Why Write Your Own Sequence Of Operation
When something fails, circumstances (and fingers) often point to the designer — unless you can prove your work.
Continuing on from last month’s discussion encouraging design engineers to create their own automatic temperature control (ATC) system flow diagram and write their own sequence of operation, let me start with the importance of this responsibility. Think about the equipment and recognize it has already been pre-engineered (e.g., 500-ton chiller). Then think about the duct distribution and pipe distribution and recognize that all of this is very stagnant and, except for the air, water, or steam passing through these conduits, there are no moving parts.
Now recognize the ATC system has moving parts and computer programming to orchestrate this systems’ movement, and accept the fact that if there is going to be an HVAC problem with the system, there is a 90% chance the problem will be the ATC. Sure, the system can experience air and water balancing issues, but for the most part, those get manually adjusted and the problem is usually solved.
That said, my experience has been proven time after time from the knowledge I have gained from sketching out my own system flow diagram and writing my own sequence by sequence (e.g., off, on-max cooling, on-nominal cooling). As can be expected, and explaining why there is a demand for commissioning, HVAC systems have problems at startup and some are quick to say, “It’s operating as designed. Must be an engineering problem.”
Because I did my homework (sketch and sequences), I was confident that the HVAC system would work, and it was my responsibility to prove it. I call this “Guilty until proven innocent,” from a major job years ago when everyone around me at the job site was confident that they had done their job and this was my problem.
Yes, I was confident I could resolve the problem because I could go back to the start of my HVAC sequence of operation initiative, where I had sketched out the flow diagram and inserted the basics (e.g., symbols for starter, disconnect, discharge air temperature, chilled water valve).
Next, I’d draft a first pass at the sequences with the first sequence always being the system “Off.” In the off position, I’d determine what reaction would occur/fail-safe: outdoor air (OA) damper closed, return air damper open, etc. I’d also consider, while the system was off, whether to require some heat within the AHU to ensure cold OA didn’t infiltrate through the OA damper, resulting in a heating coil freezing. Note, I could have simply had the heating coil go normally open/fail-safe. However, I had learned on a troubleshooting project that the design engineer had oversized the heating coil and normally open control valve, so when the system shut down, the temperature within the unit casing got very hot. As a result, when the unit turned on the hot discharge air would actually melt the first downstream fire damper fusible link and shut the fire damper.
Going on with the sequence operation draft, I would add more and more sequences, usually starting at maximum cooling and finishing with maximum heating and then thinking through the alarms and safety action-reaction. In the end, I was very confident that my design would work. I’d have another design engineer review these documents as a “fresh pair of eyes” to check my work.
Writing your own sequence and sketching the design is very valuable because when the project goes to construction, the ATC subcontractor and the various equipment manufacturers are going to dissect your design, with portions of the work remaining the ATC’s responsibility and other parts of the sequence becoming the manufacturer’s responsibility (e.g., air-handler). Here lies the beginning of a problematic HVAC system! Because the ATC sequence of operation is usually found within the ATC section of the specification and the design engineer places an ATC diagram on to the contract drawings, the equipment manufacturer quite often overlooks the ATC specification and seldom will coordinate the required interface with the ATC subcontractor.
When this happens, and it happens a lot, the solution eventually ends up at a construction meeting with the owner and the contractors saying, “We did our jobs and this was a design problem.” Guilty until proven innocent. When the dust settles, seldom does the design engineer hear, “Great job.” Everyone is more interested in forgetting the meeting had ever occurred.