This article is written by Christopher Stephens of JVS Refrigeration in California, with just a few additions by me (Bryan) in italics. Thanks, Chris!

Reach-in refrigerators are an interesting side of our industry, often looked at as frustrating and troublesome. Since we usually see reach-ins in kitchens or convenience stores, the refrigerators are never located in a convenient place to work on them, and that tends to lead to frustration on the technician’s part. Please understand that my article pertains to medium-temperature refrigerators. I also advise you to use manufacturers' OEM parts when possible, as the unit was designed to work with them. One of the more misunderstood and misdiagnosed parts is the temperature controller.

Keep in mind that some refrigeration temperature controls sense the evaporator coil temperature (not the desired box temperature). Some use intake air sensors, and some use supply air sensors. The medium being sensed—coil, return air (intake), or supply air (discharge)—will greatly impact how the controls function and what impacts them.

Personally, I break temperature controllers down into five different types. Please understand that these are generic descriptions, and you should always lean on the manufacturer if possible to understand their control strategies. 

1. Standard pressure control – These work on the principle that saturated refrigerant is a constant temperature at any given pressure. This control style is not used very much anymore as a means of temperature control because it is not very precise, and to an untrained technician, it can be hard to set the temperature correctly. To use this control strategy, you need to understand what evaporator T.D. (temperature differential) your reach-in was designed with. As far as tools go, you will need a temperature pressure chart, an accurate set of refrigeration gauges, and an accurate thermometer. With all these tools, you can take your desired box temperature and find it on your pressure chart. Read across the pressure chart, and find the corresponding pressure for your desired box temperature. That will be your cut-in pressure to set your control at. Now, we need to find the cut-out setting. Typically, we want the system to have about a 5-8 degree differential between the cut-out and cut-in to reduce system short cycling. This will likely be about 8 degrees colder than the cut-in temperature, so take your desired box temperature and subtract your differential of 5-8, then subtract your designed evaporator T.D. (specific to the equipment but likely 20-30 degrees for reach-ins) and find that number on the temperature pressure chart. Then, read across the pressure chart and find the corresponding pressure, which will be your cut-out setting. Understand that pressure controls are never exact, so you will need to adjust accordingly in the field. If you'd like to learn more about P-T charts and the P-T relationship, we wrote a related article that you can read HERE.
2. Constant cut-in control (electromechanical) – These are one of the most common temperature controls that you will find in reach-in refrigerators because they are the most economical for the manufacturers. That's because they have an off cycle defrost built into them. They work by inserting the sensing bulb into the evaporator coil, and they have a set temperature that they turn back on (cut in) at, no matter how cold you turn the dial. They work very similarly to the pressure control, as they are designed with the evaporator T.D. (temperature differential) in mind. However, instead of using pressure, they sense the evaporator temp on the surface of the coil. They have a knob to adjust the cut-out temperature, but you have no control over the cut-in temperature. That is why they are called constant cut-in. By design, they also have a built-in defrost, as the cut in temperature is usually 37 to 41 degrees (for a cooler/refrigerator), depending on the manufacturer. They rely heavily on proper superheat and proper refrigerant charge. If the charge or superheat is incorrect, the coil could get too cold, and the control could prematurely shut off. That could lead a technician to diagnose a bad control if they did not understand how they work. If you come upon a reach in that is short cycling and shutting off too soon, make sure to check the charge and measure the evaporator superheat before you diagnose a bad control.

3. Constant cut-in (digital) – These work the same as the electromechanical control, but they typically have two probes, one to be located in the coil and one to be located in the return air stream. They tend to have more available features, such as an added defrost cycle based on time (every four hours, every six hours, etc.) while still using box temperature as a failsafe. For example, say the control has a defrost every four hours if the coil temperature comes above a pre-determined temp—say, 40 degrees—the control will terminate the defrost. The controls can also shut off the evaporator fan motors during the off cycle to save energy and reduce warm air intrusion into the unit. These types of controls are on many HC (hydrocarbon) units being built today.
4. Universal electromechanical – These typically have one sensing bulb that you mount in the return air stream. They turn on and off via the temp setting.
5. Universal digital – These are usually aftermarket controls and can have several different control strategies. They can usually be customized to do anything, from heating to cooling to defrost, depending on the manufacturer.

Something to understand is that reach-in refrigerators are usually designed to perform in a certain environment. If something changes, such as the ambient temp in that environment, or if doors are left open, the box will not perform correctly. I suggest you take a step back before you start throwing parts at a reach-in and evaluate the environment. You may find your problem there!

—Chris

As always, I suggest “Commercial Refrigeration for Air Conditioning Technicians” by Dick Wirz as the bible for refrigeration training.