What Is a Rooftop Unit?

A rooftop unit is a self-contained HVAC system that houses all components — compressor, condenser, evaporator, blower, and controls — in a single cabinet mounted on the roof. Ductwork runs from the unit through the roof into the building below.

RTUs are used in commercial buildings because they're space-efficient (they don't take up interior floor space), accessible for maintenance, and available in large capacities. A single RTU can serve an entire floor of a commercial building.

How RTUs Work

RTUs operate on the same refrigeration cycle as residential AC systems. In cooling mode, the evaporator coil inside the unit absorbs heat from return air drawn from the building. The condenser coil releases that heat to the outdoor air. The cooled air is then distributed through the ductwork.

Most commercial RTUs also include gas heating (a gas burner in the unit) or electric heat strips, making them year-round HVAC systems. Some larger buildings use separate dedicated heating systems, but for most commercial applications, the RTU handles both heating and cooling.

Sizes and Capacities

Residential AC systems are typically 1.5–5 tons. Commercial RTUs start at about 3 tons and go up to 25 tons or more for a single unit. Large buildings may have multiple RTUs serving different zones.

Proper sizing is critical for commercial applications. An undersized RTU can't maintain comfortable temperatures during peak demand. An oversized unit short-cycles, which reduces efficiency and increases wear — the same problem as oversized residential systems, but at a larger scale.

Maintenance Requirements

RTUs require regular maintenance to operate efficiently and reliably. Key maintenance tasks include:

Filter replacement: Commercial RTUs typically use larger filters than residential systems. Depending on the building's use and occupancy, filters may need replacement monthly to quarterly.

Coil cleaning: Both the evaporator and condenser coils accumulate dirt and debris. Dirty coils reduce efficiency and can cause the system to fail.

Belt inspection: Many RTUs use belt-drive blowers. Belts stretch and wear over time and need periodic adjustment or replacement.

Electrical inspection: Connections, contactors, and capacitors need regular inspection — the same components that fail in residential systems fail in commercial ones, just at larger scale.

Drain pan inspection: RTUs have condensate drain pans that can accumulate algae and debris. Overflowing drain pans can cause water damage to the roof and building interior.

When to Replace an RTU

Commercial RTUs typically last 15–20 years with proper maintenance. Signs that replacement may be approaching include frequent repairs, declining efficiency (rising energy costs), difficulty maintaining temperature, and refrigerant issues (especially if the unit uses R-22, which is no longer produced).

For multi-unit properties, a planned replacement schedule is more cost-effective than reactive replacement. Replacing units before they fail allows for competitive bidding, planned installation, and minimal disruption to tenants.