A freeze stat is a temperature sensing device for HVAC that monitors a heat exchanger to prevent its coils from freezing. Freeze stats can be used on both freon-to-air, and freon-to-liquid type heat exchangers and serve different purposes with similar goals for each.
The purpose of an air coil freeze stat is to keep the freon-to-air heat exchanger (commonly called air coils) from freezing. This kind of freeze stat is typically used for heating coils which are exposed to outside air. To accomplish this, they typically shut down the flow of outside air to a mixing box when the temperature reaches a predetermined setpoint. The setpoint for air coil freeze stats is typically about 38 degrees Fahrenheit which is approximately when the dew point temperature of the air starts to drop below freezing point.
The water coil freeze stat performs a similar function to the air coil freeze stat, but is used on a different type of HVAC system. Its function is to keep the freon-to-liquid heat exchanger (commonly called liquid coils) from freezing.
Conceptually, mechanical freeze stats are constructed with a diaphragm, a capillary tube and bulb, and an electric switch. The capillary tube allows gas movement to and from the capillary bulb and the diaphragm with a fall or rise in temperature, respectively. When the temperature reaches a specific setpoint, the pressure in the diaphragm will trip a switch which typically shuts down the flow of outside air while the capillary bulb's temperature is at or below the setpoint. Mechanical freeze stats can have more than one set of contacts, and the contacts can be NO or NC.
Digital freeze stats, also known as electronic or solid state freeze stats, use an electronic circuit, microprocessor, or microcontroller in place of the mechanical freeze stat's diaphragm and switch. They also use one or more electric or electronic sensors in place of the capillary tube and bulb for temperature sensing. Digital freeze stats utilizing microcontrollers or microprocessors can also perform more advanced functions than simply opening or closing switch contacts. Digital freeze stats can perform seeminly intelligent operations such as reversing the refrigerant flow and using some building heat to thaw a freon-to-liquid heat exchanger and then restoring the refrigerant flow to its normal direction once the danger of freezing the water coil has been averted.
Freeze Stat Uses
- Heat pump
- Geothermal heat pump
- Air handlers/mixing boxes
