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Open Access Publications from the University of California
Cover page of Boiler Retrofits and Decarbonization in Existing Buildings: HVAC Designer Interviews

Boiler Retrofits and Decarbonization in Existing Buildings: HVAC Designer Interviews


In this study, we investigate methods to reduce carbon emissions from existing large commercial buildings with central natural gas-fired boilers used for space heating. This research explores opportunities to reduce natural gas use through improved building operations and through building decarbonization. We conducted one-hour interviews with 17 mechanical HVAC designers, together having over 350 years of industry experience, professional tenures at engineering consulting firms and design/build firms, and project work in California, New York, Texas, Alaska, the United Kingdom, and Canada. We asked a mix of quantitative and qualitative questions, covering four topic areas: General Background, Peak Heating Load and Boiler Selection, Boiler Controls, and Existing Building Decarbonization. The interviews yielded insight into industry practices, including determining peak heating load, equipment redundancy, boiler staging controls, Heating Hot Water temperature resets, challenges of building electrification, and design considerations for building decarbonization. From the interview results, we developed five key findings: (1) New boilers are oversized, (2) Actual building load distributions are not available, (3) Heating Hot Water temperatures are too high, (4) Boiler end-of-life is not the best electrification opportunity, (5) Reduce building emissions even if all-electric is infeasible. There are many challenges to reducing carbon emissions from existing buildings, but we conclude there are also many opportunities to make immediate positive change.

Cover page of Cooling Energy Savings and Occupant Feedback in a Two Year Retrofit Evaluation of 99 Automated Ceiling Fans Staged With Air Conditioning

Cooling Energy Savings and Occupant Feedback in a Two Year Retrofit Evaluation of 99 Automated Ceiling Fans Staged With Air Conditioning


Controlled air movement is an effective strategy for maintaining occupant comfort while reducing energy consumption, since comfort at moderately warmer temperatures requires less space cooling. Modern ceiling fans provide a 2–4 °C cooling effect at power consumption comparable to LED lightbulbs (2–30 W) with gentle air speeds (0.5–1 m/s). However, very limited design guidance and performance data are available for using ceiling fans and air conditioning together, especially in commercial buildings. We present results from a 29-month field study of 99 automated ceiling fans and 12 thermostats installed in ten air-conditioned buildings in a hot/dry climate in California. Staging ceiling fans to automatically cool before, and then operate together with air conditioning enabled raising air conditioning cooling temperature setpoints in most zones, with overall positive occupant interview and survey responses. Overall measured cooling season (April– October) compressor energy savings were 36%, normalized by floor area served (41% during summer peak billing hours). Weather-normalized changes in zone energy use varied from 24% increase to 73% decrease across 13 compressors, reflecting variation in occupant schedules and other uncontrolled factors in occupied buildings. Median weather-normalized energy savings per compressor were 21%. Staging ceiling fans and air conditioning provided comfort across a wider temperature range, using less energy, than air conditioning alone.

Cover page of Capturing Energy Savings from Correcting VAV Box Minimums on Campus

Capturing Energy Savings from Correcting VAV Box Minimums on Campus


The Office of Sustainability at UC Berkeley leads energy and water saving campaigns on campus and has set the goal to reduce energy use intensity by an average of at least 2% annually. One of the proposed energy conservation practices is to improve ventilation efficiency. Our project primarily addresses wasted fan, cooling, and heating energy through excessive air recirculation in campus buildings. By correcting the variable air volume minimum airflow setpoints, we anticipate up to 10-30% HVAC energy savings. As a pilot project, this report documented how to implement these changes step by step and lower the barrier to entry for Facilities Services to implement this change in other campus buildings. We developed a comprehensive campus building evaluation matrix and reviewed all 31 campus buildings in the building automation system. We conducted three rounds of analysis, including screening each of the buildings in the building automation system, further reviewing building candidates’ case by case, and performing sample zone minimum airflow setpoints calculation. Chavez Student Center was selected as the final building candidate to demonstrate the energy savings measure. In the end, the total minimum airflow rate savings at Chavez Student Center is 4,615 cfm based on the calculation.

