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    Dirty Coil Diagnostics Out West

    June 26, 2019
    Coil pressure drop is found to be the culprit in a commercial building with tenant thermostat wars.

    Last week I was visiting one of my sons in Nevada who recently purchased a commercial building plagued with thermostat wars. The tenants shared a single system and were stealing airflow from each other. Let’s take a look at how I bummed a set of air diagnostic test instruments from a 20-year friend and local HVAC contractor, then went scouting for the cause of this range war in Reno.

    Initial Occupant Interview
    A brief interview with each tenant found tensions were running high throughout the building because of comfort issues. The lack of comfort overshadowed the benefits of the newly painted floors, updated decor, and furniture. Comfort issues were negatively impacting the workflow and productivity of business.

    Each tenant had their own ideas, based mostly on emotion. They blamed each other for how the system was being manipulated and causing the problems. However, our interviews told a different story and pointed to airflow issues. The interviews didn’t lead to a dirty coil. They did hint at wide variations in room temperature and airflow.

    Prior to my arrival, the building receptionist assumed the thermostat sheriff role and spilled the beans about the ongoing thermostat wars. So, we assigned her the duty of participating in the diagnostics process and explaining what she learned about the system to each tenant.

    Responding to the interviews, we determined the most valuable test for all concerned would be to measure room and system airflow with a balancing hood.

    Inspect and Collect System Information
    Following a good diagnostic protocol, we inspected the system and documented its size and condition. We found a new furnace had been installed with a new 1” pleated filter.  However, the indoor coil and condensing unit of the 3.5-ton system were untouched and nearly 20-years old.

    The fan was rated at .50” w.c. (inches of water column) and its required airflow was interpreted from equipment specifications at 1225 cfm. We adjusted for the 5,000-foot altitude (divide by .83) and found the system needed 1475 cfm (cubic feet per minute).

    Measure Airflow
    Since the tenant's complaints pointed to airflow issues, we decided to measure room airflows first. The percentage of required airflow in the rooms was from 14% in one room on an outside western wall to 232% for an internal room with nearly no internal load. By comparing required to actual airflow for each room, we began to see the cause of contention.

    We found that one tenant had installed a quarter-round deflector on a supply register and added a rectangular cardboard box to force more air into one of his offices.

    We added together all the room airflows to find only 545 delivered cfm out of 1475 required cfm. We divided 545 cfm by 1475 cfm to reveal the system had only 37% of required airflow. At that moment, you could hear a tumbleweed roll across the desert as we documented what had been happening in the building.

    We found that one tenant had installed a quarter-round deflector on a supply register and added a rectangular cardboard box to force more air into one of his offices. We measured 37 cfm at the supply register as it was found. I moved his well-intended contraption and measured airflow again and watched as the hood displayed 97 cfm. “Lock ‘em up,” I told the thermostat sheriff.

    Symptoms of a Dirty Indoor Coil
    This building’s situation was pretty typical when you think about the normal symptoms of a dirty or damaged indoor coil, which include complaints of low airflow and the inability to cool properly. Another symptom is loud air noise, which was obvious when we walked into the mechanical room. Other symptoms can include high temperature drop over the indoor equipment and excessive blower motor watt consumption.

    We measured the dry bulb temperature drop over the equipment at 21.3ᵒ F. Interestingly, this is about right for a well-operating system in dry desert conditions. But it is not right for a system that is 63% low on airflow.   

    We measured blower motor watts at 642. For that sized system, the watts were at least three times higher than ideal. High utility bills? Oh yes!

    Static Pressure Testing
    Static pressure testing is normally the first test when you suspect a dirty or damaged coil. But the interests of the landlord and tenants flip-flopped our normal testing protocol.

    We measured the system’s total external static pressure (TESP). We installed a test port into the airstream and measured the furnace entering pressure at .58” w.c. and the furnace exiting pressure at 1.01” w.c. We added these two pressures together to find the system’s TESP of 1.59” w.c. Remember, the fan was rated at a maximum TESP of .50”w.c.  Eureka! We found the gold and the primary source of their problems.

    Next, we measured the pressure after the cooling coil to isolate the coil’s resistance to airflow. It was at .22” w.c. We subtracted .22” from the furnace exiting pressure of 1.01” to find a coil pressure drop of .79” w.c. This is three times the estimated coil pressure drop when it was new and clean.

    We didn’t stop there. We cracked open the coil housing and peeked in to discover a hot mess inside. Forensics lead us to conclude the unit was run without a filter during the recent demolition and building remodel project. A new furnace and filter were installed after the remodel was completed. The construction debris plugging the coil was evidence frozen in time for us to find.

    Prescription and Scope of Work
    Before we jumped in the Chevy Tahoe and headed west, towards Tahoe, we wrote a scope of work detailing needed repairs.

    Since a new furnace was the only new component installed, we recommended the coil and condensing unit also be replaced with new 16-SEER equipment matching the furnace. Any attempt to clean that coil would be futile. We included eight other improvements that would enable the system to provide the needed comfort and restore a sense of calm to the suite of offices.

    We proportionally balanced the system with a requirement for verifying airflow, then re-balancing and commissioning the system at the works completion. We wrote a stern letter for the landlord requiring tenants to be hands-off the system in the future. 

    After discovering the system problems and providing a scope of repairs, our diagnostic job was done. Perhaps what we did on this project will help you jump directly into measuring coil pressure drop to solve a low airflow problem. Then you too can ride off into the sunset after a job well done.

    Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free test procedure describing how to measure and interpret coil pressure drop, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles, and downloads.