Rooftop Unit (RTU): What It Is and How It Works

What makes a unit a rooftop unit

DOE's equipment rule defines a single package unit as a central air conditioner or heat pump with all the major assemblies enclosed in one cabinet. DOE's Federal Energy Management Program (FEMP) notes that one air-source heat pump system can provide both space heating and cooling in a building. The same rule defines a split system as one where at least one major assembly sits apart from the others. The Energy Information Administration (EIA) describes a packaged unit as factory-assembled equipment installed as a self-contained unit. EIA says these units are generally mounted on the roof, and sometimes on a slab outside the building.

EIA adds that packaged units produce warm or cool air directly and move it through ducts or a similar system. It says some electric packaged units are also called direct expansion (DX) units. DOE's rule defines commercial package air-conditioning and heating equipment as electrically operated, unitary central air conditioners and central air-conditioning heat pumps for commercial application that are air-cooled, water-cooled, evaporatively-cooled or water source (not including ground water source).

The house-scale comparison of the two layouts is on package unit vs split system. Other equipment families that a quote might call "industrial" are sorted on industrial air conditioner.

How an RTU runs through the day

PNNL's 2013 field-test report for DOE describes conventional RTU control plainly. In most packaged units, a zone thermostat runs the compressor or the gas furnace, depending on whether it calls for cooling or heating. In the units PNNL tested, the compressor or furnace cycled on and off to hold the setpoint. The supply fan ran continuously while the building was occupied, to deliver ventilation air.

The report says packaged equipment with constant-speed supply fans is designed to provide ventilation at the design rate whenever the fan runs, as building code requires. It notes that the ventilation rate cannot be adjusted easily with a constant-speed fan. The report adds that a building may not be fully occupied for a number of hours each day.

The advanced controller PNNL tested added integrated air-side economizers, supply-fan speed control and demand-controlled ventilation (DCV). Its DCV option used return-air carbon dioxide to decide how to regulate the outdoor-air damper and fan speed. Where a unit had more than one compressor, they were staged. Every vendor PNNL reviewed fitted the supply fan with a variable-frequency drive (VFD), a motor speed control. The drive can vary fan speed by mode: heating, cooling or ventilation only.

FEMP makes a related point. It says two-speed fans cut energy use in ventilation-only mode, and modulating compressors use less energy than single-speed ones at part load. How an RTU ties into a building automation system is covered on building automation and controls.

The economizer: outdoor air as a cooling source

A set of PNNL training slides quotes ASHRAE Standard 90.1-2004 for the definition. The standard calls an air-side economizer a duct-and-damper arrangement plus an automatic control system. Together they let a cooling system supply outdoor air to reduce or eliminate mechanical cooling in mild or cold weather. FEMP says that when temperature and humidity are favorable, economizers open dampers to bring in more outside air and recirculate less. Under certain conditions, FEMP says, the economizer can shut down the condenser and cool with outdoor air only.

The units PNNL tested before retrofit used a fixed dry-bulb high limit. Below 55°F outdoor air, with a call for cooling, the outdoor-air damper opened fully. The slides give a different rule of thumb for humid climates. It says to economize there when outdoor air is 5°F to 10°F below return-air temperature.

The same slides list why economizers fail: a jammed or frozen damper, a broken or disconnected linkage, or a nonfunctioning actuator or disconnected wire. It adds a faulty outdoor-air or return-air sensor, a malfunctioning controller and wrong control settings. It also says to look for dampers open at unusual times of day. Our reading: on a humid coast, ask how the high limit is set and when the economizer was last tested. Why humidity is its own problem is explained on commercial humidity control.

Size classes, efficiency ratings and the 2029 federal standard

DOE's rule sorts commercial package equipment into three size classes by rated cooling capacity. Small equipment is rated below 135,000 Btu/h. Large runs from 135,000 up to 240,000 Btu/h, and very large from 240,000 up to 760,000 Btu/h.

DOE's rule defines the integrated energy efficiency ratio (IEER) as a weighted average of mechanical cooling energy efficiency ratios (EER) at four load levels and their rating conditions, in Btu/watt-hour.

DOE's standards page says manufacturers of this equipment must meet federal energy conservation standards. DOE published amended standards for air-cooled units of 65,000 Btu/h and up on May 20, 2024. Compliance is required on and after January 1, 2029. DOE also adopted two new test metrics. They are integrated ventilation, economizer and cooling (IVEC), and integrated ventilation and heating efficiency (IVHE). DOE says testing to them will not be required until compliance is required with an amended standard based on them.

FEMP's purchasing guidance applies to commercial central air conditioners rated below 240,000 Btu/h. It says units built for more moisture removal perform better in humid climates. In hot climates, it says to consider units that exceed the minimum requirements. Our reading: for a Tampa Bay roof, ask for the moisture-removal performance of the exact model, not only its IEER.

On the roof: curb, condensate and clearances

The details below come from one maker, Carrier, in its 2011 installation manual for one rooftop line. Read the manual for your own model.

The manual says the gasket between unit and curb is critical for a watertight seal. It says a poorly applied gasket can also cause air leaks and poor unit performance. It says the curb should be level so the unit's drain works, and that ductwork must attach to the curb, not to the unit. It also tells the installer to check building codes for weight distribution on a roof mount.

Every unit in that line needs an external condensate trap at least 4 in. deep, protected from freezing. The manual says the trap should be deep enough to offset maximum unit static difference.

The manual says that although the unit is weatherproof, it should not sit where runoff from higher roofs or overhangs falls on it. For a unit that will not start, see rooftop unit not running.

Refrigerant, licenses and who works on an RTU

The Environmental Protection Agency (EPA) requires certification for technicians who maintain, service, repair or dispose of equipment that could release refrigerant. EPA says the tests are specific to the type of equipment the technician seeks to work on. Its Type II certification covers high- and very high-pressure appliances, except small appliances and vehicle air conditioners. The Carrier manual warns that R-410A systems operate at higher pressures than R-22 systems. It says pressure must be relieved and all refrigerant recovered before system repair or final unit disposal. FEMP adds that, when retiring light commercial heating and cooling equipment that contains HCFCs, the Clean Air Act requires that a certified technician recover the refrigerant on site. FEMP describes HCFCs as ozone-destroying refrigerants that were commonly used in this equipment until recently. The refrigerants coming into new units are covered on refrigerant transition.

Section 489.105 of the Florida Statutes defines air-conditioning contractor classes. A Class B contractor's services are limited to 25 tons of cooling and 500,000 Btu of heating in any one system. A Class A contractor's services are unlimited in the execution of contracts for the air-conditioning work the statute lists, if not prohibited by law. Our reading: on a building with large RTUs, check which license class the contractor holds.

FEMP says installation problems such as oversizing, improper charging and leaky ducts cause efficiency losses, discomfort and shorter equipment life. It also says automated fault detection and diagnostics can alert operators to failures such as low refrigerant charge. FEMP adds that security can almost never be networked in after the fact, so it is important to make sure networked devices are secure.