Armstrong Furnace ADFSTAT504-1 Defrost Air Tempering Thermostat FM21

The Armstrong Furnace ADFSTAT504-1 Defrost Air Tempering Thermostat FM21 is a temperature-responsive control part that regulates electrical circuits based on air-conditioning parameters during cooling and heating operations. It monitors surrounding temperatures and opens or closes contacts at predetermined set points to control connected components within the operating sequence. Mounted within air handling and heat pump HVAC systems, it operates as part of the thermal control circuit that regulates freezing-related airflow tempering. Responsive sensing enables accurate switching actions, coordinated control sequences, and balanced air humidity management during equipment operation.