There are two types of temperature controls used in commercial refrigeration:
Let’s start with thermostatic type controls. An air-sensing thermostat does just that: it senses air temperature. The control sensor tube is usually mounted in the evaporator housing. The evaporator is located inside the unit, usually at the top where the fan motor is mounted. The thermostat has a straight capillary or sensor. The capillary tube is mounted on the outside of the evaporator coil usually pushed into a tube that is mounted in the front of the evaporator.
An evaporator-sensing thermostat has a coiled capillary tube attached to it, which you can see pictured as a tight spiral to the left. The evaporative sensing capillary or coiled tube end push into a hole that is in the evaporator. It senses the temperature of the evaporator coil rather than air temperature.
These two controls are not interchangeable. If you put an air sensing control in place of an evaporative sensing control, the evaporator unit will shut off permanently, causing the temperature to rise. If you put an evaporative sensing control in place of an air sensing control the unit will continue to run, causing the evaporator to freeze up. If this happens there will be very little airflow, causing the temperature to rise.
How to spot a defective thermostat:
A commercial refrigeration thermostat can fail in two ways: in an open position or a closed position.
If the thermostat fails in the open position, the unit will not run at all. To check this, remove the screws from the evaporator housing (make sure the unit is unplugged or the breaker is off) and pull the housing down. Locate the wires attached to the thermostat and remove them. Connect the two wires together and tape with electrical tape. Turn on the breaker or plug the unit back in. If the unit runs, replace the thermostat.
If the thermostat fails in the closed position, the unit will run all the time. Running constantly will cause the evaporator to freeze up, restricting the airflow and causing the temperature to rise. Use the same procedure described above to test the unit and replace the thermostat if necessary. Also be sure to defrost the evaporator before turning the unit back on.
If the evaporator continues to freeze up after the thermostat replacement, call a service company as you may have other problems requiring a trained technician.
The other type of control is a low-pressure control. These are usually located in the compressor compartment.
A low-pressure control is connected into the refrigeration lines and controls the temperature by using the pressure of refrigerant flowing through the line. This type of control requires a service technician to replace.