Packaged Rooftop Units

This specification covers factory-assembled, self-contained packaged rooftop units (RTUs) with integral direct-expansion refrigerant cooling, mounted on a roof curb and connected to a ducted air distribution system. Equipment covered includes the cabinet, compressors and refrigerant circuits, air-cooled condenser coil and condenser fans, indoor DX evaporator (cooling) coil, supply fan and motor, integral heating section, outdoor-air and economizer dampers, filtration, condensate management, unit-mounted controls, and the roof curb and vibration isolation interface.

A packaged rooftop unit is distinguished from a central station air handler by its integral, factory-charged refrigerant system: the compressors, condenser, and evaporator are contained within a single weatherproof cabinet, eliminating the field-installed chilled water plant and the associated hydronic piping. The unit rejects heat directly to outdoor air through its condenser and delivers conditioned supply air to the building through ductwork penetrating the roof curb. This packaging makes RTUs the dominant equipment type for low- and mid-rise commercial construction.

Equipment shall comply with ANSI/AHRI 340/360 for performance rating of commercial and industrial unitary equipment (65,000 Btu/h and above), ANSI/AHRI 210/240 for unitary equipment below 65,000 Btu/h, ANSI/ASHRAE/IES 90.1 for energy efficiency and economizer requirements, ANSI/ASHRAE 62.1 for outdoor-air ventilation, ANSI/ASHRAE 15 and ANSI/ASHRAE 34 for refrigerant safety and classification, ANSI Z21.47 / CSA 2.3 for gas-fired heating sections, NFPA 90A for fire and smoke safety, and UL 60335-2-40 for electrical and refrigerant-system safety listing.

The Contractor shall coordinate RTU installation with the roof curb and structural roof framing, the roofing and flashing system, ductwork connections through the curb, electrical power and control wiring, gas piping (where gas heat is provided), condensate drainage, and vibration isolation. Coordination with Building Automation SystemBuilding Automation SystemResolves to the current adopted revision.sync/building-automation-system is required for controls integration, and ductwork shall conform to HVAC DuctworkHVAC DuctworkResolves to the current adopted revision.sync/hvac-ductwork.

Equipment, materials, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited. Where conflicts exist between referenced standards, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.

Contractor shall submit the following for review and acceptance prior to ordering equipment. Fabrication and procurement shall not proceed until action submittals have been reviewed and returned.

• Manufacturer's product data for each RTU, including model designation, configuration, capacity ratings, and a unit weight and dimensional drawing showing the roof curb interface, supply and return openings, and required service clearances
• Certified performance data per AHRI 340/360 (or AHRI 210/240 for units below 65,000 Btu/h), including net total and sensible cooling capacity at design conditions, EER, IEER, and heating capacity and efficiency
• Refrigerant designation, ASHRAE 34 safety classification, total factory charge by circuit, and, for A2L refrigerants, the calculated charge limit and the leak-detection and mitigation provisions per UL 60335-2-40
• Supply fan performance data at the design external static pressure, including fan brake horsepower and ASHRAE 90.1 fan-power compliance documentation
• Compressor data including type, number of stages or capacity-modulation range, and minimum step of unloading
• Heating section data: for gas heat, input and output capacity, turndown, and ANSI Z21.47 certification; for electric heat, kW and stage count; for heat pump, integrated heating capacity and supplemental heat
• Economizer assembly data including damper leakage class, actuator type, changeover control method, and FDD provisions per ASHRAE 90.1
• Sound power level data per AMCA 300 in octave bands, for both supply discharge and outdoor (condenser) radiated sound
• Roof curb drawing showing curb height, dimensions, gasketing, supply and return duct openings, and the curb-to-unit interface; seismic and wind restraint details where required
• Electrical data including unit minimum circuit ampacity (MCA), maximum overcurrent protection (MOCP), voltage, phase, and disconnect configuration
• Filter data including MERV rating certification per ASHRAE 52.2 and rack configuration

Contractor shall provide the following at or before substantial completion:

• Operation and maintenance manuals with manufacturer's installation, operation, and maintenance instructions for the unit and all major components
• As-built wiring diagrams and the BAS points list with terminal designations
• Factory and field test reports, including the startup report by the manufacturer's representative
• Refrigerant charge record (type, total charge by circuit, and any field charge adjustment)
• Commissioning records, including economizer and FDD functional test results
• Air balance report from the TAB agent per Testing Adjusting And BalancingTesting, Adjusting, and Balancing for HVACResolves to the current adopted revision.sync/testing-adjusting-and-balancing
• Filter media installed at substantial completion with MERV certification and a change-out date tag
• Warranty documentation, including the separate compressor and heat exchanger warranties

RTUs shall be produced by a manufacturer regularly engaged in the production of packaged unitary equipment for a minimum of ten years, with a documented quality management system and the capability to provide replacement parts and factory-authorized service within the project region for a minimum of fifteen years after the date of manufacture.

