If your walk-in cooler is freezing product on the bottom shelves while the top shelves are fine, the most likely cause is a thermostat probe that’s mounted in the wrong spot, combined with normal cold-air stratification. Cold air is dense and sinks. If your temperature sensor is reading the warmest air in the box, it keeps calling for more cooling while the coldest air pools at floor level, freezing whatever sits down there.
Why Cold Air Stratifies in a Walk-In
Evaporator fans pull warm air across the coil, drop its temperature, and blow it back into the box. That cold discharge air falls. It collects along the floor, especially near walls and under shelving where airflow is weakest. The top of the box stays warmer because that’s where return air gathers before it gets pulled back to the coil.
A cooler running at a setpoint of 38°F can have meaningful temperature variation from floor to ceiling — that’s normal in any refrigerated space. The problem is when the probe senses the warm end of that range and keeps calling for more cooling, the floor zone drops below freezing.
Most Likely Causes, in Order
Thermostat bulb or probe in the wrong location. This is the first thing I check. On older mechanical thermostats, the sensing bulb is a copper capillary tube that’s supposed to be placed where it can sense representative return air, away from direct coil discharge and away from dead air pockets near the ceiling. A lot of units get installed with the bulb zip-tied near the top of the evaporator housing for convenience. That’s wrong. The controller reads warm air, keeps running, and the floor turns into a glacier.
On digital controllers, the probe location matters just as much. Check your manufacturer’s installation docs for the recommended placement on your specific unit. As a general field rule, away from direct coil discharge and away from dead spots is the starting point.
Evaporator fan blowing cold air straight down a wall. Some walk-in layouts put shelving tight against the wall opposite the evaporator. The fan blows cold air horizontally, it hits the back wall, drops, and channels along the floor. Lower shelves in that path take the full brunt. You can confirm this with a cheap IR thermometer: scan across shelf heights and see where the temperature drops off.
Defrost cycle not completing properly. If defrost is running too infrequently or the termination thermostat isn’t working and the cycle ends before the coil is fully clear, ice builds up on the coil fins. Airflow gets restricted and the coil runs colder than it should during the next refrigeration cycle. The discharge air comes out colder, falls, and freezes what’s on the floor.
Refrigerant or expansion valve issues. Less common as a cause of freezing only the bottom shelf, but a refrigerant problem or a failing expansion valve can cause the coil to run abnormally cold. If the whole box is overcooling and not just the floor zone, this is more likely what you’re dealing with. It takes gauges and a proper superheat and subcooling check to diagnose correctly.
How a Tech Diagnoses It
The first step is a temperature map. I’ll place a couple of data loggers or a calibrated probe at floor level, mid-shelf, and near the ceiling and run the box through a full refrigeration cycle. That tells you what the actual temperature spread is and where the control point sits relative to the cold zone.
From there it’s looking at where the sensing element is located and comparing it to the manufacturer’s spec. If the docs aren’t available, checking that the probe is sensing return air rather than discharge air or a dead air pocket is the practical test.
I’ll also pull the electrical panel cover and check defrost termination, defrost interval, and whether the termination thermostat is functioning. On digital controllers, the setpoint, offset, and sometimes defrost history can be read right off the display.
Refrigerant checks come last if the temperature data suggests the coil is running abnormally cold. That means gauges and a proper superheat and subcooling check.
What to Check Before You Call
Move temperature-sensitive product off the floor shelves right now. Dairy and leafy greens belong on middle or upper shelves until this is resolved. That’s the only zero-risk thing you can do while waiting for a tech.
Take a look at where the thermostat probe is mounted. If it’s above the evaporator housing, tucked near the ceiling, or clipped to conduit away from any airflow, that’s useful information to tell your tech. Don’t move it yourself — a probe relocated to the wrong spot without knowing the manufacturer spec can make the overcooling worse.
If you can see ice buildup on the coil, note it. That’s a symptom worth reporting, not something to chip away at.
Refrigerant and wiring are not owner jobs. Refrigerant work requires an EPA 608 certification, and guessing at wiring or defrost controls on a commercial walk-in is how a probe fix turns into a compressor replacement.
Call Us
If the floor zone keeps freezing product, a tech needs to map temperatures, check the probe location against spec, and pull the defrost data. That’s a couple hours of diagnostic work on a first visit, often resolved the same trip.
If the whole box is running below setpoint, not just the floor, that points to refrigerant or a valve problem. Either way it’s not a watch-and-wait situation — continued overcooling damages product and the equipment.
Bay Area Refrigeration Service handles walk-in diagnostics and repairs across the Bay Area. Call or reach out at bayarearefrigerationservice.com and we’ll get you on the schedule fast, often same or next day when we can.