Center for the Built Environment

This multi-year project generated significant new and improved software design tools and commissioning guidelines for underfloor air distribution (UFAD) systems, new performance guidelines for radiant slab cooled buildings, and an updated advanced Berkeley thermal comfort model. This final report presents detailed results in four major task areas as summarized below.

Recommended commissioning guidelines were developed for the following three key elements affecting UFAD system performance: (1) procedures for measuring, adjusting, and optimizing room air stratification; (2) a new test protocol for determining air leakage from underfloor plenums; and (3) strategies and methods for controlling and managing thermal decay (temperature gain) in underfloor plenums. Emphasis is placed on commissioning procedures that are practical and as simple as possible for use by commissioning agents, and promote energy efficient operation while maintaining thermal comfort. The guidelines were developed through a combination of field and laboratory experiments, fundamental energy simulations, computational fluid dynamics (CFD) modeling, and simplified design tool studies.

A number of improvements were made to EnergyPlus/UFAD, a version of the publicly available whole-building energy simulation program, EnergyPlus (developed under a previous PIER contract), which greatly enhanced its capabilities for modeling the more complex heat transfer processes found in UFAD systems. The improved version of EnergyPlus was in turn used as a basis for developing a more comprehensive simplified design tool for determining design cooling loads for UFAD systems, the first of its kind.

Radiant slab cooling systems were found to demonstrate strong energy saving performance and improved occupant satisfaction in dry western U.S. climates based on a combination of occupant satisfaction surveys, two case studies, and whole-building energy simulations.

The usability of the Berkeley thermal comfort model was improved by developing a user tutorial and demonstrated by conducting a case study of a building with a radiant floor slab.

A new procedure for predicting the thermal comfort of people in naturally ventilated buildings is described. The procedure starts by obtaining, for each important wind direction, velocity ratios between points of interest inside the proposed building and the wind-measuring height outside. Th~s is best done with a wind-tunnel test of a scale model of the building, but there are also published sources of such ratios. The ratios, plus building-induced temperature changes, are applied to a weather tape representing the site, in order to produce an hour-by-hour record of indoor climate. This record is used in a program simulating human thermal com-fort. The program puts out the percentage of time, by season and by periods of day, that thermal comfort is expected in the proposed building. This information enables architects and engineers to make more rational decisions in designing naturally ventilated buildings.

Hydronic radiant heating and cooling systems are considered as an energy efficient technology to condition buildings. We performed a literature review to assess if radiant systems provide better, equal or lower thermal comfort than all-air systems. We included only peer-reviewed articles and articles published in proceedings of scientific conferences. The publications found have been classified based on research methods used. These include: (1) building performance simulation (BPS), (2) physical measurements (in laboratory test chambers and in buildings) and (3) human subject testing / occupant based surveys. This review identified eight conclusive studies: five studies that could not establish a thermal comfort preference between all-air and radiant systems and three studies showing a preference for radiant systems. Very few studies were based on occupant feedback in real buildings suggesting a significant research need. Overall, we found that a limited number of studies are available and therefore a solid answer cannot be given. Nevertheless, there is suggestive evidence that radiant systems may provide equal or better comfort than all-air systems.

Zhang’s thermal comfort model predicts that the local comfort of feet, hands, and face predominate in determining a person’s overall comfort in warm and cool conditions. We took advantage of this by designing a task-ambient conditioning (TAC) system that heats only the feet and hands, and cools only the hands and face, to provide comfort in a wide range of ambient environments. Per workstation, the TAC system uses less than 42W for cooling and 60W for heating. By reducing the amount of control normally needed in the overall building, it could be possible to save much larger amounts of energy in the building HVAC system.

We tested the TAC system on 18 subjects in our environmental chamber, at temperatures representing a wide range of practical winter and summer conditions (18-30ºC, or 65-86 ºF). A total of 90 tests were done. We measured subjects’ skin and core temperatures, obtained their subjective responses about thermal comfort, perceived air quality, and air movement preference. The subjects performed three different types of tasks to evaluate their productivity at white-collar-types of work during the testing.

