The typical evolution of an HVAC system’s sequence of operation (SOO) entails the design engineer writing the narrative for inclusion in the design documents. The controls contractor will then include a SOO in their submitted shop drawings. The worst-case scenario is that the temperature controls contractor replaces the designer’s SOO with something less-than-appropriate and it gets approved. Best case scenario is they work off the designer’s SOO, but offer improvements and clearly identify where their sequence deviates from the designer’s. The most likely scenario is they copy and paste the designer’s SOO and it is left to their field technician to fill in any gaps and work through any issues that may come from it. Depending on if, and when, a commissioning provider is brought into a project, they will likely provide feedback to further improve the SOO during either the design review or submittal review process.

The effectiveness of this typical SOO evolution process can be across the spectrum and is seldom great for complicated systems. A successful outcome is dependent on the competence of the various players and the openness of the design engineer to feedback. The efficiency of this approach is poor, as it entails drawn-out back-and-forth communications, and a lot of value is lost in translation.

For unique and complicated HVAC systems, I offer a better path for SOO development. Below is a case study highlighting a more collaborative approach.

Project Background

A new hospital was being built. The design initially included a chilled water plant with a more conventional system layout for the 4 water-cooled centrifugal chillers, along with a heat recovery chiller upstream of the centrifugal plant. When the construction team got brought on board, however, they approached the owner and design team with a recommendation to try something a little less conventional, a series counterflow arrangement for the 4 centrifugal chillers (see Figure 1). This arrangement allowed for performance improvements and the owner and design team agreed to the shift in approach. Given the complexity of this system and the general lack of familiarity the team had with the series counterflow arrangement, no single party felt comfortable putting together the SOO in a vacuum. The owner called for a collaborative effort.

Collaboration Meeting

A meeting convened with the goal to develop a comprehensive SOO for the updated chilled water plant configuration with universal buy-in from the project team. The sequence was to minimize energy consumption, but to remain sustainable (i.e. reliable, stable, and intuitive to future building operators). Additionally, this was a custom plant that will need adjustments to optimize its performance once in operation, thus clear optimization principles for continuous improvements needed to be put in place.

The participants in the meeting included, but were not limited to, the following:

• Owner’s capital projects team
• Owner’s facility operations staff
• Design engineer
• Construction manager
• Cx provider
• Mechanical contractor
• Controls contractor
• Chiller manufacturer's representative
• Cooling tower manufacturer's representative

The Process

The meeting began with a presentation from the chiller manufacturer on the benefits of such a series counterflow chilled water plant arrangement. The chillers operated most efficiently at low load and low lift, which meant the overall control strategy needed to run as many chillers as stably possible to lower the load on each chiller, and the series configuration allowed for a reduction in lift seen by each chiller.

A member of the construction management team, who had previously worked as a building operator on such a series counterflow chilled water plant, also provided his experience and some key areas of interest that he wanted addressed in the SOO.

The team then began working through developing the sequence for all aspects of the plant. It was fascinating to see the positive contributions every member at the meeting made. Below are a couple of examples.

Contributions

The design engineer obviously had the most knowledge on the design. He could quickly navigate the design set as well as his own calculations to pinpoint key information needed to inform the SOO. For instance, he verified the primary chilled water pumps were sized for N+1 redundancy, which solidified the need for a lead/lag/standby pump staging sequence. He explained how the chilled water fan coil units serving interior loads were selected for a higher chilled water supply temperature, a value which set the upper end of the chilled water supply temperature (CHWST) setpoint reset.

Regarding CHWST setpoint reset, various options were presented. A true demand-based reset sequence which monitors requests from served loads was quickly ruled out given the facility operations staff’s preferences. A simple outdoor air temperature reset raised concerns of operating rooms exceeding relative humidity requirements during milder ambient temperatures, but with corresponding higher ambient dewpoints. A compromise incorporated an outdoor air temperature reset as well as critical space humidity monitoring which would override the CHWST setpoint downwards if the upper limit of space dewpoint was being approached.

The chiller manufacturer had all the performance data on the 4 centrifugal chillers that were being installed. Their efficiency profiles, effective turndown ratios, and minimum flow requirements all informed the development of an appropriate staging strategy for the centrifugal chillers that maximized efficiency, but also promoted stability while staging. We landed with a load based staging strategy, where the load (tons cooling) on the four centrifugal chillers was monitored. Initial Stage On and Stage Off thresholds for each step were discussed, and we selected ones we believed would get us close to optimized energy performance, but not result in unnecessary cycling between stages.