Units rated 65,000 Btu/h and above shall be certified under the AHRI Unitary Large Equipment (ULE) or applicable certification program to AHRI 340/360, and published EER and IEER ratings shall bear the AHRI certification mark. Units below 65,000 Btu/h shall be certified to AHRI 210/240. Published capacities and efficiency ratings shall reflect AHRI-certified ratings, not uncertified catalog selections.

The complete unit, including all electrical and refrigerant-system components, shall be listed and labeled by a Nationally Recognized Testing Laboratory (NRTL) to UL 60335-2-40. Where the AHJ continues to accept UL 1995 listings for equipment manufactured before the transition date, UL 1995-listed equipment is acceptable. Gas-fired heating sections shall additionally be certified to ANSI Z21.47 / CSA 2.3.

A pre-installation conference shall be held before setting any RTU, attended by the Contractor, the roofing sub-contractor, the controls sub-contractor, the TAB agent, and the Owner's representative. The agenda shall include crane and rigging plan, curb setting and flashing sequence, structural load verification, utility connections, refrigerant A2L safety provisions, controls interface points, and the commissioning schedule.

RTUs shall be selected and rated for the operating conditions at the installation site. Design conditions are as indicated on the mechanical schedules. Because the entire unit, including compressors and the condenser, is exposed to outdoor weather, the cabinet, coatings, and components shall be suitable for continuous outdoor service.

The unit shall provide rated mechanical cooling at outdoor ambient temperatures up to 115°F and shall be capable of operation at the design summer ambient for the project climate zone. Where mechanical cooling is required at outdoor temperatures below 55°F (e.g., to serve interior loads in cold weather), low-ambient cooling provisions such as condenser fan cycling, condenser fan speed control, or head-pressure control shall be specified.

Where the unit is installed within approximately three miles of a saltwater coast, or in an industrial atmosphere with corrosive contaminants, the condenser and evaporator coils shall be protected with a corrosion-resistant coil treatment and the cabinet exterior shall receive an enhanced corrosion-resistant finish. Uncoated aluminum-fin condenser coils corrode rapidly in salt-laden air, reducing heat rejection and shortening compressor life.

Net total and net sensible cooling capacity at the design entering conditions shall be as indicated on the mechanical schedules. Capacity shall be the net value after deduction of supply fan heat, certified per AHRI 340/360. Sensible heat ratio shall be appropriate for the space load; selecting a unit by total capacity alone, without confirming sensible capacity, is a common cause of poor humidity control in spaces with high latent load.

Cooling efficiency shall meet or exceed the minimum EER and IEER required by ASHRAE 90.1 (or the adopted IECC) for the unit's capacity category and heating type. Integrated Energy Efficiency Ratio (IEER) represents weighted part-load performance across four load points and is the controlling metric for most commercial RTUs, which spend the majority of operating hours at part load. Units below 65,000 Btu/h are additionally rated by SEER2 and EER2 per AHRI 210/240. Selections that exceed the code minimum should be evaluated on life-cycle cost where energy rates or operating hours are high.

Under the EPA AIM Act HFC phasedown, new packaged unitary equipment manufactured for the US market has transitioned away from R-410A to lower-GWP A2L refrigerants. R-454B and R-32 are the predominant replacements; both carry an ASHRAE 34 A2L (lower-toxicity, lower-flammability) classification and a global warming potential substantially below R-410A. Specifying R-410A on new units is no longer aligned with current production and should be limited to replacement components for existing equipment.

For units charged with an A2L refrigerant, the unit shall comply with the charge-limit, leak-detection, and mitigation requirements of UL 60335-2-40 and ASHRAE 15. Where the refrigerant charge in a circuit exceeds the applicable limit, the unit shall include a factory-installed refrigerant leak detector that, on detection, initiates supply fan operation to dissipate refrigerant and de-energizes ignition sources. The Contractor shall not field-modify A2L charge or relocate detectors. Service personnel shall be trained for A2L equipment.

