20 research outputs found
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Comparison of construction and energy costs for radiant vs. VAV systems in the California Bay Area
The goal of this study was to perform a design stage cost analysis comparing a selected radiant building against an identical building with a traditional variable air volume (VAV) system. Major findings from the cost estimates include:⢠The radiant HVAC design has a total cost of 29.9/ft2 for the VAV design, representing a 9.8/ft2 higher than that for the VAV design.⢠Since labor rates are higher in the San Francisco Bay Area, for the estimated national average labor rate, the premium for radiant is $6.8/ft2, compared to the VAV system. The high installed cost for the radiant equipment is partly a reflection of the current radiant manufacturersâ pricing strategies and the contractorsâ bidding practices. The radiant market is relatively small and immature in the United States, especially compared to the well-established VAV market. Alternative design approaches are discussed that may reduce first costs and/or energy costs. Energy models of the two designs (radiant and VAV) were developed in EnergyPlus to evaluate the corresponding energy and comfort performance. In the VAV system model, the controls are generally based on the recently published ASHRAE Guideline 36 (ASHRAE, 2018), which provides high performance sequences of operation for VAV systems. However, for the hybrid radiant slab and DOAS system, there are no well-established control sequences readily available. The annual simulation results show that the total site HVAC energy use is 16.2% higher for the radiant system (2.9 kBtu/ft2) than the optimized VAV design (2.5 kBtu/ft2). The report contains further discussion of opportunities to improve the energy performance of radiant systems. For example, in mild climates, such as the Bay Area in California, radiant designs should take advantage of the benefits of free cooling as much as possible either with airside or waterside economizers
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Optimizing Radiant Systems for Energy Efficiency and Comfort
Radiant cooling and heating systems provide an opportunity to achieve significant energy savings, peak demand reduction, load shifting, and thermal comfort improvements compared to conventional all-air systems. As a result, application of these systems has increased in recent years, particularly in zero-net-energy (ZNE) and other advanced low-energy buildings. Despite this growth, completed installations to date have demonstrated that controls and operation of radiant systems can be challenging due to a lack of familiarity within the heating, ventilation, and air-conditioning (HVAC) design and operations professions, often involving new concepts (particularly related to the slow response in high thermal mass radiant systems). To achieve the significant reductions in building energy use proposed by California Public Utilities Commissionâs (CPUCâs) Energy Efficiency Strategic Plan that all new non-residential buildings be ZNE by 2030, it is critical that new technologies that will play a major role in reaching this goal be applied in an effective manner. This final report describes the results of a comprehensive multi-faceted research project that was undertaken to address these needed enhancements to radiant technology by developing the following: (1) sizing and operation tools (currently unavailable on the market) to provide reliable methods to take full advantage of the radiant systems to provide improved energy performance while maintaining comfortable conditions, (2) energy, cost, and occupant comfort data to provide real world examples of energy efficient, affordable, and comfortable buildings using radiant systems, and (3) Title-24 and ASHRAE Standards advancements to enhance the building industryâs ability to achieve significant energy efficiency goals in California with radiant systems. The research team used a combination of full-scale fundamental laboratory experiments, whole-building energy simulations and simplified tool development, and detailed field studies and control demonstrations to assemble the new information, guidance and tools necessary to help the building industry achieve significant energy efficiency goals for radiant systems in California
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Variable Air Volume Hot Water Reheat Terminal Units: Temperature Stratification, Performance at Low Hot Water Supply Temperature, and Myths from the Field
