An Alternative Approach to Writing HVAC Control Sequences

Reading and correctly interpreting a control-system sequence of operation can be difficult. Misunderstandings are common and often result in poorly programmed systems. This article discusses an alternative method of writing HVAC control sequences, one resulting in narratives at once concise—often, less than half the word count—and precise, providing absolute clarity as to how a control system operates. This method reduces complexity, optimizes control, and enhances system value while conserving cost, time, and energy.

The Output-Oriented/Prioritized Control Sequence

Output-oriented/prioritized is a structured format for writing control sequences. It is based on two principles: output-oriented organization and priority levels.

Output-oriented organization. In an output-oriented control sequence, the required operation of each output point in a system (e.g., AHU-1 preheat valve) is described completely in a single list. Every condition influencing control of the output points is discussed in the list and nowhere else.

Priority levels. Within each output point, conditions that may affect the output point are prioritized. For the highest-priority condition that exists, the required output action is executed, and any remaining conditions are ignored. For example, for an air-handling unit:

Supply-fan control

1. If the smoke detector activates, the supply fan shall be de-energized at its safety circuit.
2. If the freezestat trips, the supply fan shall be de-energized at its safety circuit.
3. If the high-static safety trips, the supply fan shall be de-energized at its safety circuit.
4. In occupied mode, the supply fan runs continuously.
5. In unoccupied mode, the supply fan remains off, cycling on only when room temperature is below the night heating setpoint.

Preheat valve

1. If the freezestat alarm trips, the preheat valve shall open 100 percent.
2. If the supply-fan status is off, the valve shall modulate to maintain unit interior temperature at 45°F.
3. The preheat valve shall modulate to maintain a minimum preheat-coil discharge-air temperature of 50°F.

Chilled-water valve

1. If the freezestat alarm trips, the chilled-water valve shall close.
2. If the supply-fan status is off, the chilled-water valve shall close.
3. The chilled-water valve shall modulate to maintain supply-air temperature.

Outside-air damper

1. If the freezestat alarm trips, the outside-air damper shall close.
2. If the supply-fan status is off, the outside-air damper shall close.
3. In unoccupied mode, the outside-air damper shall close.
4. If outside-air enthalpy is above 25 Btu per pound, the outside-air damper shall be minimally (15 percent) open.
5. If outside-air enthalpy is below 25 Btu per pound, the outside-air damper will modulate between minimally and fully open to maintain the supply-air-temperature setpoint and a minimum low-limit mixed-air temperature of 50°F.

Return-air damper

1. The return-air damper modulates inversely to the outside-air damper at all times.

Interpretation

Interpretation of the sample air-handling-unit control sequence above is demonstrated in the following examples:

1. An on-site engineer finds a supply fan is not operating. Following the sequence under “Supply-fan control,” he checks smoke-detector status. It is normal. He then checks freezestat status. It is normal as well. Proceeding down the list, he discovers the high-static safety has tripped. There is no need to go further; the high-static safety needs to be reset.
2. While reviewing trend logs, a commissioning agent sees an outside-air damper was closed during occupied mode. Following the sequence under “Outside-air damper,” she finds the freezestat tripped. There is no need to read further; the freezestat needs to be reset.

Benefits

An output-oriented/prioritized control sequence:

• Is highly structured and easily learned.
• Is condensed, shortening the process of locating and interpreting an issue.
• Is modular, allowing the use of standard libraries.
• Can be used by building owners and operators as well as HVAC contractors in troubleshooting with minimal training.
• Can be formalized as a standard practice.

With an output-oriented/prioritized control sequence:

• The likelihood of misinterpretation is minimized.
• Required actions are clear.
• Standard English is used throughout.
• Repetition is avoided.
• Programming time is reduced, reducing cost for the owner.

The output-oriented/prioritized control sequence mimics how software developers in many other disciplines of science and engineering approach complex software activities: by writing “structured English.” This method produces a high-level English-language description of the flow of logic before coding.

To address needs that arise during commissioning and testing, consider writing into specifications a provision allowing up to 10 percent modification of a sequence. Because the output-oriented/prioritized approach is very precise, omitted or mistaken sequence statements will be obvious. Such a provision will improve flexibility and aid cost control.

Conclusion

This article is meant to stimulate thought. There are a number of ways to approach the writing of control sequences, many of which have merit. If you prefer to write sequences giving exact instructions, the output-oriented/prioritized approach is worth considering. At the very least, consideration and practice of this approach will allow specifying engineers to perceive control sequences from another point of view.

Rick Meetre, PE, CEM, CBCP, is an independent HVAC controls and energy consultant with 30 years of senior-level experience. His areas of focus include HVAC-system commissioning, testing, and verification; HVAC-controls design review; energy-metering and usage evaluation; and troubleshooting and solving difficult HVAC-system problems.

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