Advanced Design Rooftop Unit
Rooftop Unit: Advanced vs. Conventional
These retrofit units offer improved energy performance over typical existing "commercial" rooftop units by enhancing the performance of individual components and configurations, including fans, coils, filters, dampers, compressors, condensers, controls, and airflow path.
Item ID: 246
HVAC--Rooftop Units & Air Handling Units
Technical Advisory Group: 2010 HVAC TAG (#3)
Average TAG Rating: 3.5 out of 5
TAG Ranking Date: 06/29/2010
TAG Rating Commentary:
- Determining a savings/rebate method for this is important.
- "One of the criteria for choosing technologies, is whether there is equipment available off the shelf. I understand some manufacturers are offering some, but not all of these features."
Technical Advisory Group: 2015-1 Commercial HVAC TAG (#11)
Average TAG Rating: 3.7 out of 5
TAG Ranking Date: 03/10/2015
TAG Rating Commentary:
- It would be helpful to get a list of qualified products (CEE may have this), as well as current cost information.
- There's a solid basis in design, competitions, early adopters, etc. Let's do it!
- Technology is being piloted now. Savings may vary. Will need trained field installlers, automated controls and monitoring; and need to overcome the landlord/tenant market barriers.
- The key for persistent savings is the ongoing monitoring and control
The advanced design rooftop HVAC unit offers improved energy performance over the typical commercial rooftop unit (RTU) by enhancing the performance of individual components and configurations. Standard commercial rooftop HVAC units capture most of the market for this technology. The standard units do not compete in terms of energy efficiency, lifespan, or maintainability; however, they are significantly less expensive.
This technology can include a variety of features to improve the performance of rooftop HVAC units. The Advanced Rooftop Unit Product Definition Report identified 36 features related to the following areas: economizer section, fan and unit cooling efficiency, refrigerant type, fan and refrigeration control, thermostats and sensors, serviceability, and diagnostics and monitoring. Features in available products vary. Energy savings will be achieved in all HVAC applications, and will depend on how many enhancements are incorporated into the unit and on the application. It is difficult, therefore, to identify cost premium, energy savings, other benefits, and drawbacks. These values generally come from commercially available units that include a set of features, chief among them variable speed and demand-controlled ventilation (DCV).
An advanced design rooftop HVAC unit can cost two to three times as much as a standard commercial-quality rooftop HVAC unit, depending on which of the enhancements are included. Although these rooftop HVAC units are more expensive, their effective life should be longer than a standard commercial-quality rooftop HVAC unit.
RTUs are estimated to be used in 46% of all commercial buildings and serve about 69% of the cooled floor space in U.S. commercial buildings. A 2013 study by Pacific Northwest National Laboratory involved installing advanced controllers on 66 RTUs on eight different buildings involving retail, office space, food sales, and healthcare. Of the 66 RTUs, 17 were packaged heat pumps and the rest were packaged air conditioners with gas heat. The advanced controllers provided a reduction in normalized annual RTU energy consumption between 22% and 90%, with the average being 57% for all RTUs.
Baseline Description: Packaged Rooftop HVAC Meeting Code Efficiency
Baseline Energy Use: 10.5 kWh per year per square foot
The 2009 Commercial Building Stock Assessment (CBSA) gives the actual electrical building Energy Use Index (EUI) for various types of heating and cooling systems (CBSA Table D-EA5). Office buildings with electric heating and cooling have an EUI of 20.1 kWh/sf/year. Office buildings with no electric heating or cooling use only 8.2 kWh/sf/year, indicating that the combined HVAC heating and cooling energy use is 11.9 kWh/sf/year. (For all commercial buildings, the corresponding values are 19.9 and 9.4 kWh/sf/year, respectively.) Because this technology can be applied to many types of non-office buildings, a baseline energy use of 10.5 kWh/sf/year is assumed (NEEA, 12/21/2009).
Manufacturer's Energy Savings Claims:
"Typical" Savings: 30%
Savings Range: From 10% to 70%
The percentages above are from a manufacturer, Reznor, that went through the Air-Conditioning, Heating, & Refrigeration Institute (AHRI) testing. Values for the Rebel from Daiken McQuay are similar.
Transformative Wave estimates energy savings of 25% to 50% (TransformativeWave, 2015). Bes-Tech estimates energy savings of 40% to 60% and estimates peak demand reductions of 30% to 60% (Bes-Tech,2013).
Best Estimate of Energy Savings:
"Typical" Savings: 60%
Low and High Energy Savings: 24% to 90%
Energy Savings Reliability: 5 - Comprehensive Analysis
Advanced RTUs can include various combinations of energy-saving features, so performance will vary. The studies cited below looked at commercially available RTUs with various equipment combinations, none of which likely included all of the options listed in the Detailed Description. Performance also varies with operation and climate. These values are, therefore, rough estimates of what future users may experience.