As the commissioning provider, I brought up that those Stage On and Stage Off Setpoints could be further optimized once the system was in operation. The Stage On Setpoint was therefore to be made adjustable for future optimization. However, to prevent the possibility of an inappropriate adjustment to the Stage Off Setpoint, we agreed to tie it to the Stage On Setpoint. For example, the plant went from operating 1 centrifugal chiller to 2 when the centrifugal plant load exceeded the Stage On Setpoint (750 tons, adj.), and it dropped from 2 chillers to 1 when the centrifugal plant load dropped below Stage Off Setpoint (a hard-coded 200 tons below the adjustable Stage On Setpoint).

The process of optimizing the Stage On Setpoints for chiller staging had to be intuitive as it would be the responsibility of the future building operators. The controls contractor demonstrated how multiple trending points could be combined on the BAS’s graphical user interface. The total plant power draw could be plotted against the number of operating chillers. Review of trends for when the plant staged on another chiller could identify if there was a collective increase or decrease in power consumption, which would inform the operators if the next-in-line chiller was coming on prematurely or too late, and the Stage On Setpoint could be adjusted accordingly.

Calculation of the centrifugal plant’s load requires measurement of the entering and leaving temperatures, along with flow. One member of the mechanical contractor’s team was quick to remind the group of a previous change in the location of one of the flow meters (FM-2 in Figure 1) due to space limitations. It was actually located downstream of the system bypass pipe (i.e. to the right of where it is shown in Figure 1). That resulted in multiple flow meter readings (FM-1 and FM-2) needing to be brought into the load calculation performed by the BAS to capture total flow through the plant.

One memorable aha moment was when the question was raised as to how to ensure load was evenly distributed when two chillers were operated in series. The sequence was updated to clarify that the downstream chiller was to be written the output from the CHWST reset sequence discussed previously, but the upstream chiller would be written a setpoint halfway between the CHWST setpoint the downstream chiller got, and the real-time temperature of the water entering the centrifugal plant.

The member of the construction manager’s team who had previously operated such a plant had specific concerns regarding cold weather startup and stable operation in low ambient conditions. This plant was located in ASHRAE Climate 7A and there would be plenty of instances of this during the shoulder seasons. Keeping that in mind, along with pursuing the energy savings opportunity of spreading condenser water flow amongst more operational cooling towers than chillers, a condenser water control strategy was devised. The strategy makes use of all the controlled devices at our disposal for temperature and flow control (condenser water pump VFDs, a modulating cooling tower bypass valve, 2-position isolation valves on the inlet and discharge of each tower, 2-position bypass valves around each tower’s fill, and cooling tower fan VFDs). Flow setpoints for each portion of the sequence were informed by equipment limitations identified by the chiller and cooling tower manufacturer representatives. For example, the cooling tower manufacturer representative was available to clarify the minimum and maximum flow rates for each tower, which informed condenser water flow setpoints in various scenarios as well as limited us on how many towers in excess of operational chillers could operate without creating dry spots in the tower fill and promoting scale accumulation.

The final discussion of the day included stable equipment rotation. Much consideration was needed, as this is often overlooked and can create instability with equipment tripping offline if done inappropriately. Such instability may prompt future building operator overrides which will handcuff this whole control strategy. The final sequence included provisions for when certain rotations can occur to allow for seamless rotation of primary equipment.

Implementation

This was one of the most complicated systems I had ever commissioned. However, the functional testing went exponentially smoother than any other central plant I have worked on. I attribute this not only to a stellar controls team, but much to the collaborative nature in which the sequence was developed. There was buy-in from all parties. The controls contractor was at the table, they knew the reasoning behind each aspect of the SOO. So many what-if scenarios were discussed at the collaboration meeting, and we proactively sought to address those potential issues with a robust sequence from the get-go. There were not any areas left open for interpretation and the expectations for how the plant needed to operate were clear. The testing itself was quite a spectacle, with many construction team members present to witness all their efforts come to fruition. This included shadowing from the future plant operators, which assisted them learning the system prior to taking ownership of it.

Our firm has been retained to facilitate an ongoing commissioning process for the new hospital. We have worked with facilitates staff during the first year of occupancy to work out any remaining kinks in the operation of the building’s mechanical systems. Aside from a couple adjustments to how the heat recovery chiller performed its internal staging, there have not been any other major issues with the plant. It continues to operate very well, and we are working with the team to optimize the various setpoints the plant utilizes.

Conclusion

Not every HVAC system’s SOO needs some collaborate task force. However, for those complicated, unconventional systems, I am convinced there is no better approach. Writing SOOs is not easy, and for complex systems, no single person is going to have all the answers. A collaborative approach obtains buy-in from all the parties. It gives the controls contractor a seat at the table and allows them to better understand the designer’s intent. It gives the future facility operator a chance to better understand the system, but also an opportunity for the sequence to be tailored to their level of experience and their operational practices. A collaborative approach is the best chance at setting a complicated system up for long-term success.