Writing Your Own Sequence Of Operation
Multiple benefits await once you kick the habit of handing off the sequence of operation duties.
When I first started in the HVAC industry working in a design consulting engineering firm, I came to accept that the HVAC sequence of operation and its associated contract specification, Automatic Temperature Controls (ATC), were provided by an ATC sales engineer. I’d go so far as to say this was the industry standard for designing HVAC systems. Heck, you could get some very knowledgeable sales engineer to come into your office and you’d explain the job and he (never met a woman sales engineer back in the 1960s) would help you out.
Of course, that section of the specification would state, “based on Honeywell Controls” or some other sales engineer’s company. This didn’t assure him he’d get the job, but many contractors would try and buy out the controls using the “base” ATC manufacturer.
For the first few years, I came to rely on a couple of different sales engineers back then because it was convenient and I’d learn more about the controls from these individuals. Then electric heat became very popular, and with this I had to spend more time coordinating the electrical engineer’s portion of the design in addition to drawing on the ATC sales engineer’s suggested sequence of operation. It was at this point that I was working with an in-house electrical engineer who bordered on being a tyrant with HVAC engineers. I think he enjoyed the intimidation especially if you were a young, inexperienced design engineer. That’s when I stopped using a sales engineer to do my work, although many other design engineers still relied on the convenience of a sales engineer.
I had this great mentor, Jim McGrath, who sat me down and began my education into writing my own HVAC sequences of operation. Going forward, when it came to providing the electrical data associated with HVAC to the electrical engineer, things changed drastically. Instead of my usual completed electric data sheet (unit number, horsepower, starter required, and interlock with associated other piece of equipment) I was now providing the same electric data sheet along with individual system flow diagram sketches with control devices, control panel, and electrical power and interlock wiring shown and noting “wired by electrical sub-contractor or ATC sub-contractor.” If I had 10 different systems (e.g., central air-handlers, cabinet unit heaters, hot water and chilled water systems), it was all there on individual system flow diagrams and sequences of operation.
At this point in time, I wrote my own sequences of operations beginning with “system OFF,” and that would allow me to make the decisions pertaining to normally open or closed ATC devices in fail-safe mode. These flow diagrams gave everyone, including the Tyrant, a picture of the complete design intent and who was responsible for what work. I also would include the set points, alarms, etc., along with what I wanted to see on the ATC control panel cover.
A lot has changed since those pre-CAD days. Sales engineers don’t usually call on engineers and instead provide computer-generated, standard ATC design drawings along with the associated sequences of operation. Another change has been equipment manufacturers designing and mounting their own ATC controls pre-packed and pre-wired as part of their equipment, thus eliminating the ATC subcontractor from furnishing and installing their control system.
In the end, the building operator will receive the ATC subcontractor’s record drawings that will be very detailed from years of uses and continuous improvements. The HVAC equipment manufacturer usually doesn’t provide the same quality level for ATC flow diagrams and sequence of operations, and the owner will most likely not see a complete ATC record document. Instead, the owner will have to draw from various record documents to find the rest of the ATC documents (e.g., ATC for central air-handling system will be in the equipment record submittal). If the owner has a chiller, make-up air unit, central AHU, and overall ATC, then she will need to compile three different record submittals to pull out the ATC information and place it with the overall ATC record document.
Next month, I’ll discuss how the design engineer should create his own system flow diagrams and write the needed sequences of operation and then maybe highlight which segments of the system design are ATC subcontractor and which are furnished and installed by the equipment manufacturer. I’ll also note why this should be done.