Rooftop Package Unit Product

Overview

Rooftop package units are compact, self-contained HVAC systems designed to mount directly on commercial building roofs, eliminating the need for a separate mechanical room or central plant. Combining a direct-expansion (DX) cooling circuit, gas furnace or heat pump heating, economizer damper, and integrated controls in a single weathertight enclosure, the package unit delivers cooling and heating to a building floor or zone through a single duct connection. Capacities range from 3 to 15 tons, making them ideal for smaller office buildings, retail centers, warehouses, and modular expansion of larger facilities.

The unit operates by drawing outdoor and return air through an economizer mixing assembly, filtering it through cartridge media, running it across an evaporator coil for cooling (with refrigerant pumped by a reciprocating or scroll compressor), and pushing the conditioned air into ductwork via a centrifugal fan. When heating is required, a gas furnace or electric heating coil warms the supply air. The entire system responds to a simple setpoint thermostat mounted in an occupied space or a networked building automation interface.

How it works

Economizer damper operation: Outdoor and return air dampers respond to a thermostat-mounted or integrated sensor comparing outdoor air enthalpy (temperature and humidity) to the return air. When outdoor conditions are mild and dry, the outdoor damper opens and the return damper closes, allowing "free cooling" without running the compressor. During periods of high outdoor humidity or temperature, both dampers reposition to recirculate interior air.

Cooling cycle: When cooling is required, the economizer damper closes to outdoor air. Refrigerant in the evaporator coil absorbs sensible and latent heat from the supply air, cooling and dehumidifying it. The now-gaseous refrigerant travels to the rooftop condenser coil, where it rejects heat to outdoor air and is liquefied by the compressor discharge pressure. The compressor, driven by a three-phase motor, continuously circulates refrigerant between the evaporator and condenser.

Heating cycle: In heating mode, a gas furnace ignites in the discharge air path. The burner head mixes fuel gas with combustion air and ignites via direct-spark ignition. Flame sensors confirm successful combustion; if flame is lost, solenoid gas isolation valves de-energize, shutting off fuel flow. The heat exchanger transfers combustion heat into the supply air stream. Alternatively, in air-source heat pump models, the reversing valve redirects refrigerant flow to heat the indoor coil, while the outdoor condenser becomes the evaporator.

Air handling: A centrifugal supply fan driven by a belt or direct-drive motor pulls filtered air across the evaporator, through the gas furnace or heating coil section, and pressurizes it into the building ductwork. The fan's output is typically modulated via a capacitor run motor or soft-start to reduce energy waste during partial-load conditions.

Filter and drain: Slip-in cartridge filters are accessible via a hinged door on the unit casing. Evaporator condensate drains via a sloped pan and piped condensate trap to a building drain or designated location. Filter pressure drop is monitored and signaled via a visual light or electrical switch.

Subsystems

Direct Expansion Circuit is the heart of the cooling cycle and is hermetically sealed. Gas Heat Section provides draft-free heating with built-in safety interlocks. Economizer Assembly enables free cooling and reduces compressor runtime during shoulder seasons. Roof Curb and Flashing provides structural support and weatherproofing, critical since the unit sits directly on the building envelope.

Common failures

Refrigerant leaks at solder joints or valve stems reduce cooling capacity and require EPA-compliant recovery and re-pressurization. Gas valve pilot orifices clog from carbon deposits, preventing ignition; the valve cartridge must be replaced. Condenser coil fins become blocked with dust and pollen, degrading heat rejection and raising discharge temperature. Evaporator pans leak due to freeze-thaw cycling or corrosion. Filter cartridges bypass if gaskets shrink; cartridge replacement is the remedy.

Maintenance

Annual service includes cleaning or replacing filter cartridges, inspecting and flushing the condensate pan, checking for refrigerant leaks (electronic leak detector), verifying gas pressure and flame color during heating, and testing the economizer damper response to temperature changes. Soft-start or contactor contacts should be monitored for pitting and replaced if they remain drawn when de-energized. The curb and flashing should be inspected for rust or open seams after severe weather.

Installation

Rooftop units are delivered fully charged and factory-tested. Installation requires a structurally adequate curb and flashing to the roof penetration, electrical connection (typically 460 V, three-phase, 60 Hz), a single main duct from the unit to the building, and a condensate drain line to a trap and building drainage. The unit should be positioned to avoid wind-driven snow or rain ingestion and to facilitate maintenance access. Ductwork should be sealed and balanced to avoid negative pressure that draws unconditioned air through the roof.