Cover page of Artificial Intelligence for Efficient Thermal Comfort Systems: Requirements, Current Applications and Future Directions

Artificial Intelligence for Efficient Thermal Comfort Systems: Requirements, Current Applications and Future Directions


In buildings, one or a combination of systems (e.g., central HVAC system, ceiling fan, desk fan, personal heater, and foot warmer) are often responsible for providing thermal comfort to the occupants. While thermal comfort has been shown to differ from person to person and vary over time, these systems are often operated based on prefixed setpoints and schedule of operations or at the request/routine of each individual. This leads to occupants’ discomfort and energy wastes. To enable the improvements in both comfort and energy efficiency autonomously, in this paper, we describe the necessity of an integrated system of sensors (e.g., wearable sensors/infrared sensors), infrastructure for enabling system interoperability, learning and control algorithms, and actuators (e.g., HVAC system setpoints, ceiling fans) to work under a governing central intelligent system. To assist readers with little to no exposure to artificial intelligence (AI), we describe the fundamentals of an intelligent entity (rational agent) and components of its problem-solving process (i.e., search algorithms, logic inference, and machine learning) and provide examples from the literature. We then discuss the current application of intelligent personal thermal comfort systems in buildings based on a comprehensive review of the literature. We finally describe future directions for enabling application of fully automated systems to provide comfort in an efficient manner. It is apparent that improvements in all aspects of an intelligent system are be needed to better ascertain the correct combination of systems to activate and for how long to increase the overall efficiency of the system and improve comfort.

Cover page of Ceiling-fan-integrated air conditioning: Airflow and temperature characteristics of a sidewall-supply jet interacting with a ceiling fan

Ceiling-fan-integrated air conditioning: Airflow and temperature characteristics of a sidewall-supply jet interacting with a ceiling fan


Ceiling-Fan-Integrated Air Conditioning (CFIAC) is a proposed system that can greatly increase buildings’ cooling efficiency. In it, terminal supply ducts and diffusers are replaced by vents/nozzles, jetting supply air toward ceiling fans that serve to mix and distribute it within the room. Because of the fans’ air movement, the system provides comfort at higher room temperatures than in conventional commercial/ institutional/retail HVAC. We have experimentally evaluated CFIAC in a test room. This paper covers the distributions of air-speed, temperature, and calculated comfort level throughout the room. Two subsequent papers report tests of human subject comfort and ventilation effectiveness in the same experimental conditions. The room’s supply air emerged from a high-sidewall vent directed toward a ceiling fan on the jet centerline; we also tested this same jet on a fan located off to the side of the jet. Primary variables are: ceiling fan flow volumes in downward and upward directions, supply air volume, and room-vs-supply temperature difference. Velocity, turbulence, and temperature distributions are presented for vertical and horizontal transects of the room. The occupied zone is then evaluated for velocity and temperature non-uniformity, and for comfort as predicted by the ASHRAE Standard 55 elevated air speed method. We show that temperatures are well-mixed and uniform across the room for all of the fan-on configurations, for fans both within or out of the supply jet centerline. The ceiling fan flow dominates the CFIAC airflow, and even though non-uniform is capable of providing comfortable conditions throughout the occupied area of the room.

