If you’re walking into your restaurant in the morning and the walk-in is reading 45°F or higher, something failed overnight. Here’s what usually causes it and how to figure out which one you’re dealing with.
Door Gaskets and Door Alignment (Most Common)
Start here before anything else. A bad gasket is the single most common reason a walk-in loses temperature slowly over a long, unattended period. During the day, the compressor catches up to small losses. At night, with nobody opening and closing the door, a leaking gasket bleeds cold air for eight hours straight.
Run your hand along the full perimeter of the door with it closed and feel for cold air. You can also hold a dollar bill against the gasket and close the door; if it slides out without resistance, the seal isn’t making good contact. Check the corners especially, they crack and pull away first.
Also check door alignment. If the door is sagging or the self-closer is weak, it may not be sealing even when it looks shut. Hinges wear out. If the door isn’t hanging square or the closer is dragging, that’s a quick fix for a tech during the same visit.
Evaporator Fan Failure
The evaporator fan inside the cooler circulates air across the coils and keeps the temperature even throughout the box. If one of those fan motors seizes up, the compressor can still run and the coils can still get cold, but the air isn’t moving. You’ll see the box struggle to maintain temp and often get stratification: cold near the coil, warm higher up or in the back.
Open the cooler and listen. You should hear the fan(s) running. If you don’t hear airflow, that’s a strong indicator. Ice on the fan blade usually points to a separate defrost issue rather than the motor itself. Either way, a tech needs to pull the component and test it.
Defrost System Failure
Walk-in coolers run a defrost cycle to melt frost off the evaporator coils. If that cycle fails, frost builds up over days until the coils are essentially encased in ice. At that point, no air can move across them and the cooler stops cooling efficiently.
Look at the evaporator coils. If they’re covered in a thick layer of frost or ice, defrost failure is likely. The cause could be the defrost heater, the defrost timer or control board, or the defrost termination thermostat. A technician tests each component individually to find which one failed. Don’t chip at the ice yourself; you’ll damage the coils.
Refrigerant Loss
Low refrigerant is probably the most feared cause but actually not the most common. When refrigerant leaks, the system can’t absorb enough heat and the box can’t pull down to proper temp. Symptoms include the box running continuously without reaching setpoint and the compressor running hot. A technician confirms this by checking system pressures at the service ports.
Refrigerant handling requires an EPA Section 608 certification. More importantly, topping off without finding the leak is money down the drain. A tech finds the leak, repairs it, then recharges to spec.
Condenser Coils and Airflow Around the Compressor
The condenser is the outdoor (or remote) unit that dumps heat out of the system. If the coils are dirty or the area around the unit is cramped, the system can’t reject heat efficiently. It’ll run longer, work harder, and eventually not keep up overnight.
You can check that nothing is physically blocking airflow around the condensing unit. Cardboard, stored items, grease buildup near a kitchen exhaust vent. That’s worth clearing yourself. Actual coil cleaning involves chemicals and technique that can damage the fins if done wrong; it’s part of a routine service call and not something to improvise.
How a Tech Diagnoses This
When I send a tech out for a walk-in that’s not holding temp overnight, the process is systematic. Pull the temperature log if the unit has one, then check door seals, fan operation, and evaporator coil condition visually. Then test the defrost heater, thermostat, and timer individually. Then check refrigerant pressures at the service ports.
Most of the time, it’s not refrigerant. It’s a gasket, a fan motor, or iced-up coils from a failed defrost. Those are faster and cheaper fixes. Even refrigerant work isn’t complicated when you have the right equipment and an EPA cert.
What You Can Check Right Now
Before calling, run through these:
- Door gasket (dollar bill test, feel for cold air leaks along the perimeter)
- Whether the door is hanging square and the self-closer is engaging
- Whether the evaporator fans are running (open the door and listen)
- Whether the coils are visibly coated in frost or ice
- Whether anything is blocking airflow to the condensing unit outside
If you find something on that list, report it when you call. It helps the tech bring the right parts.
Everything else (refrigerant diagnosis, defrost component testing, electrical work, anything inside the refrigerant circuit) needs a certified technician with the right tools.
Call Before It Gets Worse
If the door checks out and the fans are running and the coils aren’t visibly iced over, the problem is likely refrigerant or a defrost component. The longer the compressor runs at the wrong pressures, the more wear you’re putting on it. A diagnosis call now is cheaper than a compressor replacement later.
The FDA Food Code requires most potentially hazardous foods to be held at 41°F or below. If your walk-in is running warm overnight, you have a food safety issue, not just an equipment issue.
We do commercial refrigeration work in the Tri-Valley and East Bay alongside HVAC and appliance repair. Call us or book at adriumservice.com and describe what you’re seeing. We’ll get you on the schedule fast, often same or next day when we can. If a morning high-temp alarm has happened more than once, get someone out before product loss forces the issue.