Afternoon Comfort Complaint at a Bakery
A 1,150-square-foot bakery calls in July because the sales area never drops below 78°F after 2 p.m. The space is served by one 2-ton packaged rooftop unit, 9 years old, that also conditions the production area. Since last summer the owner added a 12-foot refrigerated display case and a second deck oven, and afternoon staffing rose from 8 to 20 people. On a 96°F day at 2:30 p.m. the technician measures 76°F return air, 58°F supply air at the register closest to the unit, and 55% indoor relative humidity. Compressor current is at nameplate rated load amps, the condenser coil is clean, the filter is new, and the blower is on the correct speed tap. The owner says the unit "just needs another 3 pounds of refrigerant" and wants the technician to add it and leave.
1. Is the rooftop unit failing, or has the building load changed? Support your answer with the measured data and identify two additional measurements that would separate the two possibilities.
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The data point to a load problem, not an equipment failure. A supply-to-return temperature difference of 18°F (76°F minus 58°F) sits inside the normal 16-20°F range, compressor amps are at rated load, and the coils and filter are clean. The unit is removing roughly the heat it was designed to remove. What changed is the heat gain: a self-contained display case dumps its condenser heat into the sales area, a second deck oven adds sensible and latent load, and 12 extra people add about 250 Btu/h sensible each plus moisture. A 2-ton unit delivers only 24,000 Btu/h, while 1,150 ft² of bakery at 30-35 Btu/h/ft² already needs 34,000-40,000 Btu/h before the new appliances.
To confirm, measure total external static pressure and actual blower cfm, then calculate superheat and subcooling to prove the charge is correct. A room-by-room load calculation and a check of supply air delivery to the sales area complete the picture.
2. How should the technician answer the owner's request to add 3 pounds of R-410A and leave?
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Adding charge without a verified superheat or subcooling reading is guesswork that can make the complaint worse. On a TXV system, charge is set by subcooling at the liquid line; on a fixed orifice it is set by superheat. With the compressor already at rated amps and an 18°F temperature split, the unit is not visibly undercharged, and 3 pounds on a 2-ton system is a large fraction of its total charge. Overcharging raises condensing pressure and amperage, reduces capacity, and can shorten compressor life, all while the real problem, the added internal load, remains.
A defensible response is to explain, in writing if needed, that no leak was found and no charging indicator justifies refrigerant. Offer to document the measured capacity, perform the load calculation, and quote options such as a dedicated system for the production area, added ventilation, or relocated appliance heat. That protects the technician, the company, and the customer.
3. Outline the recommendation you would give the owner, in priority order.
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First, document what the existing system actually does: static pressure, cfm, superheat, subcooling, and temperature split. Second, complete a load calculation that includes the display case, oven, occupancy, and door traffic, because equipment selection without that number is a guess. Third, address load before capacity. Can the display case condenser be vented or relocated? Can the oven exhaust be captured? Can the production area be isolated with a partition and its own unit, such as a ductless mini-split? Fourth, verify the duct system can handle any added airflow; a larger rooftop on undersized ducts will simply trip on high limit or deliver poor throw. Finally, if a replacement is justified, size it to the calculated load, not to the old nameplate.