Center for the Built Environment

There is an increasing focus on the time at which energy is used in buildings both to reduce utility costs and carbon emissions in response to time-dependent grid signals. One method to shift electrical load out of peak pricing hours is to use batteries, but they have high first costs and also incur an energy penalty due to round trip efficiency and other losses. Another method is to use thermal storage to offset heating and cooling. Similarly, mechanical ventilation systems can also be controlled to shift energy use to periods of the day with lower energy, cost, and environmental impacts by varying the ventilation rate while still meeting ventilation code requirements. Mechanical ventilation systems in large multi-family residential buildings are mostly central air systems with either manually balanced dampers or constant airflow regulator (CAR) dampers that aim to provide a constant ventilation airflow rate to each apartment. ASHRAE 62.2 allows for dynamic ventilation rate systems in these buildings as long as the average relative exposure rate and the peak relative exposure rate during occupied periods are no more than 1 and 5, respectively, for any time interval that cannot exceed an hour. In this study, we used EnergyPlus simulations to examine energy end-use profiles for a large multi-family building under design in San Jose, California. We considered a balanced ventilation system using a central dedicated outdoor air supply (DOAS) system. We tested different load-shifting scenarios with multiple parameters to explore how the ventilation airflow rate can be varied to shift load, while also assessing energy and utility cost impacts. The parameters we assessed in each scenario were: the presence of a centralized ERV system or not; ventilation design sizing; and length of load shifting time period. All dynamic ventilation cases, with and without ERV systems, resulted in energy and operational cost savings relative to the constant ventilation cases when compared to providing the same amount of load shifting using batteries, and all tested strategies met ASHRAE 62.2 requirements. The results show that after accounting for the battery penalty typically associated with load shifting, all dynamic ventilation cases reviewed result in improved energy savings when compared to the constant ventilation strategy.

We studied a combination of heating system measures in two large commercial officebuildings in San Francisco (110,000 and 120,000 ft 2 respectively) within a project funded by the California Energy Commission’s Public Interest Efficiency Research program. We retrofitted theexisting heating plants and updated the HVAC controls to ASHRAE Guideline 36-2021 as closely as possible while retaining the existing controller hardware. These measures decreased annual natural gas consumption by about 70 percent while also reducing HVAC electricity consumption. The results reinforce previous work showing significant natural gas reductions in 3 other buildings that underwent full controls retrofits (including controller hardware), and large savings from another 3 buildings that underwent partial controls upgrades. We show that on today’s electricity grid, which is quite dirty during the winter and early morning hours when most heating occurs, the carbon emissions reduction from these measures exceeds the reduction from fully electrifying the existing heating system’s load with today’s air-to-water heat pumps. More importantly, these solutions are mutually beneficial. Acknowledging that we also need to electrify HVAC loads to meet our climate goals, replacing controls first will reduce the size, weight, first cost, and ongoing operating cost of the subsequent heat pump installation requiredto fully electrify, and will make it more feasible to do so. This paper highlights an overlooked opportunity for enormous decarbonization in the existing commercial building stock using a solution that is available, cost effective, and scalable. We should prioritize these measures first,and then electrify, rather than focusing solely on electrification.

Conventional wisdom and standard industry practice is to setback zone temperature setpoints when commercial buildings are unoccupied at night. The HVAC systems then operate in warmup mode to recover zone temperatures prior to the start of occupancy, sometimes with an optimal start algorithm. These strategies were intended to reduce HVAC energy consumption when originally developed decades ago but are due for re-examination given the significant changes in HVAC systems that have since occurred. In particular, the changes currently underway with the movement toward electrification present new design considerations and priorities. Warming up a building as fast as possible may not be the best strategy in terms of energy use, operating cost, or carbon emissions. This article discusses some of the downfalls of conventional morning warmup practices, suggests an improved strategy, and shows the results from a pilot field demonstration test.

Natural gas combustion to serve space heating hot water systems causes approximately one third of large commercial building energy use in California. This project evaluated an innovative set of non-proprietary, cost-effective methods to reduce energy consumption and associated emissions from these systems. The project demonstrated 70% natural gas savings and substantial electricity savings in two large office buildings, yielding total utility cost savings of approximately $110,000 (or $0.5/ft²) per year. The project also conducted detailed studies on distribution losses and boiler efficiency in several buildings; measured performance of key components in laboratory tests; gathered and analyzed data from hundreds of buildings to evaluate actual performance of these systems; and provided a public dataset to inform future retrofits, research, and code development. The research also highlighted characteristics that make a building a good candidate for retrofit so these results can be scaled. Market transformation activities included 10 journal and conference publications, policy recommendations and a design guide. Based on these findings and other recent work, the opportunity for similarly large emissions reductions appears to be common within the existing large commercial building stock. The resources provided by this project can aid stakeholders in achieving California’s goals to decarbonize buildings.

