Refrigerated Display Case Problems: Warm Product, Fog and Leaks

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Display cases are the hardest-working refrigeration in a grocery store, market, deli or convenience store. Open multi-deck cases hold temperature with nothing but a curtain of cold air, and glass-door cases fight fog every time a customer lingers. When product warms, glass fogs or water pools in the aisle, the cause is usually airflow, stocking, a heater or defrost fault, or a problem back at the condensing unit or rack. This guide sorts the symptoms and marks what store staff can check and when to call. It is part of our Commercial Refrigeration Help Center.

Open cases and glass-door cases work differently

An open multi-deck case blows cold air from a grille at the top front (the discharge air grille) down across the shelf fronts to a return grille at the bottom. That moving air curtain is the only thing separating the product from the store. Anything that breaks the curtain, such as product stacked past the load line, a shelf tag hanging in the airstream, a floor fan, or an HVAC diffuser blowing across the case, lets warm air fall in. A glass-door case keeps the cold behind doors but has to keep the glass clear, so its frames and often the glass carry anti-sweat heaters. Either type can be self-contained, with its own compressor, or remote, fed from a condensing unit or a compressor rack in a back room or on the roof. Our Bay Area commercial refrigeration systems guide explains the system types.

Symptom finder

Warm product in an open case

Start with stocking. Every open case has a load limit line marked on the end panels or shelves; product above it sits in the path of the air curtain and pushes cold air out into the aisle. Keep the return air grille at the bottom front clear of product, price signs and spills. Look for a floor fan, an entrance door or an HVAC supply vent blowing across the case front, which can tear the curtain apart. Then check that the case fans are running (you should feel steady air at the discharge grille) and that the discharge honeycomb is not clogged with dust or labels. If all of that is right and the case is still warm, the evaporator may be iced or the case may not be getting enough refrigerant, both technician jobs. Our grocery store maintenance guide covers the routine cleaning that prevents most of this.

Know your targets. CalCode requires a thermometer in each refrigeration unit, placed to read the warmest part (HSC 114157); in an open case, that is usually the front of the top shelf. The same section of CalCode that sets the 41°F rule allows some items, including raw shell eggs and pasteurized milk and milk products in original, sealed containers, to be held at or below 45°F (HSC 113996). Everything else potentially hazardous in the case stays at 41°F or below.

Fogging, sweating and anti-sweat heaters

Fog on the inside of a glass door right after it closes is normal and should clear quickly. Fog on the outside, or fog that stays, means the glass or frame is colder than the store air's dew point. The usual cause is an anti-sweat heater that has failed, come unplugged or been switched off to save energy. Many cases use anti-sweat heater controls that switch the heaters by store humidity; ENERGY STAR notes that certified models use components such as hot gas anti-sweat heaters and ECM fan motors (ENERGY STAR), and PG&E's business rebate catalog lists anti-sweat heater controls for both coolers and freezers as rebate measures (PG&E rebate catalog). A control that is set too lean leaves doors fogged on humid days. Door gaskets matter here too: a torn gasket lets warm air in and puts frost on the inside of the frame. Heater and control repairs involve line-voltage wiring in the door frame, so they are technician work.

Night covers and store hours

An open case loses cold all night to an empty store. Pull-down night covers, used every night after close, slow that loss and give the case an easier start to the morning. They only help if they are fully down and not caught on product. They also matter in an outage: the California Department of Public Health tells retailers to cover open refrigerated display cases or consolidate products to lessen heat gain when power fails (CDPH). Our power outage page covers what to do with product when the lights go out.

What store staff can safely check

1. Probe product and log it. Check the warmest spots, usually top shelf front and near the case ends, and write down the time and readings.
2. Fix the stocking. Pull product back below the load line and away from the return and discharge grilles.
3. Find the draft. Turn off or redirect floor fans, and note whether a door or HVAC vent blows across the case.
4. Check the doors. On glass-door cases, make sure every door closes and seals, and that no one has propped one open for stocking.
5. Look for alarms. Check the case display, the store's refrigeration controller or monitoring system, and the breaker panel. Reset a tripped breaker once only.
6. Handle water safely. Clean the aisle, sign it, and look for whether water comes from the case drain, the front, or the ceiling. Our refrigeration water leaks page covers drains and pans.

Stop and call when

• More than one case, or a whole lineup, warms at the same time.
• The evaporator coil inside the case is iced, or the case fans have stopped.
• Glass stays fogged with the doors closed.
• You hear hissing, see oil at fittings under the case, or smell refrigerant.
• The compressor or condensing unit is tripping breakers, running constantly, or silent.

Staff should not remove case bottoms or electrical covers, and should never adjust refrigeration valves or controllers on a remote system. Opening a refrigerant circuit is a job for an EPA Section 608 certified technician (40 CFR 82.161). Many newer self-contained cases use R-290 (propane); EPA's 2024 rule allows up to 494 g of R-290 in open cases and 304 g in units with doors (EPA). Keep flames and sparks away from any case you suspect is leaking.

When the problem is back at the rack or condensing unit

Remote cases share their refrigeration. When several cases warm together, a technician looks at the condensing unit or rack first: a dirty or blocked condenser, a failed condenser fan, a tripped compressor, a refrigerant leak, or a controller fault. Leaks matter for more than cooling. As of September 2026, the federal HFC leak repair rule has applied since January 1, 2026 to appliances holding 15 pounds or more of HFC refrigerant, with a 20 percent leak-rate trigger for commercial refrigeration and 30 days to repair (40 CFR 84.106). In California, CARB's Refrigerant Management Program covers facilities with a system holding more than 50 pounds of high-GWP refrigerant and calls for leaks to be fixed within 14 days of detection (CARB RMP). Our guide to how technicians detect refrigerant leaks explains the tools we use.

Older cases and racks often run on R-404A or R-507A. California has prohibited those refrigerants in new remote condensing units and supermarket systems, and in retrofits of them, since January 1, 2019 (CARB), and since January 1, 2025, SB 1206 has barred the sale of newly produced bulk refrigerants above a 2,200 GWP, which CARB says covers R-404A and R-507 (CARB). Certified reclaimed refrigerant can still be sold, so existing systems can be serviced. Our refrigerant rules page covers what that means for repair and replacement, and our condensing unit replacement service handles the equipment side.

Keeping cases reliable

• Train stockers on load lines and keeping grilles clear.
• Use night covers every night on open cases.
• Clean discharge honeycombs, return grilles and case drains on a schedule, and have coils and drains checked during commercial refrigeration service visits.
• If open cases cost too much to run, our energy cost guide covers doors, controls and current incentive programs.

Schedule display case service

Frequently asked questions

Written and reviewed by Jim McKinny, owner of Cool Aid Air Conditioning & Refrigeration — California C-20 and C-38 licensed, EPA Universal certified, second generation of a family business serving the Bay Area since 1966. Every job we complete is backed by our 1-year parts & labor warranty.