This is a companion to last month’s “Art and Science of Point Selection” column. The sequence of operation (“sequence”) is the most important aspect of BAS design. BASs require extensive sequence programming by the installer to transform it from a pile of electronics into the brain of an HVAC operation. As with point selection, sequence writing: 1) Requires an “art and science” approach, and 2) Is an iterative process with the point selection. So this column won’t provide much detail about these aspects (revisit last month’s column, if necessary) but instead will focus on the key aspects unique to sequences.
There are four objectives that a sequence needs to address:
1. Equipment protection: The sequence must include protection to minimize damage to equipment, such as freezing coils, short-cycling motors, duct damage due to over/under pressurization, etc. This objective is not optional and cannot be traded off for more easily meeting the remaining objectives.
2. Reliable operation: Each building and building use will have a different definition of “reliable” (i.e., ranging from a high tolerance to periods of degraded interior conditions to the nearly 0 percent system downtime for a top-tier data center). The sequence needs to address the project’s definition of reliability, which is also not an option.
3. Comfort: Indoor temperatures, rh, and air quality need to be maintained at reasonable levels. These attributes have a range of acceptable values, which, along with owner’s willingness to tolerate an even wider range, can be balanced with the next objective.
4. Energy efficiency: Increasing system efficiency can be a balancing act with comfort. Unfortunately, it’s not always clear which sequence choices will improve efficiency and/or if they will be so complicated that it can affect reliability or even equipment protection.
The Process of Writing Sequences
1. Start with a schematic diagram and initial point list of the systems to be controlled.
2. The initial point list should take into account:
- Which factory-provided equipment safeties or controls will be used as is vs. bypassed for control by the BAS. If used as is, the only points included should be for monitoring safeties and controls as well as for providing supervisory control of the factory controls (on/off, set point reset, etc.); and
- Which factory-provided controls are to be interfaced via points vs. digital communications (e.g., BACnet).
3. As with the point list, develop the sequences “top down.” List all unique systems, their components, and then the controlled devices (VFDs, damper/valve actuators, etc.).
4. For each component/device (or a logical group of components/devices), describe the normal mode of operation (e.g., unoccupied vs. occupied) and abnormal modes (e.g., failures, alarms). The latter needs to address the equipment’s safety requirements, while all modes need to meet the project reliability objective while achieving the project’s comfort vs. energy efficiency balance.
5. Review the point list to ensure it can fully address the sequence, then review the schematic diagram to ensure the sequence and points match the mechanical design.
6. Continue iterating this sequence vs. objectives vs. the point list and schematic diagram comparison and edit them further until they’re all in agreement and meet the project’s objectives.
The following is an abbreviated example of what the above process might result in for a simple single-boiler heating plant:
- a. Occupied mode start/stop
i. Start when OAT is under 65ºF
- b. Unoccupied mode start/stop
i. Start when OAT is under 45ºF
- a. Constant volume primary pump
i. Interlock to boiler
- b. Variable volume secondary pump
i. Interlock to boiler and modulate VFD to maintain loop diff. pressure
3. Mixing valve
- a. When boiler is on modulate to maintain the HWST to the reset schedule
4. Failure alarm
- a. Issue alarm when boiler failure is detected
Sequence development, as with point list selection writing, has become more challenging over the years. This column has previously discussed the hope that ASHRAE Guideline 36 – “High Performance Sequence of Operations” can help with this challenge. Keep your fingers crossed.