Refrigeration technicians are often called upon to service and repair reach-in coolers. The refrigeration systems used on these units are very similar among models from various manufacturers. Most of these coolers are self-contained with an air-cooled condensing unit, use capillary tubes as their metering device, and use a constant cut-in temperature control to maintain case temperature.

A common repair on these coolers is to replace the temperature control. Using the wrong type of temperature control can cause problems down the road.

The temperature controls used on most reach-in coolers are designed to sense the temperature of the evaporator coil, not air temperature. There is a relationship between the coil temperature and temperature of the air entering the evaporator. Most evaporators are designed for a 10 to 15 degree F temperature difference between the evaporator coil temperature and its entering air temperature.

This being the case, it is easy for a manufacturer to control the case temperature by monitoring the coil temperature. When the coil reaches a specific temperature, it will correspond to a specific case temperature. For example, if the desired case temperature is 35 degrees and the evaporator has a 10 degree temperature difference, then when the evaporator coil reaches 25 degrees, it corresponds to a 35 degree case temperature.

Easier Isn’t Better

Why do manufacturers design their systems this way? Wouldn’t it be easier to just sense air temperature?

Although it would be easier, it would cause problems with the system. Because the evaporator normally operates below 32 degrees, frost will develop on the evaporator surface. This frost must be removed, otherwise it will continue to build and eventually block off the airflow through the evaporator. If the unit senses coil temperature, the manufacturer can design the system to completely defrost each time the unit cycles off on temperature. The typical temperature control used on these systems is a constant cut-in type. The cut-in temperature is fixed at a temperature that will ensure the evaporator is completely defrosted before allowing the compressor to cycle back on. The actual cut-in temperature can vary between different manufacturers and designs.

Several replacement temperature controls are available with a cut-in of either 38 or 41 degrees. These temperatures seem to work well for a universal replacement temperature control. It is usually best to obtain an OEM temperature control when possible, to ensure that the temperature cut-in is appropriate for the design. However, a universal temperature control will normally work well.

The cutout values of these temperature controls are adjustable to allow for colder and warmer case temperatures. Since the temperature control is actually sensing coil temperature, the cutout values of these temperature controls are adjusted to the coil temperature, which matches the desired case temperature.

When replacing these temperature controls, make sure that the sensing element is secured properly to the evaporator. Most evaporators are designed with a well, into which the sensing element can be inserted. Make sure the sensing element is inserted fully into the well to ensure that it will sense the proper temperature.

Replacing temperature controls on these coolers is not difficult as long as the right temperature control is used. Using the wrong temperature control will lead to problems.

Marchese is owner of Coldtronics in Pittsburgh. He can be reached at 412-734-4433,, or

Publication date: 03/03/2003