Summary: A tube mounted normally closed bi-metal low limit Frost Stat is located on the suction line close to the air coil. When the suction line temperature drops below ~36 deg F, the low limit opens generating an 'In Defrost' alarm. The In Defrost alarm disables the compressor(s) and the fan continuously operates. The unit remains in this condition until the low limit automatically resets at ~51 deg F. The unit returns to normal operation after the low limit has reset.
Troubleshooting: An In Defrost alarm is most likely to occur during the cooling mode. Check for issues related to related to low airflow, low charge, water side economizer valve leaking by/precooling the air, or a refrigerant restriction.
Notice: The unit may be equipped with a Freeze Stat located on the suction line of the coax or the suction line of the compressor. The Freeze Stat opens at ~36 deg F and closes at ~51 deg F is wired in series with the high- and low-pressure switches. When the Freeze Stat opens, the control generates a Compressor 1 or 2 failure. See What Causes Diagnostic: Compressor 1 or 2 Failure on WSHP? for more information.
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.