The TAC system was able to maintain positive comfort levels across the entire temperature range tested. TAC did not significantly affect the task performance of the occupants compared to a neutral ambient condition. Whenever air motion was provided, perceived air quality was significantly improved, even if the air movement was re-circulated room air. There was no dry-eye discomfort with the head ventilation device as designed, even at 1 m/s in the breathing zone. The acceptable thermal sensation levels were from –2.2 to 2. In our tests, subjects found thermal environments acceptable even if they were judged slightly uncomfortable (-0.5).

Simulated annual energy savings with the TAC system in Fresno, Oakland, and Minneapolis were each about 40% with intensive use of TAC (allowing 18-30ºC ambient interior temperature), and 30% with a moderate use (in 20-28ºC ambient temperature).

Air conditioning and mechanical ventilation systems may be oversized in commercial buildings in the Tropics. Oversized cooling coils may lead to reduced dehumidifying performance, indoor air quality and thermal comfort and increased energy consumption. In this paper, an adaptable cooling coil design is assessed with a general-purpose coil selection software tool, in which the number of active rows changes as a function of the load. For a 100% oversized coil, it is shown that the adaptable cooling coil is able to provide small but relevant improved humidity control down to 25% of the design load. This was obtained without affecting energy performance in typical variable air volume design and control. For specific applications, where variable air volume systems mainly control space humidity, there are also energy savings. The adaptable cooling coil could be seen as providing additional flexibility in the operation of HVAC systems, particularly in the tropics.

Radiant cooling systems extract heat from buildings differently than all-air cooling systems. These differences impact the time and rate at which heat is removed from a space, as well as the total amount of thermal energy that a mechanical system must process each day. In this article 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 (operative temperature). The results show that radiant cooling must remove more heat than all-air cooling – 2% more in an experiment with constant internal heat gains, and 7% more with periodic scheduled internal heat gains. Moreover, the peak sensible space heat extraction rate for radiant cooling (heat transfer at the cooled surface, not the cooling plant) must be larger than the peak sensible space heat extraction rate for all-air systems, and it must occur earlier. The daily peak sensible space heat extraction rate for the radiant system was 1–10% larger than for the all air system, and it occurred 1–2 hours earlier. These findings have consequences for the design of radiant systems. In particular, this study confirms that cooling load estimates for all-air systems will not represent the space heat extraction rates required for radiant systems.

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 $38.9/ft² compared to $29.9/ft² for the VAV design, representing a $9.0/ft² premium for the radiant design.

• The higher costs for the radiant system can largely be attributed to higher piping labor costs for piping and radiant equipment, which itself is $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 manufacturers’ pricing strategies and the contractors’ 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.

Underfloor air distribution systems use the underfloor plenum beneath a raised floor to provide conditioned air through floor diffusers that creates a vertical thermal stratification. Thermal stratification affects energy, indoor air quality and thermal comfort.

The purpose of this study was to characterize the influence of linear bar grilles and VAV directional diffusers on thermal stratification in perimeter zones by developing theoretically and empirically based models. Forty seven laboratory experiments were carried out in a climatic chamber equipped with a solar simulator.

Linear bar grilles tend to produce less stratification than VAV directional diffusers and, in some cases with high airflow rates, may generate reverse stratification. Models to predict temperature stratification for the two tested diffusers have been developed.

This multi‐year project developed EnergyPlus/UFAD, a version the publicly available wholebuilding energy simulation program EnergyPlus that adds the capability for modeling underfloor air distribution systems. The project also developed a practical design tool for determining the cooling airflow quantity for underfloor air distribution systems. EnergyPlus/UFAD and the cooling airflow design tool are the first validated underfloor air distribution system tools of their kind. As such, they represent a significant advance in the state of the art of the design and energy analysis of such systems. This highly collaborative effort involved experts and facilities from four organizations, including the Center for the Built Environment at University of California, Berkeley; University of California, San Diego; Lawrence Berkeley National Laboratory; and York International.

This final report and seven appendices present experimental testing and analytical and computational fluid dynamics modeling on room air stratification and underfloor plenum distribution—critical efforts that informed the development of models for EnergyPlus. Also discussed are new implementations of heating, ventilation, and air conditioning systems to support underfloor air distribution system modeling in EnergyPlus and the development of a practical design tool for such systems.