How Should Rooftop Unit (RTU) HVAC Systems Be Designed?

Rooftop unit design starts with an accurate building load calculation, then selects packaged units with the right capacity, efficiency rating, and features — economizers, demand-controlled ventilation, gas or heat pump heating. The design must coordinate curbs and structural support with the roofing system, provide code-required ventilation and exhaust, and define controls sequences so the units actually operate at their rated efficiency. RTUs dominate single-story commercial work because they're simple to install and service — everything is in one box on the roof. But that simplicity is deceptive. The design decisions around sizing, ventilation, and controls determine whether the owner gets two decades of quiet service or a chronic comfort and energy problem.

The roof is where RTU projects succeed or fail, and it involves three trades. Structural needs the unit weights and curb locations early to verify the roof framing — concentrated loads near midspan can require reinforcement. Roofing needs curb dimensions, heights, and locations to plan penetrations, flashing, and drainage paths. Electrical needs the MCA and MOCP values for feeder and disconnect sizing. Ventilation compliance is the other coordination point. The RTU has to deliver the code-required outdoor air for the occupancy, which means the design must show ventilation calculations, not just assume the unit's minimum damper position handles it. For restaurants and other high-exhaust occupancies, the makeup air strategy has to be designed alongside the RTU, or the building runs negative and the doors won't close properly.

RTU design looks simple on the surface, which is exactly why the details get skipped. I run every packaged-unit design through the same checks before it goes out. These are the items that catch the expensive mistakes.

• Load-based sizing: unit capacity tied to a real calculation, not a rule of thumb
• Economizer and ventilation compliance: code-required features actually specified and shown
• Curb and structural coordination: weights, locations, and curb heights resolved with structural and roofing
• Controls sequences: written sequences of operation, not just a thermostat schedule
• Service and replacement access: clearances, crane picks, and a realistic path for the unit 15 years from now