A grocery store manager in Warren couldn’t figure out why the front of the store stayed cold enough for customers to keep their coats on while the back of the store, near the deli counter, ran uncomfortably warm. Both zones were served by the same rooftop system. Neither zone had a mechanical problem. The open refrigerated display cases running the length of the front aisles were the real thermostat in the room, and nobody had accounted for that when the store’s HVAC was last serviced.

Why Open Cases Change the Whole Load Calculation

Open refrigerated and frozen display cases constantly spill cold air into the surrounding aisle to keep product temperature down. In winter, that cold air actively fights the store’s heating system. In summer, it does part of the cooling system’s job for free. That interaction isn’t a bug in the store’s design. It’s a documented characteristic of supermarket HVAC. A National Renewable Energy Laboratory report prepared for the U.S. Department of Energy describes how HVAC designers account for it directly through what the industry calls “sensible case credits,” negative loads applied to the space to represent the cooling effect of the cases, and notes that in many supermarkets, refrigerated cases provide the majority of the sensible cooling in the sales area during the cooling season. In the heating season, that same cooling effect has to be counteracted rather than relied on. A comfort-focused system, sized the way an office or retail space would be, never accounts for that dynamic.

What Was Actually Happening in Warren

The front-of-store rooftop unit had been running a standard comfort setpoint without ever factoring in the cold air spilling from a full run of open dairy and produce cases nearby. That spillage pushed the front aisles well below the thermostat’s target. The same system, trying to average conditions across the whole floor, ended up underserving the deli area at the back. Refrigeration cases were fewer there, and the ambient temperature ran warmer.

The Fix: Re-Zoning Around the Refrigeration Load

The fix wasn’t a bigger rooftop unit. It was re-zoning the store’s HVAC controls to treat the areas near heavy refrigeration case concentration differently from areas without them, instead of running one blended setpoint across the entire sales floor. Additional destratification and circulation fans near the deli and bakery counters helped even out the temperature the open cases weren’t naturally cooling in that part of the store.

Why This Gets Missed During Routine Service

A technician doing a standard seasonal maintenance visit checks the rooftop unit’s operation against its programmed setpoint and calls it good if the unit is performing to spec. That check misses the store-specific load created by refrigeration equipment. That refrigeration equipment sits outside the HVAC system entirely. It drives a meaningful share of the store’s actual temperature pattern anyway. Refrigeration systems account for approximately one-half of the total energy consumed by a typical grocery store, according to ASHRAE’s Advanced Energy Design Guide for Grocery Stores, and the guide specifically calls out the heating load created by open refrigerated cases as one of the ways that system interacts with the rest of the building. Grocery and supermarket HVAC service requires someone who understands that interaction specifically. Standard rooftop unit maintenance applied to a building that happens to sell groceries isn’t enough.

Common Questions

Does adding more refrigerated cases always make the HVAC problem worse? It depends on where the cases go. Any change to the case layout shifts the store’s actual cooling and heating needs, and that shift should trigger a review of the HVAC zoning rather than an assumption that the existing setup still applies.

Can the refrigeration system and the HVAC system share any equipment? In some newer supermarket designs, heat rejected from refrigeration compressors gets recovered and used to help heat the store in winter, though this requires a system specifically designed for that integration rather than being something an existing store can add without real engineering.

Why does the front of a grocery store often feel colder than the back? It’s usually the concentration of open refrigerated cases near the entrance and produce section. Those cases spill cold air into the aisle no matter what the store’s thermostat is set to.

What Happened at the Warren Store

The re-zoning work at the Warren store cost $6,200: separate thermostat control and damper adjustments to split the front-of-store zone from the deli and bakery zone. Four destratification and circulation fans near the deli added another $2,600. The deli area, which had been running around 65 degrees, settled in around 70 degrees within the first week. The coat-wearing complaints at the front of the store stopped. The following winter’s heating bill came in about 12 percent lower than the season before, saving roughly $2,000 over the season. That’s enough to pay back the $8,800 re-zoning and fan project in a little over four winters at that rate. The rooftop unit was no longer fighting cold air spillage across a zone it was never actually responsible for.

Sources

Hirsch, NREL/DOE: Energy-Efficient Supermarket HVAC in Humid Climates in the United States (Sensible Case Credits)

ASHRAE: Advanced Energy Design Guide for Grocery Stores