Hot water coils are common in commercial building HVAC systems. Nevertheless, their design, installation, and control are frequently sub-optimal, with respect to maximizing heat exchange effectiveness and air temperature setpoint control. For example, conditions on-site sometimes lead to coils being installed in parallel flow instead of counter flow configuration, and temperature stratification in the leaving air can lead to control issues. Additionally, low hot water supply temperatures (HWST) of ~120â°F (49â°C) are becoming more common with the rise of heat pump and efficiency retrofits. As hot water systems are typically designed for high HWST (160 - 180â°F, 71 - 82â°C), lower waterside âdelta Tâ temperature differences (HWST â HWRT) would occur using low HWST in retrofits of conventional hot water heating systems. If buildings retain existing coils for the low-HWST systems common to efficiency retrofits, they will be unable to maintain the same design heat capacity without replacing terminal units. This creates challenges for retrofit projects throughout the industry, and low-HWST designs also present challenges to new construction. We present the background, methods, and findings of an experiment conducted in 2022 at the Price Industries Laboratory in Winnipeg, Canada. In this experiment, we tested multiple VAV HW reheat terminal units across a range of test factors, including VAV box sizes and number of coil rows. The performance of each coil setup was compared at both high and low HWSTs, and at multiple damper positions. We also performed several additional tests to determine the best solutions to common field installation and operation issues and to gauge the impact of varying coil insulation. In addition to tests we ran with stock-manufactured coils, we also ran several tests using coils of our own custom designs, focusing on symmetry and limited circuit count. The intent of these tests was to better understand the factors in VAV HW reheat systems that may be overlooked in typical system design and coil selection processes, especially as parameters such as HWST and water side temperature differences begin to change. Understanding these factors is important to the design and operation of these systems as sub-optimal performance in the terminal unit systems has cascading effects both for retro-fitted low-HWST systems and existing boiler systems. Overall, the results from this experiment serve to inform recommended changes to VAV terminal unit design, selection, and control to improve whole-building performance
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Quantifying energy losses in hot water reheat systems
We developed a new method to estimate useful versus wasted hot water reheat energy using data obtained from typically installed instrumentation that applies to all pressure independent VAV terminal units with discharge air temperature sensors. We evaluated the method using a year of 1-minute interval data for a 11,000 m2 building with 98 terminal reheat units, and found a 14% upper bound for the uncertainty associated with this method. We found that just 21% of gas energy is converted to useful reheat energy in this building. The distribution losses alone were 44% of the heat output from the boiler. The results raise questions regarding the tradeoffs between hot water heating systems, which have significant distribution losses, and electric heating systems, which effectively have zero distribution losses. In this building, and likely many others, an electric reheat system supplied by a small photovoltaic panel system would have a lower operating energy cost and a lower initial cost than the hot water reheat system. Further investigations using this method will be relevant to designers and standards developers in deciding between electric and hot-water reheat, particularly for modern designs using dual-maximum controls and low minimum airflow setpoints
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Quantifying energy losses in hot water reheat systems
We developed a new method to estimate useful versus wasted hot water reheat energy using data obtained from typically installed instrumentation that applies to all pressure independent VAV terminal units with discharge air temperature sensors. We evaluated the method using a year of 1-minute interval data for a 11,000 m2 building with 98 terminal reheat units, and found a 14% upper bound for the uncertainty associated with this method. We found that just 21% of gas energy is converted to useful reheat energy in this building. The distribution losses alone were 44% of the heat output from the boiler. The results raise questions regarding the tradeoffs between hot water heating systems, which have significant distribution losses, and electric heating systems, which effectively have zero distribution losses. In this building, and likely many others, an electric reheat system supplied by a small photovoltaic panel system would have a lower operating energy cost and a lower initial cost than the hot water reheat system. Further investigations using this method will be relevant to designers and standards developers in deciding between electric and hot-water reheat, particularly for modern designs using dual-maximum controls and low minimum airflow setpoints
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Time-averaged ventilation for optimized control of variable-air-volume systems
Typical Variable Air Volume (VAV) terminals spend the majority of time at their minimum airflow setpoints. These are often higher than the minimum ventilation requirements defined by code, resulting in excess energy use and a risk of over-cooling the spaces. We developed and tested a Time-Averaged Ventilation (TAV) control strategy in an institutional building on the UC Berkeley campus to address this issue. Whenever a zone does not require cooling, TAV alternates the VAV damper between partially open and fully closed so that the average airflow matches a predefined ventilation setpoint. Compared to the existing, base case scenario using single-max VAV logic, this strategy reduced the mean zone airflow fraction from 0.44 to 0.27 during the intervention period. The corresponding reductions in average heating, cooling, and fan power were 41%, 23%, and 15% respectively. In addition to being programmed directly in a native control system, TAV may be applied via sMAP as a low-cost retrofit strategy in any building that has a BACnet network and direct digital control (DDC) to each VAV terminal
