Walk-In Cooler Troubleshooting: Back to Basics

A Veteran Tech’s Guide to Systematic Diagnosis

We have all seen a walk-in cooler. They come in every size imaginable and more. Walk-ins can be used to store food, ripen fruit, or even age products. The equipment in them can range from basic mechanical thermostats to complex boards with a multitude of electronic components. Yet they all have one basic thing in common: keep product below a certain temperature.

Anatomy of a Walk-In Cooler

Modern-day walk-in boxes are foam-filled panels with a durable metal outer sheathing. They offer fully customizable color coatings, finishes, shapes and sizes. Old school coolers were wooden boxes and poorly insulated, often just multiple layers of wood to help with insulating the cavity. Before refrigeration, people would cut blocks of ice from frozen lakes and rivers and put them into insulated boxes to keep food longer.

Basic components of a walk-in cooler are like what you would find in any AC system:

• Condensing unit consisting of:
  • Compressor
  • Coil
  • Fan
  • Controls
  • Sight glass (hopefully)
• Evaporator assembly including:

For a deeper understanding of how these components work together, check out Gary’s article on The Refrigeration Cycle Explained.

The Basics

Walk-in coolers are designed to keep cold food cold for extended holding. Here are the key temperature requirements:

• Food temperature should be below 40 degrees Fahrenheit
• Air temperature should range from 34 to 38 degrees
• This ensures product temperature stays in the safe zone

Important: Walk-in coolers are designed to be loaded with chilled or cold product. They are not sized properly to handle the extra BTU load needed to chill hot products. I have some customers who insist on loading trays of hot, steaming pasta into a walk-in cooler and wonder why it cannot keep up.

Evaporator Configurations

Inside of the box, you’ll find your evaporator. They come in several configurations:

• Normal evaporators mounted to the ceiling (usually on one side closer to a wall)
• Low profile units
• Center mount systems
• Encapsulated systems mounted on top of the walk-in

These all share common components including fans to move the air, metering device to control refrigerant flow, the coil itself, and control systems. For insights into how evaporator issues can develop, check out Gary’s guide on Why Do Evaporator Coils Freeze.

Outside of the box, either on top, in another room or outside of the building, you will have your condensing unit containing your compressor, condensing fan and coil, controls and more.

Troubleshooting

ABC (Airflow Before Charge) is a critical principle that many technicians don’t follow. You need to give the system every opportunity to run on its own before you gauge up. This means checking:

1. Are both evaporator and condenser fans running?
2. Is your evaporator frozen up?
3. Are your coils clean and free of debris?

If any of those problems exist, correct them and see if your problem is fixed. In my experience, 95 percent of the time you do not need to put gauges on a system. For more modern diagnostic approaches (eg without gauges), see Jennifer Manzo’s guide to Non-Invasive System Testing.

Temperature Control Troubleshooting

If basic checks don’t reveal the issue, start with the evaporator side. You’ll typically find a temperature control device that can be:

• Powering a solenoid valve (in pump-down systems)
• Controlling the condensing unit contactor (on smaller systems)

With standard mechanical thermostats:

• Contacts should open below setpoint and close above setpoint
• Numbers can be misleading – I’ve seen units 10 degrees off that run perfectly
• Others can be 40 degrees off and need replacement

Modern Electronic Controllers

Electronic temp controllers are becoming the new standard, offering:

• Programmable defrosts
• Differential setpoints
• Minimum compressor off times
• More control over your system

Note: Most electronic thermostats use “dry” style contacts – no power supplied. You must provide the power source you want switched.

Refrigerant Considerations

When dealing with refrigerants in walk-in systems, there are several important factors to consider. Different refrigerants have unique properties and characteristics – for more details on how refrigerant blends behave differently, see our article on Azeotrope Refrigerants vs Zeotrope.

For this instance, let’s use R448, as that is what is becoming prevalent in walk-in coolers here lately. For a cooler, ideal evaporator temperature is 25 degrees. eSo in order to confirm that, you take your suction vapor pressure and at 50 psi converted to temperature is 25 degrees. Remember that every pressure is just converted to a temperature.

Working with Superheat

Let’s say your condensing unit is operating properly, airflow checks good, but you have a weird frost pattern and a suction pressure that is not adding up. You may have an issue metering refrigerant flow into your evaporator. Superheat is used to maintain proper, effective and efficient evaporator operation.

The majority of walk-in coolers will utilize a TXV to maintain proper superheat in the system. Here’s what you need to know:

• Superheat is measured by taking suction vapor pressure converted to temperature minus saturation temperature
• You ideally want to measure superheat at the outlet of the evaporator
• For a walk-in cooler, superheat at the evaporator should be 6 to 10 degrees
• Don’t adjust superheat until the box is close to normal operating temperatures

https://www.youtube.com/watch?v=_-Du1QToEE0

Adjusting the TXV is a slow process. A small adjustment can make a huge change. It is best to make a small adjustment and give it time to settle out before making another change.

Also, once the cooler superheat is properly set, I like to check it at the suction inlet at the condensing unit. This also is vital to ensure you are not allowing liquid to return to the compressor and possibly cause damage.

Closing Thoughts

In conclusion, troubleshooting a walk-in cooler requires a systematic approach and attention to detail. Understanding the fundamentals of refrigeration and airflow is key to diagnosing and resolving issues effectively. Always start with the basics—ensuring proper airflow, checking for blockages, and confirming system components are operational. From there, methodically work through the control systems, evaporator, and condensing unit. Remember, tools like pressure-temperature charts, superheat, and subcooling measurements are your best allies in ensuring the cooler operates efficiently and safely.

By following these principles and leveraging your knowledge, you’ll keep your walk-in coolers performing at their best, ensuring the products inside remain fresh and safe. Stay curious, stay safe, and keep learning—there’s always more to master in the world of refrigeration!