Recently, we were involved in a project were a simple Lead/Lag controller was started up by the mechanical contractor. During the install there were a few items where the commissioning agent requested some changes to the programming of our controller. Since the job is local, I ended up going on site to install the new program and provide some courtesy training. As I watched the system work, it did not “feel” right. The outlet of the hot water boilers was around 190°F but the system loop was stuck well below setpoint. The boilers were cut back on their high limits, but the water was never making it to the main supply sensor. I found myself walking around the boiler room staring at the ceiling, tracing the piping in my mind. It all looked right. The offline boilers recirculate into the boiler loop. The online boilers go to the secondary loop. There is a way for it to cross over which I assumed was where the temperature was dropping, but I mentioned on both occasions to the mechanical contractor to check the flow, because there was definitely flow problems. Like most contractors, he was simply happy to be getting off the job.
After the building was handed over to the end user, their chief operator called asking for help. He saw that there was a problem. When an event happened that cause instability (boiler fault, large change in load, etc.) the system would lose control and never really recover. He was prepared to rip the controls off the wall. Of course this was a controls problem.
I asked our VP of Service to go to the job. I knew there was something wrong, but I did not see it. I explained the flow problems that I had noticed and asked him to keep an eye out. Once on site, Darrel started examining the install like I did. What my untrained eyes missed, his saw. The isolation valves were wired correctly in our panel, but went through relays which were wired inverted. The offline boilers were actually piped into the header, while the online boilers were recirculating. Presumably, heated water was flowing back to the boiler loop, then out through the offline boilers.
It was very satisfying personally to return to the site and see the temperatures working as they should. Everyone involved, the installer, mechanical contractor, commissioning agent, the end user and I missed this. Once fixed, the controller worked well.
It is easy to assume the controls are at fault. They are often the least understood, and therefore bear the brunt of the blame. However in times like these, the controls are simply responding to some other mechanical error. Not every problem is caused, nor can be fixed by controls.