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 ~120⁰F (49⁰C) are becoming more common with the rise of heat pump and efficiency retrofits. As hot water systems are typically designed for high HWST (160 - 180⁰F, 71 - 82⁰C), lower waterside “delta T” temperature differences (HWST – 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.

Variable air volume systems with hydronic reheat at terminal units are a common Heating Ventilation Air Conditioning (HVAC) system type in medium and large commercial buildings. This study measured HHW heat loss in detail in a 66,000 ft² (6,200 m²)​ office and lab building, built in 2000, in Davis, California. We used methods adapted from Raftery et al. (Raftery, Geronazzo, et al. 2018) to calculate the HHW distribution losses from BAS measured data, and then measured unintentional heat loss at the whole building level including losses from distribution and passing HHW valves. We further measured HHW distribution losses in greater detail on a single HHW distribution branch removing loss contributions from other potential issues, such as passing HHW valves.

For the whole building, using newly installed, calibrated water flow meter and matched pair calibrated RTD HHW supply and return temperature sensors, typical HHW setpoints, with all air handlers turned off, the steady-state unintentional heat loss was 4.4 W/m² (1.4 Btu/h.ft²) when all VAV terminal unit HHW valves were commanded shut, and 3.2 W/m² (1.0 Btu/h.ft²) when one HHW valve was commanded open.

Focusing on one HHW branch, during normal building operation over a two-month period in the heating season, we used BAS readings for air flow rate, supply air temperature, and discharge air temperature and measured a distribution heat loss of 2.86 W/m² (0.91 Btu/h.ft²) and 40% HHW distribution efficiency. Using separately installed, calibrated temperature sensors yielded a similar result (2.43 W/m² (0.77 Btu/h.ft²), 49%), and further correcting air flow rates with passive flow hood single point calibration of BAS reported flow rates also yielded a similar result (2.76 W/m² (0.87 Btu/h.ft²), 42%). The close agreement between the results using BAS and calibrated sensors suggest that existing buildings can be screened for heat loss reduction interventions using only BAS data.

The magnitude of the measured HHW losses are small compared to design day loads, but they occur for a large number of hours so reducing these losses can save substantial energy. Further, during the cooling season the losses both waste heat and increase cooling loads. Paths forward include adopting aggressive heating hot water supply temperature resets, reducing unnecessary reheat operation, improving HHW pipe insulation practices, and/or changing design strategies to seasonal switchover or electrically driven distributed systems such as electric resistance or terminal unit heat pump equipment.

Excessively high minimum airflow setpoints for Variable Air Volume (VAV) boxes, caused by outdated energy codes stipulating they should be 30% or higher of the maximum airflow, led to significant energy waste. Lower setpoints meet the ventilation code requirements while minimizing recirculation and reheat energy waste. ASHRAE RP-1515 showcased this by correcting VAV minimums in 1,000,000 ft² (92903 m²) of California office space which yielded 10-30% HVAC energy savings and improved thermal comfort. Consequently, the Title 24 Energy Standards and ASHRAE 90.1 were updated to mandate minimum airflows match ventilation requirements. Beyond increased reheat energy waste caused by elevated VAV minimums, boiler operation issues can also contribute to avoidable energy waste. Despite energy codes mandating low VAV minimums for several years, these issues remain common in new construction and existing buildings. Our goal is to simplify retrofit decision-making for owners and operators by developing a screening method to assess extensive or small-scale building portfolios, using easily accessible data encompassing building type, age, size, and monthly gas consumption. The method entails applying a series of filters to a list of potential buildings to identify those with heating system challenges that should be prioritized for system upgrades. The main filter highlights buildings with elevated summertime gas consumption, as well-functioning systems lacking a major gas end-user should exhibit minimal gas usage during the cooling season. This filter employs a threshold for summer gas consumption we calculated based on standard design parameters, assumptions, and past case studies to serve as a benchmark and pinpoint problematic buildings. We applied this filter, among others, to over a decade of gas consumption data for 22 buildings at California State Polytechnic University, Humboldt. Collaborating with operators enabled us to identify 2 high priority buildings from the data set and validate the filtering process by cross-referencing floor plans and schedules to verify that these issues do in fact exist. Additionally, we applied this methodology to monthly gas data for 3318 buildings in Washington, DC to gauge its applicability on a larger scale. This process prioritized 30 potential buildings that could significantly reduce fossil fuel consumption, elevate thermal comfort, and realize gas bill savings through economical retrofits. While the screening method does not identify all buildings needing heating system upgrades, the results demonstrate how effective they are at highlighting buildings which should be prioritized to see the largest savings from the lowest cost interventions.