Rooftop Unit Changeout in a Strip Center
A property manager for a 2,500 ft² retail suite in Tulsa authorizes replacement of a 10-ton gas/electric rooftop unit with a 12-ton unit because the suite 'never keeps up' after a new server room was added. The existing curb is 10 feet from a make-up air intake and 4 feet from a plumbing vent. The new unit has a 1/2-inch gas connection; the existing black-iron line is 1/2 inch, 60 feet long, with four elbows and a 5-foot rise, serving only this unit. Electrical is 60-amp, 208/230-volt single-phase. The building official requires a mechanical permit and plans. The contractor orders the 12-ton unit before checking the load or the flue. The suite has 10-foot ceilings, the server room adds 6 kW of sensible load, and the existing 8-inch single-wall flue terminates 2 feet above the roof surface.
1. What calculations and code checks must be completed before the order is released?
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The contractor should complete a commercial load calculation that includes the server room's 6 kW sensible load, the envelope, occupancy, lighting, and ventilation. Only then can the 12-ton selection be justified; the existing 10-ton unit may be failing for reasons other than capacity, such as low airflow, refrigerant charge, or economizer failure. The contractor must verify the curb and roof structure can carry the new unit and that the unit's weight and dimensions match the curb. Gas piping must be sized by the longest-length method using the new unit's input rating and the available pressure drop. Electrical must be checked against the unit nameplate minimum circuit ampacity and maximum overcurrent protection, not just the existing 60-amp disconnect. The flue must be listed for the appliance category and installed with the required clearances and termination height. Plan review, permit issuance, and inspection sequencing must be scheduled before installation.
2. The contractor wants to reuse the 1/2-inch gas line and 60-amp circuit. Evaluate both.
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Neither can be accepted merely because it served the old unit. A 12-ton package unit with gas heat will have a much higher input than the old 10-ton unit, often in the range of 200,000 Btu/h or more. A 1/2-inch black-iron line at 60 feet equivalent length cannot carry that input at normal available pressure drop; the contractor must use the gas code table or calculation method, include equivalent length for the four elbows and rise, and likely upsize the line substantially. On the electrical side, the existing 60-amp circuit may be inadequate depending on the new unit's nameplate. The contractor must compare the nameplate minimum circuit ampacity and maximum overcurrent protection to the conductor size, disconnect rating, and breaker. If the nameplate requires a larger circuit, the contractor must install it. Reusing undersized gas or electrical components creates a fire, carbon-monoxide, or nuisance-trip hazard and will fail inspection.
3. What inspection sequence and documentation should the contractor use?
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The contractor should obtain the mechanical permit before starting, submit the load calculation and equipment cut sheets if required, and request rough-in inspections for the gas piping, electrical connections, condensate drain, and flue before concealment or startup. The final inspection should verify clearances, combustion air, vent termination, gas pressure, electrical protection, condensate disposal, and equipment access. The contractor should retain the load calculation, gas-sizing calculation, nameplate data, AHRI or manufacturer match information if applicable, and commissioning readings such as static pressure, temperature rise, and refrigerant charge. Any deviation from the approved plans should be documented and approved before the work is covered. This sequence protects the contractor if the building official questions the size increase or the reused infrastructure.