Underfloor air distribution (UFAD) is a mechanical ventilation strategy in which the conditioned air is primarily delivered to the zone from a pressurized plenum through floor mounted diffusers. Compared to conventional overhead (OH) mixing systems, UFAD has several potential advantages, such as improved thermal comfort and indoor air quality (IAQ), layout flexibility, reduced life cycle costs and improved energy efficiency in suitable climates. In ducted OH systems designers have reasonably accurate control of the diffuser supply temperature, while in UFAD this temperature is difficult to predict due to the heat gain of the conditioned air in the supply plenum. The increase in temperature between the air entering the plenum and air leaving through a diffuser is known as thermal decay. In this study, the detailed whole-building energy simulation program, EnergyPlus, was used to explain the fundamentals of thermal decay, to investigate its influence on energy consumption and to study the parameters that affect thermal decay. It turns out that the temperature rise is considerable (annual median = 3.7 K, with 50% of the values between 2.4 and 4.7 K based on annual simulations). Compared to an idealized simulated UFAD case with no thermal decay, elevated diffuser air temperatures can lead to higher supply airflow rate and increased fan and chiller energy consumption. The thermal decay in summer is higher than in winter and it also depends on the climate. The ground floor with a slab on grade has less temperature rise compared to middle and top floors. An increase of the supply air temperature causes a decrease in thermal decay. The temperature rise is not significantly affected by the perimeter zone orientation, the internal heat gain and the window-to-wall ratio.
Radiant chilled ceilings with displacement ventilation (DV) represent a promising integrated system design that combines the energy efficiency of both sub-systems with the opportunity for improved ventilation performance resulting from the thermally stratified environment of DV systems. The purpose of this study was to conduct laboratory experiments for a typical US. interior zone office to investigate how room air stratification is affected by the ratio of cooling load removed by a chilled ceiling to the total cooling load eta, for two different chilled ceiling configurations. The experiments were carried out in a climatic chamber equipped with radiant panels installed in the suspended ceiling. In the first test configuration representative of thermally activated slab applications, 12 panels covering 73.5% of the ceiling were used. During the second series of tests, 6 panels covering 36.7% of the ceiling were used, representing a typical installation of metal radiant panels. The cooling load removed by the panels varied between 0 and 73 W/m(2) (0-23.1 Btu/(h ft(2))) (based on radiant panel area) or between 0 and 28 W/m(2) (0-8.9 Btu/(h ft2)) (based on room area). The average mean water temperature of the panels varied over a more moderate range of 20 degrees C-24 degrees C (60 degrees F-75.2 degrees F) for the 12-panel tests and over a colder range of 16.50 degrees C-22.6 degrees C (61.7 degrees F-72.7 degrees F) for the 6-panel tests. The displacement ventilation airflow rate varied between 1.65 and 4.03 l/(s m(2)) (0.32-0.79 cfm/ft(2)), and the supply air temperature was kept constant at 18 degrees C (64.4 degrees F). The results showed that increasing eta, the relative amount of the cooling load removed by the chilled ceiling, reduced the total room stratification. However, a comparison between the colder 6-panel tests and the warmer 12-panel tests indicated that average radiant surface temperature (mean chilled water temperature in panels) was a stronger predictor of stratification performance. When smaller active radiant ceiling areas are used (e.g., for a typical radiant ceiling panel layout), colder radiant surface temperatures are required to remove the same amount of cooling load (as a larger area), which cause more disruption to the room air stratification. Despite the impact that the chilled ceiling has on stratification, the results indicate that a minimum head-ankle temperature difference of 1.5 degrees C (2.7 degrees F) in the occupied zone (seated or standing) will be maintained for all radiant ceiling surface temperatures of 18 degrees C (64.4 degrees F) or higher.
Underfloor air distribution system (UFAD) is a mechanical air distribution strategy in which the conditioned air is primarily delivered to the zone from a pressurized plenum through floor-mounted diffusers. It has several potential advantages compared to conventional overhead (OH) mixing systems. However, most of the energy simulation programs widely used by the industry are not able to represent two distinct features of UFAD systems: room air stratification and the underfloor supply plenum. The situation has been improved with the development of a UFAD module in EnergyPlus. The Center for the Built Environment developed the modeling methods, tested them extensively, and conducted numerous studies of UFAD energy performance. This paper summarizes lessons learned related to UFAD specific issues such as thermal decay, sizing, terminal units, room air stratification and thermal comfort.
In this paper, an EnergyPlus simulation model was used to simulate the operation of a novel integrated HVAC system. This system combines an underfloor air distribution system with a cooled radiant ceiling slab. A cooling tower supplies water to pre-cool the structural slabs during the night and early morning period. The paper compares the performance of this system to both an overhead system and an underfloor air distribution system in the cooling season for the Sacramento, California climate. The UFAD/Radiant hybrid system shows an energy reduction of between 21-25% during the peak cooling months, an electricity demand reduction of 27% during the peak hour, and improved occupant thermal comfort.
Building occupants are a valuable source of information for indoor environmental quality (IEQ) and its effects on health, comfort, satisfaction, self-reported performances, and building performance. There are no standardized methods to survey occupants. A brief literature review has been conducted to collect and describe features of IEQ questionnaires. Ten surveys have been identified and analyzed in terms of type of evaluation, objectives, investigated topics, number of applications, integration with physical measurements, questionnaire structure, types of questions and answers, length of time to complete, languages, and distribution and gathering strategies.
