MSC Diagnostics: Freeze Stat Trips

When temperatures drop into the teens and single digits, many buildings experience nuisance tripping of air handling units via freeze stats. This is a common occurrence on 100% outside air and recirculating units, as well. While it is never a welcome situation to receive a midnight phone call, or discover a BAS screen full of alarms declaring that multiple air handlers are shut down, it is particularly dire in a mission critical environment where temperature and pressure are monitored.

When freeze stats trip, many facility managers will simply reset them, only to experience repeated freeze trips until warmer weather prevails. One facility decided they’d had enough during the recent Arctic freeze and called MSC in to locate and fix the problem. The technician’s initial review found many freeze stats improperly strung across the downstream coils. Using a bright light, he also discovered a small gap between the upper and lower preheat coils, allowing cold air to bypass the coils and run toward the freeze stats.

Once the freeze stats were properly restrung and small gaps around the coils were sealed, MSC was ready to get into the important phase of testing. Using a special thermograph video, MSC was able observe flow and preheat valve position under normal sequence of operation with 19F incoming air, and the problem soon became clear. As the preheat control valve operated to steadily maintain the AHU discharge air temperature, flow in the coils decreased significantly. This loss of critical heat resulted in the repeated freeze stat tripping.

MSC ruled out quick fixes. Raising preheat temperature would only compound problems by closing the preheat coil control valve even further and decreasing flow. Overriding the control valves to a higher open position would jeopardize proper discharge air temperature.

The solution: specifically-sized freeze protection pumps for the coils. Programming the new freeze protection pumps to come on when outside air temperatures hit 30F provided greatly improved hot pressure and water flow across all coils and manifolds, thus providing even heat distribution and uniform flow, which was then verified by the thermograph video. Problem solved.