Compressors shall be hermetic or semi-hermetic scroll type. Capacity control determines part-load efficiency and comfort: single-stage compressors cycle fully on and off, multiple staged compressors provide stepped capacity, and variable-capacity (digital scroll or inverter) compressors modulate continuously to match load. Variable-capacity compression delivers the highest part-load efficiency and the tightest temperature and humidity control but at higher first cost; staged compression is the common choice for most commercial applications. Each refrigerant circuit shall be independent, with its own compressor, expansion device, and condenser section, so a single circuit failure does not disable all cooling.

The integral heating source shall be selected for the building's heating load, available fuel, and energy strategy. Gas-fired heating is the most common selection for commercial RTUs where natural gas is available. Electric resistance heat is used where gas is unavailable or for small heating loads. Heat pump (reverse-cycle) units provide efficient heating above the balance point and typically include supplemental electric heat for cold weather. A hydronic heating coil allows the unit to be served by a building hot water plant where one exists.

Gas-fired heating sections shall be certified to ANSI Z21.47 / CSA 2.3 with a stainless steel or aluminized steel heat exchanger as specified. Burners shall be staged or modulating to provide turndown that reduces temperature swing and short cycling. Combustion air and flue products shall be handled per the manufacturer's listing; the heat exchanger shall be warranted separately against failure from thermal cracking.

Electric resistance heating elements shall be staged and protected by automatic-reset and manual-reset high-limit thermal cutouts. Heat pump units shall report the integrated heating capacity at the design heating ambient and shall include supplemental electric resistance heat sized for the balance-point shortfall and for defrost recovery. Supplemental heat shall be locked out by the controls above the balance-point temperature to avoid unnecessary electric heating.

Where required by ASHRAE 90.1 Section 6.5.1 (or the adopted energy code) for the unit's capacity and climate zone, the unit shall include an integrated air-side economizer capable of modulating outdoor-air dampers to 100% outdoor air for free cooling when outdoor conditions are favorable. The economizer shall include a barometric or powered relief path to maintain building pressure at high outdoor-air fractions. A low-leakage outdoor-air damper is essential: a leaky economizer damper admits unconditioned air during mechanical cooling and heating, wasting energy year-round.

Differential enthalpy control captures the most economizer hours in humid climates but requires enthalpy sensors that need periodic calibration. Fixed or differential dry-bulb control is simpler and more reliable in dry climates. The design team shall select the changeover strategy appropriate to the climate zone and the Owner's maintenance capability.

Where required by ASHRAE 90.1 Section 6.4.3.12, the economizer shall include fault detection and diagnostics (FDD) capable of detecting and reporting faulty changeover sensors, damper-not-modulating conditions, excess or deficient outdoor air, and economizer-disabled-when-it-should-be-enabled conditions. Faults shall be reported to the BAS or annunciated locally so service personnel can act on them. Economizers without FDD are a frequent source of silent energy waste because a failed damper or sensor goes unnoticed for years.

The unit shall deliver the minimum outdoor airflow required by ASHRAE 62.1 for the served occupancy at all operating conditions, including reduced supply airflow on VAV systems. Reliable minimum outdoor-air control on a VAV unit generally requires an outdoor-air measurement and control method rather than a fixed damper position, because the outdoor-air fraction at a fixed damper position varies with supply fan speed and building pressure.

Where required by ASHRAE 90.1 Section 6.4.3.8 for densely occupied spaces, the unit shall provide demand-controlled ventilation (DCV) that resets outdoor airflow based on space CO2 concentration in proportion to actual occupancy. DCV reduces the energy penalty of ventilating for peak occupancy when a space is partly occupied, while maintaining ASHRAE 62.1 ventilation per person. DCV shall not reduce outdoor air below the area-based (base) ventilation rate required for the space.

The supply fan moves conditioned air through the evaporator coil and the building ductwork against the design external static pressure. Constant-volume units use a fixed-speed fan and are appropriate for single-zone applications. VAV units modulate supply airflow to match zone loads and shall use a variable-speed fan. Electronically commutated motor (ECM) and variable-frequency-drive (VFD) driven fans deliver the part-load energy savings that ASHRAE 90.1 requires for VAV and for multi-stage units. Forward-curved fans remain common in smaller packaged units, but backward-curved and ECM plenum fans provide higher efficiency and lower sound in larger units.