RTUs with DCV and economizer control features alone are found to save about 30% of energy use for most occupancies and locations. Energy savings of 60% are noted in a report by the Oregon Energy Office and the Northwest Energy Efficiency Alliance (NEEA) (Stipe, P.E., 2013). BPA, in conjunction with Peninsula Power and Light, conducted an emerging technology field test on two 15-ton RTUs serving a small retailer. The measurement and verification (M&V) indicated a 45% reduction in total HVAC electrical energy use. A 2013 study by Pacific Northwest National Laboratory involved installation of advanced controllers on 66 RTUs on eight different buildings involving retail, office space, food sales, and healthcare. Of the 66 RTUs, 17 were packaged heat pumps and the rest were packaged air conditioners with gas heat. The advanced controllers provided a reduction in normalized annual RTU energy consumption between 22% and 90%, with the average being 57% for all RTUs (Wang, 2013). A demonstration project involving 35 RTUs with a combined 202.5 tons of cooling at the Lawrence Middle School and the Los Angeles Center for Enriched Studies (part of the Los Angeles Unified School District) showed 45% HVAC energy savings with Transformative Wave's Catalyst.
A California State Partnership for Energy Efficient Demonstration project found HVAC energy savings of 51% at a California State University Long Beach dance complex and 29% at a San Diego State University aquaplex (Grupp, 2013).
Omaha Public Power District tested Digi-RTU with 30 RTUs and found 52% kWh savings and better humidity control (Sunde, 2011).
Snohomish County PUD tested CATALYST at one facility and found 48% kWh savings.
HPAC Engineering Journal (Aug 1, 2011) published a case study about an Enerfit installation with electrical savings of 53%. Some of these savings came from air leakage repairs performed when the Enerfit was installed.
The U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE) will soon post a "Retail Energy Alliance" guide for retrofitting to VAV. An energy savings calculator for big box and grocery stores in 16 climate zones will show fan energy savings of 50% to 75%.
Energy Use of Emerging Technology:
4.2 kWh per square foot per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Potential number of units replaced by this technology:
This technology applies to both new and retrofit applications. Estimates are based on preliminary updated numbers from the 2013 update to the CBSA using the estimates for 2014 (Ecotope, Inc., 2014). Using market shares from the CBSA (NEEA, 2009), the percentage of commercial space that is conditioned (85%) and served by packaged heating/DX cooling units (an estimate of RTUs, 35%) results in: 3,118,000,000 sf x 0.85 x 0.35 = 927,605,000 sf. For the purposes of estimating savings potential, we only consider electrically heated buildings, which account for approximately 30% of conditioned floor space (this is also the most cost-effective application): 927,605,000 x 0.3 = 278,281,500.
The CBSA estimates growth of approximately 1% in the commercial building stock over the next 20 years. To account for additions to the building stock, 10 years of growth was used to account for the potential, which is 10% of the existing housing stock. Using the market shares above results in: 3,118,000,000 x 0.1 x 0.85 x 0.35 x 0.3 =27,828,150.
Thus, the total potential = 278,281,500 + 27,828,150 = 306,109,650.
Regional Technical Potential:
1.93 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Installed first cost per: square foot
Emerging Technology Unit Cost (Equipment Only): $12.50
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $1.00
Baseline Technology Unit Cost (Equipment Only): $6.25
A custom rooftop HVAC unit can cost two to three times as much as a commercial-quality rooftop HVAC unit, depending on which enhancements are included. Additional costs may result from higher-quality components, additional components and controls, and larger unit casings.
Assume that the installed cost of a standard RTU less than 20 tons is $2,500/ton and the advanced unit is $5,000/ton. Using 1 ton per 400 sf results in $6.25/sf for the standard unit and $12.50/sf for the advanced unit.
Simple payback, new construction (years): 11.0
Simple payback, retrofit (years): 23.8
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.
Use of this equipment can be quite cost effective, especially given the much longer service life. The actual cost-effectiveness will vary with the particular features chosen and the application. The Public Interest Energy Research Program estimated a 4.9 to 6.9 year simple payback for an advanced rooftop unit in new construction (PIER, 2007).
Provide a combination of optimized RTU controls and monitoring via a web interface. To provide electric savings, minimum requirements are: supply fan control with either a variable-frequency drive (VFD) or fan cycling control, upgraded economizer controls, and remote web control and monitoring. Minimum and desired features are detailed below. As you can see, when studies report incremental cost and energy savings from “advanced RTU controls,” exactly what that means can vary. However, the estimates provided reflect the most commonly adopted set of features.
Minimum Control Requirements:
-Supply fan control [variable speed drive (VSD) or cycling]
-Demand controlled ventilation
-Digital integrated economizer control
-Differential economizer high limit
Minimum Monitoring Requirements:
-Web-based setpoints and scheduling
-Time series point monitoring (trending)
-Monitored point threshold alerts
Desirable Control Options:
-Occupancy vent and temperature standby
-Night flush cooling
-Demand management or response
-Split DX coil flow control
-Compressor variable control
-Condenser fan variable control
-Optimum start with outdoor air temperature input
Desirable Monitoring Options:
-Fault detection and diagnostics
-Time-series energy monitoring
-RTU energy benchmarking (single unit)
-RTU energy benchmarking (multiple units)
-Demand response measurement and verification
For the addition of a VSD, the motor does not need to be replaced in most cases. A VSD addition works best for three-phase motors. Small RTUs (less than 7 tons) tend to have single-phase motors, and cycling is more likely to be successful for these fans. In all cases where the fan speed is managed or cycled, provisions to vary ventilation with fan speed are required. DCV provides the most successful approach.
