Condensation on a display case usually means one of three things: the door gasket is letting warm humid air in, the anti-sweat heater has failed, or the defrost cycle isn’t finishing cleanly. Most of the time it’s the gasket. Here’s how to work through it.
The Most Likely Culprit: Door Gasket Gaps
The gasket is the rubber seal around the door perimeter. It keeps the cold air in and the warm, moist store air out. When it starts to fail, warm air seeps into the case, hits the cold glass, and leaves condensation or fog.
You can test it yourself. Close the door on a dollar bill and try to pull it out. If it slides out easily, the seal at that spot is weak. Do this all the way around, paying extra attention to the corners where the rubber tends to crack or pull away first.
You’ll also catch problems visually: look for cracked, torn, or compressed rubber, or a gasket that isn’t sitting flush. Sometimes the frame has warped slightly and the rubber can’t seat right even if it looks okay.
If the dollar-bill test fails or the gasket looks damaged, that’s your answer. A tech can confirm it and handle the replacement in one visit. It’s a cheap fix compared to the spoilage risk of running a case that won’t seal.
Anti-Sweat Heaters
Most commercial display cases have thin electric heaters built into the door frame and sometimes the mullion (the vertical divider between doors). Their job is to keep the frame just warm enough that the glass doesn’t sweat. When one fails, you’ll see condensation concentrated at the frame or along one edge of the glass rather than spread across the whole panel.
You can do a quick check: touch the door frame when the store is at normal humidity. It should feel slightly warm. If it’s cold, the heater element or its control circuit has stopped working.
Replacing the element means disassembling part of the door frame and wiring in a new one. That’s live electrical work inside a commercial appliance. Getting it wrong can cause shorts or create a safety hazard, and most operators don’t have the test equipment to verify the circuit is clean afterward. Call a tech.
Defrost Cycle Problems
Every refrigerated case runs a defrost cycle, usually two to four times a day, to melt frost off the evaporator coil. If the cycle isn’t completing or terminates too early, ice builds up on the evaporator and blocks airflow. In some cases, a defrost that overshoots can briefly raise the interior temperature, which shows up as moisture at the glass.
Signs this is the issue: the sweating appears on a schedule (same time of day, usually after a defrost), you’re seeing ice buildup near the vents, or the case is struggling to hold temperature alongside the condensation.
Defrost problems trace back to a failed heater element, a faulty termination thermostat, or a control board issue. Diagnosing and replacing those components requires test equipment and access to the electrical compartment. This one needs a licensed tech.
Ambient Humidity
Sometimes it’s not the equipment. If you’ve had a hot stretch of weather, left loading dock doors open, or had higher-than-normal foot traffic, the ambient store humidity may be high enough to overwhelm the case even when everything’s working.
Most display cases are rated for roughly 55% relative humidity or below. Once store RH climbs noticeably above that, sweating happens on well-maintained equipment too. Check your store thermostat if it shows humidity. HVAC adjustment or a dehumidifier in the affected area can help.
What a Tech Actually Checks
When we come out, we check the gasket first. Then we check frame and mullion temperatures with a contact thermometer to verify heater function. We pull the defrost log from the controller if the case has one, or watch a cycle manually. We also inspect the evaporator coil for ice buildup and check the drain pan and line, since a blocked drain can back up and freeze inside the case, which shows up as moisture problems.
The assessment usually takes 30 to 45 minutes on a single unit. Most single-cause problems, a gasket or heater replacement, can be resolved the same visit.
What You Can Check Yourself
- Dollar-bill test around the full door perimeter
- Visual inspection for cracked, compressed, or displaced gasket rubber
- Touch the door frame to feel whether the heater is warm
- Check ambient store humidity if you have a reading
If all of that checks out and the case is still sweating, the problem is past what’s visible from the outside. Don’t guess at defrost components or anti-sweat wiring. Call a tech.
When to Call
If the gasket looks fine (or you replaced it and the sweating came back), something else is going on. Same if the case is struggling to hold temperature alongside the moisture issue, because that usually points to ice on the evaporator or a refrigerant problem.
We work on commercial display cases across the Bay Area: walk-in units, deli cases, bakery reach-ins, prep tables. If your case is fogging or sweating and you want a straight answer, reach us at bayarearefrigerationservice.com. We aim for same or next-day on most service calls.