Cover page of Design and control of high thermal mass radiant systems

Design and control of high thermal mass radiant systems


Heating, ventilation, and air-conditioning (HVAC) systems play a key role in providing healthy, productive, and thermally comfortable built environment for the occupants. Improper HVAC design will degrade occupants’ satisfaction with the built environment, potentially affecting their performance which can be valued up to 200 times the building’s energy costs. In the top two energy consuming countries, the US and China, over 40% of the energy use in buildings with HVAC systems can be attributed to those systems. Moreover, 13% of total greenhouse gas emissions in the US can also be ascribed to HVAC systems. On a global scale, electricity demand for space cooling could increase by up to 210% by 2050 from 2016 levels. This rapid growth prediction is driven by the fact that most of the world’s population and wealth growth is happening in the tropics and in middle-income countries where air-conditioning has relatively small penetration in buildings. There are serious implications to electrical grid systems and most importantly, to our ecosystems if HVAC design is left unchecked.Therefore, in this dissertation we investigate high thermal mass radiant systems (HTMR) as a promising strategy to address the challenges and strain imposed by HVAC systems, with the focus on space cooling. HTMR, and other radiant systems in general, deliver 50% or more of the design heat transfer through thermal radiation, have large heat transfer areas, and have high heat transport efficiency. The “high thermal mass” in HTMR comes from the fact that there is a significant time delay, measured in hours, between a control action and the temperature response observed in the zone as a result of the thermal inertia in the concrete. This property has presented obstacles to the adoption rate of HTMR in the building stock in the US. In general, building designers are unfamiliar with how to design and control HTMR without adversely affecting occupants’ thermal satisfaction while also balancing other performance objectives such as capital and operational costs. Yet, because of the thermal response delay property, HTMR presents building designers and other stakeholders with innovative and beneficial design and control options that are difficult to implement in the more typical all-air systems to reduce equipment and electricity costs while maintaining acceptable indoor temperatures.The development of most building standards, guidelines, and tools have focused on all-air HVAC systems. One example is the standard design procedure for sizing cooling systems. The standard design procedure includes a definition of space cooling load which serves as the basis to size HVAC components from the zone level to the central cooling plant. However, that space cooling load definition is too narrowly constrained and omits fundamental principles that are essential to the operation of various cooling systems, including HTMR. We provide a critical review of the standard design procedure for sizing cooling systems to identify fundamental flaws, explain how it has influenced building energy modeling, system sizing and operation in practice, and propose a new definition for space cooling load along with an associated cooling system design procedure that better suits a variety of systems and control strategies. We conduct whole building energy simulations with the focus on HTMR to demonstrate the consequences of the standard procedure and compare it to our recommended procedure. The results show that following the standard design approach for HTMR can lead designers to underestimate the peak space cooling load by 100%, yet also select cooling plant equipment that is 100% larger than necessary due to its large thermal inertia. The standard design obscures considerable opportunities to reduce costs and improve energy efficiency and thermal comfort.For example, large heat transfer areas allow HTMR to take advantage of high-temperature cooling, i.e. using higher than typical supply water temperature to perform space cooling, and potentially eliminating the use of the vapor-compression refrigeration cycle. In lieu of this energy- and cost-intensive cycle, more sustainable cooling plants that use adiabatic cooling with cooling towers or fluid coolers can provide cool water production for HTMR. We used whole building energy simulation to determine the warmest supply water temperature that is able to still maintain comfortable temperatures for various building, HTMR, and control strategy designs. We used single zone models that represent ASHRAE 90.1-2016 and Title 24-2016 code-compliant buildings in 14 US and 16 Californian representative climates during the climates’ cooling design day. We found the warmest supply water temperature to be 18.2, 21.4, 23.4 °C for the first quartile, median, and third quartile, respectively, among all test cases. Cooling towers can generate these required supply water temperatures during nighttime periods when their performance is at their highest. There is great potential to avoid installing a compressor-based refrigeration system in most climates, while only a few will require more than code-compliant designed buildings.A key determinant to the successful implementation of HTMR is the control system. Improved HVAC control can improve energy, cost, and thermal comfort performance over typical control strategies, but improper control and faults can penalize them on a similar scale. We developed and experimentally tested a new HTMR control strategy that independently adapts to each radiant zone’s observed indoor temperatures in two California buildings located in distinct, contrasting climates. The results show that the new HTMR control strategy reduces the number of hours that zone dry-bulb temperatures exceed predefined thermal comfort limits from 9.1% to 1.6% as a proportion of total occupied hours when compared to the buildings’ existingcontrols. We verified that the new control strategy did not have adverse effects on occupant thermal comfort satisfaction through a detailed “right-now” satisfaction survey. The new strategy also reduces the number of average daily minutes HTMR manifold valves open for water flow through the slab, a proxy for energy consumption, by up to 93%.Finally, we created an interactive web-based tool for the early design of HTMR. The primary aim of this design tool is to provide an interface for estimating the performance of HTMR under steady-state and transient conditions. It allows users to estimate the impact of innovative control strategies such as nighttime pre-cooling on indoor temperature response. The tool website not only contains resources and lessons learned through the investigations presented in this dissertation but also from the overarching investigations on radiant systems undertaken by the Center for the Built Environment, which this Ph.D. study was part of.In this dissertation, we contributed on revising the fundamental cooling load definition and associated design procedure for applicability to a broader range of systems and applications, demonstrated the potential of using HTMR coupled with more sustainable cooling plants in a diverse set of US climate zones, developed and tested adaptive control strategies that take advantage of HTMR’s high thermal inertia to shift the building’s cooling load to more beneficial periods, and facilitate mechanical designers’ decision making with respect to HTMR systems through our early design web-based tool. These innovations will help achieve reductions in energy and greenhouse gas emissions attributed to HVAC systems and therefore support our global shift towards a more sustainable built environment.