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Time-averaged ventilation for optimized control of variable-air-volume systems
Typical Variable Air Volume (VAV) terminals spend the majority of time at their minimum airflow setpoints. These are often higher than the minimum ventilation requirements defined by code, resulting in excess energy use and a risk of over-cooling the spaces. We developed and tested a Time-Averaged Ventilation (TAV) control strategy in an institutional building on the UC Berkeley campus to address this issue. Whenever a zone does not require cooling, TAV alternates the VAV damper between partially open and fully closed so that the average airflow matches a predefined ventilation setpoint. Compared to the existing, base case scenario using single-max VAV logic, this strategy reduced the mean zone airflow fraction from 0.44 to 0.27 during the intervention period. The corresponding reductions in average heating, cooling, and fan power were 41%, 23%, and 15% respectively. In addition to being programmed directly in a native control system, TAV may be applied via sMAP as a low-cost retrofit strategy in any building that has a BACnet network and direct digital control (DDC) to each VAV terminal
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Evaluation of a cost-responsive supply air temperature reset strategy in an office building
This paper describes a new supply air temperature control strategy for multi-zone variable air volume systems. We developed the strategy with the intent that it is simple enough to implement within existing building management systems. At 5-minute intervals, the strategy estimates the cost of fan, heating and cooling energy at three different supply air temperatures (current, higher, lower), and chooses the one with the lowest cost as the setpoint. We then implemented this strategy in a seven floor, 13,000 m2 office building and compared the energy costs to the industry best practice control strategy in a randomized (daily) controlled trial over a 6-month period. We showed that the new control strategy reduced total HVAC energy costs by approximately 29%, when normalized to the typical annual climate data for this location and operating only during typical office hours. These findings indicate that the current industry best practice control strategy does not find the optimal energy cost point under most conditions. This new control strategy is a valuable opportunity to reduce energy costs, at little initial expense, while avoiding more complex approaches, such as model predictive control, that the industry has been hesitant to adopt. We describe the new control strategy in language common to the industry (see sequence of operations included as supplemental material) so that readers may easily specify and implement this immediately, in new construction or controls retrofit projects
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Re-Envisioning RCx: Achieving Max Potential HVAC Controls Retrofits through Modernized BAS Hardware and Software
Most existing buildings have sub-optimal heating, ventilation, and air conditioning (HVAC) controls, resulting in wasted energy and occupant discomfort. Retro-commissioning (RCx) addresses many of these issues, but it is a lengthy and highly customized process. Limited capabilities of existing building automation system hardware restricts the scope of most RCx projects. Incentive programs consider building automation system (BAS) hardware retrofits to be high-capital investments and do not allow them in typical RCx programs.
This paper describes work that the authors are leading to facilitate technical and market innovation in the BAS industry to unlock large savings in existing commercial buildings through deep retrofits of BAS hardware and software. California and New York research projects are demonstrating BAS retrofits leveraging the American Society of Heating, Refrigeration, and Air Conditioning Engineersâ (ASHRAE) new Guideline 36 high performance sequences of operation to achieve greater than 20 percent whole building energy savings, while saving costs and reducing risk through streamlined processes and standardization across BAS manufacturer product lines and across implementation practices.
This paper describes market barriers that impede achieving deep savings from BAS retrofits in custom incentive and traditional RCx programs and presents a new maximum potential BAS retrofit model that addresses these barriers. The new model leverages the authorsâ efforts in market enablement through open standards, BAS industry partnerships, and tools for cost-effective scaling that includes tools for project screening, savings calculations, and measurement and verification (M&V). This approach is widely applicable and will be ready for at-scale implementation within two years