From the outside, HVAC design might look like a lot of duct and pipe routing, moving air and water around a building. It might conjure thoughts of mechanical engineers politely (or not so politely) asking architects for more ceiling space. And sure, it’s a big part of what we learn in school, along with energy theory and thermodynamics.

In practice, however, one of the most overlooked parts of being a mechanical engineer is documenting how the whole system is supposed to work. We’re talking about chillers, boilers, pumps, air handlers — the whole lineup.

At the end of the day, it’s our control sequences and direct digital control (DDC) points lists that get handed off to the controls contractors, who turn our words into code that makes the system run. If those words aren’t understandable, the system won’t be either.

The value of clarity

Control sequences are the critical link that translates our design intent into real-world system behavior. They must be clear, logical and easy to follow, as controls contractors rely on this documentation to write the underlying programming logic. 

When sequences are vague, overly complex or treated as mere boilerplate, the risk of misinterpretation rises dramatically. This can lead to programming errors, which manifest as comfort complaints, energy waste and costly troubleshooting during commissioning. 

Conversely, a well-written sequence acts as a precise set of instructions. It enables accurate programming, streamlines the commissioning process and ultimately ensures the system performs as designed in the field.

Start with the basics

Writing control sequences starts with a basic understanding of HVAC and controls terminology. A basic control function consists of three devices:

1. Sensor (input)

2. Controller

3. Control device (output)

Sensors monitor parameters such as temperature, pressure, humidity and flow. These feed data into the controller, which interprets the input and sends commands to control devices: fans, pumps, valves, dampers, etc.

The controller is the brain of the operation. When everything works together, the system responds intelligently to changing conditions, just as we designed it to.

Digital vs. analog: What’s the difference?

Like computer language, digital means binary: 1 or 0, on or off. In HVAC terms, that’s things such as fan status or flow switches. Either it’s happening or it’s not.

Analog means continuous and varying. These are your temperature readings, humidity levels, pressure differentials — anything that changes gradually over time.

Here in Wisconsin, we’ve all seen a hot water cabinet unit heater in a building’s main entrance vestibule. Inside the cabinet is a small fan and a hot water control valve. Usually, a digital thermostat is on the wall, and somewhere nearby (or hidden in a closet) is a DDC controller.

This gives us the three main pieces of a control system:

1. Sensor: thermostat

2. Controller: DDC controller

3. Control devices: Fan and valve

The thermostat provides an analog input — it senses temperature. The fan is a digital output (on/off), and the valve is an analog output, modulating to maintain the desired temperature.

How it’s written

The control sequence for this system might look like: As the temperature in the vestibule drops below 60 F (adjustable), enable the cabinet unit heater fan and modulate the control valve open. Upon the space reaching the setpoint, the fan shall be disabled and the control valve shall close.

Is it really that easy? Is that all there is to writing DDC control sequences?

Well, yes and no.

What other parameters might you incorporate into a cabinet unit heater control sequence? What other systems are affected by this heater? How does the boiler plant and pumping system function with all the terminal equipment?

Sometimes controls feel more like an art than a science. You could take that same simple cabinet heater sequence and:

• Add a time delay on the valve to open after the fan has been enabled.

• Add a discharge air temperature sensor to prove satisfactory supply air temperature.

• Use that same sensor to trigger a low discharge temperature alarm back to the building automation system’s (BAS) front end.

Writing sequences as design documentation

Control sequences serve two purposes: as instructions and as documentation of design philosophy. When written well, they reflect how you think about comfort, efficiency and system behavior. They become part of the building’s operational DNA, guiding future engineers, operators and technicians long after the project is complete.

In many cases, the control sequence is the only part of the design that is revisited during troubleshooting or retro-commissioning. Years down the road, someone will open the BAS and ask, “What was this supposed to do?” If your sequence is clear, they’ll know. Whether it’s vague or missing, they’ll guess — and that guess might not align with your original intent.

That’s why writing sequences with care is about getting the system running today and setting the building up for success tomorrow. It’s a form of professional stewardship.

Another part of the design legacy is what you pass on to younger colleagues. Clear sequences help younger engineers and technicians understand how systems work. They’re also a great way to pass on institutional knowledge and design philosophy. In a small team such as the one I’m a part of, where mentorship happens organically, control sequences become a shared language. It helps everyone learn, troubleshoot and improve.

A living document

Treat your sequences as living documents. They should evolve based on lessons learned, feedback from commissioning and operational realities. If something didn’t work as expected, revise the sequence. If a technician found a better way to describe a mode or condition, incorporate it. This mindset turns your sequences into a shared resource that improves over time.

Encouraging this kind of iterative improvement also supports team growth. Junior engineers learn how the systems work and how to think critically about them. Senior engineers can share their experience in a practical, impactful format.

Ultimately, well-written control sequences reflect your team’s values: clarity, collaboration and continuous improvement.

Glossary of Common Terms

Control sequences are only as good as the clarity of their language. Following is a glossary of common terms.

Among these, the following six terms are the most misunderstood.

1. Setpoint vs. control point

• Setpoint. The desired value for a system parameter (e.g., 72 F room temperature).

• Control point. The actual value being adjusted to meet the setpoint (e.g., valve position).

Tip: Think of the setpoint as the goal, and the control point as the action taken to reach that goal.

2. Enable vs. command

• Enable. Grants permission for a device or sequence to operate.

• Command. The actual instruction sent to a device to perform an action.

Tip: Enabling is passive; commanding is active.

3. Occupied vs. scheduled

• Occupied. Indicates that a space is actively in use.

• Scheduled. Refers to programmed time periods for system operation.

Tip: Occupied is for real-time usage; scheduled is for time-based logic.

Dale Garfield is a senior mechanical engineer and team lead at EUA, specializing in HVAC system design and controls integration. With a passion for mentorship and a knack for continuous improvement, Garfield helps bridge the gap between design intent and real-world performance. He’s also an advocate for continuous learning and collaborative problem-solving within small, high-performing teams. Garfield is a U.S. Marine Corps veteran and holds an executive MBA from the University of Wisconsin-Oshkosh and a bachelor’s degree in architectural engineering from the Milwaukee School of Engineering.