Simple or Complex Sequences?
Should engineers create a complex sequence that utilizes freely programmable controllers or implement a simple sequence that utilizes the controls that are embedded within the equipment?
It’s the heating season here in North America and that means building operators and controls contractors will soon be scrambling to ensure their heating systems are working properly. It’s very likely some of these systems will not function as expected and will need to be replaced. This leaves the specifying engineer with a decision: Create a complex sequence that utilizes freely programmable controllers or implement a simple sequence that utilizes the controls that are embedded within the equipment?
Now, this may seem like a strange topic for me to write about. After all, my entire company exists to provide training to controls contractors, engineers, and building operators on how to install, design, and program building automation systems.
However, throughout my career, I have seen too many needlessly complex control sequences. This month, I’m going to give you three questions that will help you determine the complexity of your sequence.
The Three Questions
- Is the system independent?
- Is the system a mix between new and old equipment?
- Is the system controlling to a single variable?
Is the System Independent?
Complexity is linked to independence. If you have an air-cooled chiller that is producing 42°F water independent of the rest of the systems inside the building, then you can often get away with utilizing the chiller’s own internal controls. I’ve seen this done for water treatment plants and other remote buildings, where the owner wanted the system to operate independently of a control system.
A complex water-cooled chiller that has variable-speed cooling towers and a decoupled loop staging sequence is on the opposite end of the complexity spectrum. This setup is quite complex and has multiple control variables (something we will discuss in detail later in the article). This is a perfect example of a system that is dependent on several other systems (namely the cooling towers and secondary loop).
The general rule is that if a system cannot operate independently, then you will most likely have to develop a complex sequence that requires free programmable direct digital control (DDC) controllers.
Is the System a Mix Between New and Old Equipment?
Next up, we need to look at the system itself. Are we intermixing existing and new equipment? This scenario is becoming increasingly common as the number of buildings undergoing a retrofit is increasing.
Here’s an example. Let’s say you have a series of legacy variable air volume (VAV) boxes, and you’re introducing a new VAV air-handling unit (AHU). The intermixing of pneumatic and modern systems will require a more complex sequence than, say, a DDC VAV box/air-handler setup. This is because of the inherent difficultly of controlling a system (like a DDC AHU) that is dependent on feedback from the VAV boxes themselves. In order to consume the data from the pneumatic systems, you will need an intermediary device. The most common intermediary device is a building automation system and its freely programmable controllers.
And the reality is that freely programmable controllers necessitate sequences that are, on average, more complex.
Is the System Controlling to a Single Variable?
It would be nice if all we had to worry about was a single control variable; however, this is often not the case. Even if your only control variable is zone temperature, you can still run into complexities. States like California are requiring increasingly stringent monitoring and energy-efficient sequences. Sequences are becoming increasingly complex with the introduction of load shedding, demand ventilation, and trim and response sequences, just to name a few.
And this is not even considering the effect of IoT and sensor networks on the sequencing of equipment. The reality is, as you begin to introduce more variables, your sequences will need to become more complex.
But What is Complex?
At this point, you may find yourself saying, “You know, Phil, this makes sense, but what is a complex sequence?”
Don’t worry, my friend, we will be covering how to structure sequences and what makes a sequence simple or complex in the next few articles.
However, let me leave you with this: If you are encountering any of the three scenarios I mentioned in this article, then I highly recommend that you are very thorough in your evaluation of any prepackaged controls. I’ve seen too many projects where the embedded controls could not achieve the sequence of operations.
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