Supply airflow and design external static pressure shall be as indicated on the mechanical schedules. Fan brake horsepower at design conditions shall comply with the ASHRAE 90.1 fan-power limits. The fan shall be selected at the design point with margin for the loaded-filter condition so that the unit delivers design airflow throughout the filter service interval.

The cabinet shall be weatherproof, of double-wall insulated panel construction for all conditioned-air sections, with a galvanized steel structure and a corrosion-resistant exterior finish suitable for continuous outdoor exposure. Double-wall construction prevents exterior sweating, protects the insulation from erosion and moisture, and improves cabinet rigidity and sound attenuation. Panels shall be insulated to limit thermal loss and surface condensation.

Exterior cabinet panels shall receive a factory-applied, UV-resistant, corrosion-resistant finish demonstrated to withstand a minimum 1,000-hour salt-spray exposure per ASTM B117. Fasteners and hardware exposed to weather shall be corrosion-resistant. All field cuts and penetrations made during installation shall receive touch-up coating.

Hinged or removable access panels shall be provided at the filter, evaporator coil and drain pan, supply fan, compressor, and heating sections. Panels shall be gasketed and shall seal against air leakage. Access shall permit filter change, compressor service, and component replacement without removing the unit from the curb.

The unit shall include a filter section upstream of the evaporator coil with a rack sized for the specified filter. Air filtration shall comply with ASHRAE 52.2. MERV 8 is the practical minimum for commercial occupied spaces; MERV 13 is the recommended selection for general indoor air quality where the unit fan and cabinet can accommodate the additional pressure drop, and is increasingly required by code and green-building programs. The fan selection shall account for the final (loaded) filter resistance.

A sloped, corrosion-resistant condensate drain pan shall be provided beneath the evaporator coil, extending downstream of the coil face to capture carryover. The pan shall fully drain after each cooling cycle to prevent the standing water that promotes biological growth. A condensate trap sized for the negative static pressure at the pan shall be provided so the pan drains freely and unconditioned air is not drawn in through the drain.

The unit shall be set on a factory-supplied roof curb matched to the unit footprint and supply and return openings. The curb shall be a fully welded or mechanically fastened galvanized steel assembly with integral wood nailer for roof flashing, internal cross-supports under the duct openings, and gasketing at the unit-to-curb joint. The curb height shall provide the roofing flashing height required by the roofing system and shall be coordinated with HVAC DuctworkHVAC DuctworkResolves to the current adopted revision.sync/hvac-ductwork for the supply and return duct transitions. Curb dimensions, height, and duct opening locations are as detailed on the roof plan and curb detail.

Where the unit serves noise-sensitive spaces or is mounted over occupied areas with limited structural mass, vibration isolation shall be provided between the unit and the curb (isolation curb with spring or elastomeric rails) or integral to the unit base. Rigid mounting transmits compressor and fan vibration into the roof deck and the structure below; an isolation curb breaks that path. For units over heavy structural slabs remote from occupancy, a standard curb with the unit's integral isolation is generally adequate.

Where required by the applicable building code and ASCE 7 for the seismic design category and wind exposure, the unit and curb shall be restrained to resist the design seismic and wind forces while preserving the function of any vibration isolation. Restraint details and anchorage are as detailed on the structural drawings.

Unit electrical characteristics, including minimum circuit ampacity (MCA), maximum overcurrent protection (MOCP), voltage, and phase, shall be as published by the manufacturer for the selected configuration and shall be coordinated with the branch circuit per NFPA 70. A factory-installed, lockable unit-mounted disconnect shall be provided. Field power, branch circuit sizing, and overcurrent protection are as indicated on the electrical drawings.

A unit-mounted GFCI service receptacle shall be provided where required by NFPA 70 (Article 210.63) for HVAC equipment on rooftops. A service receptacle powered independently from the unit disconnect allows safe servicing with the unit de-energized.

The unit shall be furnished with a factory-installed, microprocessor-based unit controller that sequences cooling, heating, economizer, and ventilation, and that communicates with the project building automation system per Building Automation SystemBuilding Automation SystemResolves to the current adopted revision.sync/building-automation-system. The Contractor shall coordinate the communication protocol and the physical interface points with the BAS contractor before submittal. All field control terminations shall be labeled to match the points list.

