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Troubleshooting Guide: Frozen HVAC Coils

by | Sep 25, 2020

Have the HVAC coils in your commercial building frozen? Use this as a guide to troubleshoot and determine the root cause of the freezing event.


*Note each end-user sets ATC programming logic as they see fit. Situations will vary from coil to coil. The following list reviews the basics in determining the root cause of coil freezes and can be used as a troubleshooting checklist.* 

// Failure of Preheat Coil:

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:

If your AHU utilizes a steam preheat coil:

  • Steam PRV or control valve failure
  • ATC failure
  • Blockage inside the steam or condensate piping
  • Loss of steam pressure from source (i.e. boiler, campus energy, etc.)

If your AHU utilizes a hot water preheat coil:

  • Hot water supply temperature should be between  120°F-180°F, designed leaving water temperature should be well over freezing temps
  • Check that the preheat hot water system has the proper GPM flow rate and pressure
  • Ensure the preheat coil is not air-bound – air vent if necessary



// Failure of Freeze Stat:

Customer-installed freeze stats sometimes fail to shut down the fan above the freezing point.

  • Check the set point of the freeze stat
  • Investigate freeze stat control circuit to make sure it is powered and functioning properly

// Rooftop AHUs / RTUs

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.

  • Make sure all side panel doors are closed
  • Check that no insulation has been removed from side panels
Split Frozen HVAC Coil and Freeze Block Coil

The Best Freeze Protection

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.


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