Products are manufactured by Carrier/Bryant, Daikin/McQuay, Lennox, and Trane.
The packaged rooftop HVAC unit is the most common way to provide HVAC to commercial buildings in the marketplace today. This has been the case for several decades, so there is a large market for both new units and replacement units. Actual market penetration is unknown.
Typical commercial quality rooftop HVAC units are designed to meet minimum energy code requirements. The market is very competitive and cost-driven, and there is limited incentive to produce units that exceed minimum code requirements. Options for some high-efficiency components are available from a limited number of manufacturers.
High-efficiency components currently exist, and are typically used in custom-built air handling and air conditioning units. Engineers currently work directly with custom unit manufacturers to design and specify high-efficiency units for projects with aggressive energy conservation goals. This equipment is more expensive, not only because it is higher quality, but because it is designed and manufactured in a "one-off" manner. If the market for higher efficiency units was broadened, manufacturing could become more efficient and costs would drop accordingly.
There is the Rebel (Daikin McQuay, 2012), from Daikin McQuay that has many of these features and that has gone through AHRI testing. The Rebel is 10-70% more efficient than required by code, depending on size. The Rebel is available in 3-15 ton sizes (Daikin McQuay, 2012).
Variable-speed operation is generally quieter than constant-speed equipment and improves equipment life. Earlier detection of equipment anomalies can prompt maintenance adjustments rather than more expensive equipment failures.
End User Drawbacks:
The greatest barrier to implementation is the initial cost. With financial incentives and volume production, these rooftop HVAC units could become more cost effective.
This equipment will weigh more so may require a structural analysis of the building, adding cost.
Operations and Maintenance Costs:
Given the 'soft-start' feature of the compressor and fans, it is logical to expect this equipment to last longer, as has been the case with other HVAC systems that have these features.
Anticipated Lifespan of Emerging Technology: 30 years
If maintained well, the effective life of a custom rooftop HVAC unit is at least twice as long as a commercial quality rooftop HVAC unit, up to thirty years or more. Additional lifespan results from higher quality components and greater maintainability.
Standard commercial rooftop HVAC units capture most of the market for this technology. The standard units do not compete in terms of energy efficiency, lifespan, or maintainability; however, they are significantly less expensive. High-end institutional buildings often have custom air handling units of similar technology housed within the building, coupled to chillers and boilers.
For a retrofit application, using the CATALYST Efficiency Enhancing Controller by Transformative Wave Technologies as an add-on to existing equipment may be more cost-effective than replacing the unit with a new advanced design RTU (See E3TNW #338 Advanced Rooftop Unit Controls (ARC) Retrofit).
Reference and Citations:
Katipamula, et al.,
Advanced RTU Campaign Webinar
U.S. Department of Energy, Better Buildings Alliance
A Better Buildings Alliance webinar presentation
Rebel Product Summary
Rebel Provides Quick Payback with World Class Energy Savings and Unbeatable Part-Load Efficiencies!
Rooftop Unit Savings Research Project
New Buildings Institute
Website of advocate organization.
AAON Heating and Cooling Products
AAON is a reputable manufacturer
Huntair, A CES Group Brand
Mammoth, a Nortek Air Solutions Brand
High Performance Rooftop Unit Specification
U.S. Department of Energy, Better Buildings
Rooftop Unit Comparison Calculator
Pacific Northwest National Laboratory
Advanced Automated HVAC Fault Detection and Diagnostics Commercialization Program - ARTU Product Definition Report
California Energy Commission, Public Interest Energy Research Program
Hancock, et. al.,
Advanced Rooftop HVAC Unit Controls Pilot
Minnesota Center for Energy and Environment
Final Rooftop Unit Working Group Phase 4 Recommendations
New Buildings Institute
Advanced Variable Air Volume System Design Guide
California Energy Commission
Advanced Automated HVAC Fault Detection and Diagnostics Commercialization Program - ARTU Cost Benefit Analysis
California Energy Commission, Public Interest Energy Research
HVAC Quality Installation Specification: Residential and Commercial Heating, Ventilating, and Air Conditioning (HVAC) Applications
Air Conditioning Contractors of America
Coolerado Wins UC Davis Western Cooling Challenge
High Performance Rooftop Unit Challenge
U.S. Department of Energy, Better Buildings
High Performance Rooftop Unit Specification
U.S. Department of Energy, Better Buildings
New Building Institute,
Variable Rate Rooftop Unit Test (VRTUT) Report
Northwest Energy Efficiency Alliance
Northwest Commercial Building Stock Assessment (CBSA): Final Report
Prepared by the CADMUS Group for the Northwest Energy Efficiency Alliance
Advanced Rooftop Unit Concept AHRI Initial Feedback
Air-Conditioning, Heating and Refrigeration Institute
Advanced Rooftop Unit (ARTU): CEE Initiative Concept
CEE Industry Partners Meeting
The Most Promising Emerging Technologies for the C&I Sector