At minimum, the unit controls interface shall make the following points available to the BAS:

• Supply (discharge) air temperature
• Return air temperature
• Mixed air temperature (economizer-equipped units)
• Outdoor air temperature and humidity (economizer changeover and DCV)
• Space CO2 (DCV-equipped units)
• Supply fan start/stop, speed command (VAV), and status
• Compressor stage / capacity command and status by circuit
• Heating stage / modulation command and status
• Economizer damper position and FDD fault status
• Outdoor-air damper position or measured outdoor airflow
• Filter differential pressure / dirty-filter alarm
• Refrigerant leak-detection alarm (A2L units)
• General unit alarm and lockout status

Duct smoke detection shall be provided per NFPA 90A and the IMC. Smoke detection shall initiate unit shutdown. Where return-air quantity exceeds the code threshold, a return-air duct smoke detector shall be provided. Smoke detectors shall be listed for in-duct use and installed per the manufacturer's instructions for the duct cross-section.

Every unit shall be factory run-tested before shipment. The refrigerant circuit(s) shall be evacuated, charged, and leak-tested; each compressor and condenser fan shall be operated; the heating section shall be fired or energized and checked; and the controls shall be verified. A factory run-test report shall be furnished as a closeout submittal. Units that fail any test shall be corrected and retested before shipment.

The unit shall be rigged and set on the curb per the manufacturer's instructions using the furnished lifting points and spreader bars; rigging shall not bear on cabinet panels or coils. Before lifting, the Contractor shall verify the structural roof framing and curb are adequate for the unit operating weight, confirm the crane reach and pick plan, and verify the unit and curb supply and return openings align. After setting, the unit shall be level so the condensate pan drains, and the unit-to-curb gasket shall be continuous and sealed.

Roof flashing and counterflashing at the curb shall be completed by the roofing trade and shall be watertight before startup. Duct, refrigerant (where applicable), electrical, gas, and condensate penetrations through the roof and curb shall be flashed and sealed weathertight. The Contractor shall coordinate the flashing sequence so the curb is dried-in before the unit is set.

Supply and return ductwork shall connect to the unit through the curb per HVAC DuctworkHVAC DuctworkResolves to the current adopted revision.sync/hvac-ductwork, independently supported so duct weight is not carried by the cabinet. Gas piping to gas-fired units shall include a shutoff valve, sediment trap, and union at the unit, and shall be leak-tested before firing. The condensate drain shall include a trap sized for the unit's negative static pressure and shall be routed to an approved point of disposal with the slope and freeze protection required by the climate.

Initial startup shall be performed by a factory-trained representative of the manufacturer. After startup, TAB shall be performed by an independent certified agency per Testing Adjusting And BalancingTesting, Adjusting, and Balancing for HVACResolves to the current adopted revision.sync/testing-adjusting-and-balancing to set supply, return, and outdoor airflow to design. Startup and commissioning shall verify refrigerant charge, compressor and condenser-fan operation by stage, heating operation and turndown, economizer modulation and changeover, FDD fault reporting, minimum and demand-controlled outdoor air, safety lockouts (high-pressure, low-temperature, A2L leak detection), and correct reporting of all BAS points.

The manufacturer shall warrant the unit against defects in materials and workmanship for one year from substantial completion, covering the complete assembly. Factory-authorized service shall be available within the project region during the warranty period.

Each compressor shall carry an extended warranty separate from the base equipment warranty.

The gas-fired heat exchanger shall carry an extended warranty against thermal failure.

The Contractor shall warrant the installation workmanship — including curb flashing, duct and utility connections, vibration isolation, sealing, and refrigerant and condensate connections — for one year from substantial completion.

Spare filters shall be stored on site in sealed packaging labeled with the unit designation and MERV rating. The O&M manual shall include a maintenance schedule with recommended filter change frequency, coil cleaning, condensate system service, and A2L leak-detector check intervals.

Each unit shall bear a permanent nameplate listing the manufacturer's model and serial number, nominal cooling capacity, heating input or capacity, supply airflow, refrigerant type and total charge, electrical characteristics (voltage, phase, MCA, MOCP), and date of manufacture. Units charged with an A2L refrigerant shall bear the flammable-refrigerant marking required by UL 60335-2-40 and ASHRAE 15. Access panels shall be labeled to identify the section behind each panel.