Center for the Built Environment

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.

Using fans alone or in coordination with HVAC systems to cool people offers several significant enhancements compared to conventional HVAC systems, including improved thermal comfort, indoor air quality, air distribution, energy savings, and initial cost savings. Despite the numerous benefits of fans and fan-integrated systems, comprehensive resources are unavailable to guide engineers and architects in designing and implementing such systems. The purpose of this guideline is to address this gap and provide practitioners with valuable materials and answers to common questions. What are the available fan options? Various fan types are available in the market, such as ceiling fans, desk fans, and pedestal fans. This guideline provides a comprehensive overview of the criteria for fan type selection. These criteria cover blade characteristics, fan size, airflow patterns, fan performance metrics, motors and drives, power and efficiency, and control strategies. Ceiling fans are generally preferred for their higher efficiency, control and effectiveness, but cost, flickering and fixed location are limitations. This guideline can assist users in selecting suitable fan types based on individual building characteristics and specific application needs. How to integrate fans with my existing AC system? Adequate fan choice mainly depends on design intents, space characteristics, and HVAC operation strategies. This guidebook discusses which HVAC systems can be integrated with fans. With well-defined design intent, the guidebook provides a step-by-step process for determining the number and size of fans and their layout, ensuring proper fan installation, and integrating the fans with the control of the HVAC system. How do we implement and operate fans-integrated systems? Designing a successful fans-integrated system involves more than adding fans to a building. This guidebook assists building designers and operators by helping them adjust appropriate settings in chillers, air handling units, environmental conditions, and operation strategies. This optimization maximizes energy savings while ensuring occupants' comfort within the building. Additionally, the guidelines explore HVAC systems design that can enhance air distribution effectiveness and minimize construction costs. Most importantly, the guidelines outline a transformation strategy for transitioning from conventional air conditioning to a fan-integrated system with minimal disruption to occupants. This comprehensive approach ensures a smooth and efficient integration of fans into the existing air-conditioning infrastructure. Which design tools and case studies are available? This guidebook recommends using the and the CBE Thermal Comfort Tool CBE Ceiling Fan Design Tool. These tools enable users to define the comfort zone with elevated air speed and determine the optimal arrangement of ceiling fans based on room conditions specified by the users. In addition, the guidebook highlights relevant codes and standards related to environmental conditions, fan testing procedures, fire safety, and seismic requirements (subject to variations in different countries' regulations). Furthermore, we presented several case studies of buildings successfully implementing the fan-integrated HVAC system. We released two versions of our guidebook online: the Practitioner Summary and the Full Guide. These resources support selecting, designing, constructing, operating, and implementing fans and fans integrated HVAC systems. The Practitioner Summary offers a concise overview (~15 pages) of key considerations for building practitioners, providing brief descriptions of the fan-integrated system. In contrast, the Full Guidebook provides a more comprehensive exploration (~70 pages) of the fan and fan integrated system, including real building references, catering to users from diverse backgrounds. So we expect that many readers, after studying the Practitioner Summary, will move to sections of the Full Guidebook that are relevant to their work. Fans and fans-integrated HVAC systems will result in more sustainable and healthy buildings.

Using fans alone or in coordination with HVAC systems to cool people offers several significant enhancements compared to conventional HVAC systems, including improved thermal comfort, indoor air quality, air distribution, energy savings, and initial cost savings.Despite the numerous benefits of fans and fan-integrated systems, comprehensive resources are unavailable to guide engineers and architects in designing and implementing such systems. The purpose of this guideline is to address this gap and provide practitioners with valuable materials and answers to common questions.

What are the available fan options?Various fan types are available in the market, such as ceiling fans, desk fans, and pedestal fans. This guideline provides a comprehensive overview of the criteria for fan type selection. These criteria cover blade characteristics, fan size, airflow patterns, fan performance metrics, motors and drives, power and efficiency, and control strategies. Ceiling fans are generally preferred for their higher efficiency, control and effectiveness, but cost, flickering and fixed location are limitations. This guideline can assist users in selecting suitable fan types based on individual building characteristics and specific application needs.

How to integrate fans with my existing AC system?Adequate fan choice mainly depends on design intents, space characteristics, and HVAC operation strategies. This guidebook discusses which HVAC systems can be integrated with fans. With well-defined design intent, the guidebook provides a step-by-step process for determining the number and size of fans and their layout, ensuring proper fan installation, and integrating the fans with the control of the HVAC system.How do we implement and operate fans-integrated systems?Designing a successful fans-integrated system involves more than adding fans to a building. This guidebook assists building designers and operators by helping them adjust appropriate settings in chillers, air handling units, environmental conditions, and operation strategies. This optimization maximizes energy savings while ensuring occupants' comfort within the building. Additionally, the guidelines explore HVAC systems design that can enhance air distribution effectiveness and minimize construction costs. Most importantly, the guidelines outline a transformation strategy for transitioning from conventional air conditioning to a fan-integrated system with minimal disruption to occupants. This comprehensive approach ensures a smooth and efficient integration of fans into the existing air-conditioning infrastructure.Which design tools and case studies are available?This guidebook recommends using the and the CBE Thermal Comfort Tool CBE Ceiling Fan Design Tool. These tools enable users to define the comfort zone with elevated air speed and determine the optimal arrangement of ceiling fans based on room conditions specified by the users. In addition, the guidebook highlights relevant codes and standards related to environmental conditions, fan testing procedures, fire safety, and seismic requirements (subject to variations in different countries' regulations). Furthermore, we presented several case studies of buildings successfully implementing the fan-integrated HVAC system.We released two versions of our guidebook online: the Practitioner Summary and the Full Guide. These resources support selecting, designing, constructing, operating, and implementing fans and fans integrated HVAC systems. The Practitioner Summary offers a concise overview (~15 pages) of key considerations for building practitioners, providing brief descriptions of the fan-integrated system. In contrast, the Full Guidebook provides a more comprehensive exploration (~70 pages) of the fan and fan integrated system, including real building references, catering to users from diverse backgrounds. So we expect that many readers, after studying the Practitioner Summary, will move to sections of the Full Guidebook that are relevant to their work. Fans and fans-integrated HVAC systems will result in more sustainable and healthy buildings.

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.

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.