Cover page of A review of advanced air distribution methods - theory, practice, limitations and solutions

A review of advanced air distribution methods - theory, practice, limitations and solutions


Ventilation and air distribution methods are important for indoor thermal environments and air quality. Effective distribution of airflow for indoor built environments with the aim of simultaneously offsetting thermal and ventilation loads in an energy efficient manner has been the research focus in the past several decades. Based on airflow characteristics, ventilation methods can be categorized as fully mixed or non-uniform. Non-uniform methods can be further divided into piston, stratified and task zone ventilation. In this paper, the theory, performance, practical applications, limitations and solutions pertaining to ventilation and air distribution methods are critically reviewed. Since many ventilation methods are buoyancy driving that confines their use for heating mode, some methods suitable for heating are discussed. Furthermore, measuring and evaluating methods for ventilation and air distribution are also discussed to give a comprehensive framework of the review.

Cover page of Side-by-side laboratory comparison of radiant and all-air cooling: How natural ventilation cooling and heat gain characteristics impact space heat extraction rates and daily thermal energy use

Side-by-side laboratory comparison of radiant and all-air cooling: How natural ventilation cooling and heat gain characteristics impact space heat extraction rates and daily thermal energy use


For radiant cooling to maintain equivalent comfort conditions as all-air cooling it must remove more heat from a space, the peak space heat extraction rate must be larger, and the peak must occur earlier. In this article, we assess how the magnitudes of these differences are influenced by heat gain characteristics and by the use of natural ventilation night precooling. We present measurements from a series of multi-day side-by-side comparisons of radiant cooling and all-air cooling in a pair of experimental testbed buildings, with equal heat gains, and maintained at equivalent comfort conditions. In a five-day experiment with mixed internal heat gains, solar gains, and natural ventilation night precooling, radiant cooling had to remove 35% more heat than the all-air system in equivalent circumstances; and the peak heat extraction rate was 20% larger (median difference on multiple days). In a similar experiment with highly convective internal gains the differences were smaller (26% more thermal energy, 12% larger peak), while in an experiment with highly radiant gains the differences were larger (40% more thermal energy, and 21% larger peak). The differences were much smaller in an experiment without natural ventilation night precooling (7% more thermal energy, 5% larger peak). These findings have consequences for the choice, design, and control of mechanical cooling systems, especially in buildings that also use passive cooling strategies such as natural ventilation night precooling.

Cover page of Ceiling fans: Predicting indoor air speeds based on full scale laboratory measurements

Ceiling fans: Predicting indoor air speeds based on full scale laboratory measurements


We measured indoor air speeds generated by ceiling fans in 78 full-scale laboratory tests. The factors were the room size, fan diameter, type, speed, direction (up or down), blade height, and mount distance (i.e. blade to ceiling height). We demonstrated the influence of these factors, showing that the most significant are speed, diameter and direction. With other factors fixed, the average room air speed in the occupied zone increases proportionally with fan air speed and diameter. Blowing fans upwards yields lower but far more uniform air speeds than downwards. We show that for the same fan diameter and airflow, fan type has little effect on the air speed distribution in the region outside the fan blades. We developed several new dimensionless representations and demonstrate that they are appropriate for comparisons over a wide range of fan and room characteristics. Dimensionless linear models predict the lowest, average, and highest air speeds in a room with a median (and 90th percentile) absolute error of 0.03 (0.08), 0.05 (0.13), and 0.12 (0.26) m/s respectively over all 56 downwards tests, representing common applications. These models allow designers to quickly and easily estimate the air speeds they can expect for a given fan and room. We include all measured data and analysis code in this paper.

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Cover page of Eliminating Overcooling Discomfort While Saving Energy

Eliminating Overcooling Discomfort While Saving Energy


A large percentage of commercial buildings in North America use variable air volume (VAV) systems with reheat, and this system type is also common around the world. Summertime overcooling is widespread in such buildings and has received considerable media attention over the past few years. ASHRAE Research Project RP-1515, reported in this article, shows that much of today’s overcooling originates in unsubstantiated engineering assumptions about the performance of VAV boxes and diffusers at low-flow setpoints. These assumptions are that low flows will cause diffusers to dump cooled air and create drafts around occupants, ventilation air will be poorly mixed, and VAV airflow control will become unstable or inaccurate. Together, they have resulted in VAV minimums being commonly set at 20% to 50% of maximum. ASHRAE RP-1515 and other recent research have shown each of these assumptions to be unwarranted, and that far lower minimums are desirable.