads to a negative result, *R* becomes zero. *R* is used in the SAT reset calculation below.

#### 3.2.2. Code for "Improve AHU Supply Air Temperature Setpoint Reset"

The supply air temperature cooling setpoint (SAT\_spt) is continually reset using "Trim and Respond" logic between a minimum and maximum setpoint (SATmin = 12.8 ◦C and SATmax = 18.3 ◦C). When the supply air fan is turned on, the initial setpoint is set to SAT0 = 18.3 ◦C and the reset logic is active immediately. When active, for every time step t = 5 min, the net cooling request from the downstream zones (*R'*) is calculated using Equations (3)–(5) above. If the *R* is above zero, SAT\_spt is decreased by a defined respond amount (SATres = 0.06 ◦C for each request) until the SAT\_spt reaches SATmin; if *R* is equal to zero, the SAT\_spt is increased by a fixed amount (SATtrim= 0.12 ◦C) until the SAT\_spt reaches SATmax.
