Many of the older fractional horsepower refrigeration systems, such as reach-in coolers or prep table coolers, use a mechanical constant cut-in control to regulate the air temperature within the case. This style of control directly measures the temperature of the evaporator coil to control the case temperature and provides a method of defrosting the coil during each off cycle. Each time the control cycles the compressor off, the control will not cycle the compressor back on until the evaporator is void of frost and reaches the desired cut-in temperature of the case.

Today’s newer systems do not use this style of temperature control. Most manufacturers today are designing systems using an electronic temperature controller, which directly senses air temperature. These controllers have many advantages over the older mechanical controls, including the following:

  • They allow for a more precise control of the case temperature;
  • They typically use an adjustable, timed initiated/temperature terminated defrost cycle, with an additional temperature sensor measuring the coil temperature;
  • They display the case temperature, so owners can observe the actual case temperature more easily;
  • Some incorporate an alarm function to alert the owner when a case temperature is too high; and
  • Many have short cycling protection for the compressor, using a minimum off time delay.

Since these controls also measure air temperature instead of coil temperature, they allow the system to operate with a slightly low refrigerant charge and potentially still maintain the case temperature. The older mechanical constant cut-in controls are more affected with a slight loss of refrigerant since the coil temperature at the sensing bulb will be different.


Field Modifications

When replacing an older mechanical temperature control, an option for some systems is to replace the older control with a newer electronic version. With some field modifications, it is possible to install an electronic controller on an older system. This will allow a system to operate with all the benefits of the newer electronic design.

The electronic controller requires a constant voltage, so one field modification is to determine how to provide this constant voltage to the new controller. On older cases, the evaporator fan motors were designed to be powered on all the time, so a good power source for these controllers is to wire them in parallel with the evaporator fan motor(s).

Another necessary field modification is the mounting of the new controller. Sometimes this is an easy task and other times it is very difficult. A few times, I have mounted the controller on the outside of the case and run the temperature sensor into the case to make the mounting and powering of the controller possible. Not a very desirable approach, but it can be done. Both of these required modifications may make the replacement infeasible, so before committing to this replacement, analyze the system to see if the new controller can be easily powered and mounted.

Also make sure the newer controller can handle the ampacity of the components being controlled. And finally, consider where to mount the temperature sensor — typically in the return air path of the evaporator works well.

Replacing a mechanical temperature control with an electronic controller is definitely a great option if you can make the required field modifications work.