When a preheat coil fails to warm up the frigid outside air coming into the AHU the air can reach vulnerable coils causing them to freeze, bulge and split. If you suspect your preheat coil has failed check the following potential issues:
Customer installed freeze stats sometimes fail to shut down the fan above the freezing point.
When using blended air, OA dampers stuck wide open can trigger a freeze.
If outside air enters a side panel door on a rooftop AHU, frigid outside air can bypass the preheat coil and cause coils to freeze.
Even with safeguards in place on a standard coil, mechanical failure, human error, and extreme weather conditions can allow freezing air to reach vulnerable coils causing them to freeze, bulge and split.
Until now better technology for freeze protection was unavailable. For superior HVAC coil freeze protection consider replacing critical coils with Cooney Freeze Block Coils.