Radiant chilled ceilings (CC) with displacement ventilation (DV) represent a promising integrated system design that combines radiant systems energy efficiency with the opportunity for improved ventilation performance typical of DV. The purpose of this study was to conduct laboratory experiments to investigate how the radiant surface temperature and the ratio of cooling load removed by CC over the total cooling load affects the air change effectiveness (ACE) measured according to ASHRAE Standard 129. Three experiments were carried out in a climatic chamber equipped with radiant panels covering 36.7% of the suspended ceiling, representing a typical installation of metal radiant panels. The results showed that ACE higher than one is maintained in the occupied zone even when more than half (54 percent) of the heat load is removed by a CC and the radiant surface temperature is 18.7°C.

Radiant chilled ceilings (CC) 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 U.S. 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, η, 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/m2 [0-23.1 Btu/(h ft2)](based on radiant panel area) or between 0 and 28 W/m2 [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 20-24°C [60-75.2°F] for the 12-panel tests and over a colder range of 16.5-22.6°C [61.7-72.7°F] for the 6-panel tests. The displacement ventilation airflow rate varied between 1.65 and 4.03 l/(s m2) [0.32-0.79 cfm/ft2], and the supply air temperature was kept constant at 18°C [64.4°F]. The results showed that increasing η, 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°C (2.7°F) in the occupied zone (seated or standing) will be maintained for all radiant ceiling surface temperatures of 18°C (64.4°F) or higher.

There is great energy-saving potential in reducing variable air volume (VAV) box minimum airflow setpoints to about 10% of maximum.  Typical savings are on the order of 10-30% of total HVAC energy, remarkable for an inexpensive controls setpoint change that properly maintains outside air ventilation. However, there has long been concern whether comfort and room air mixing are maintained under low flows through diffusers, and this concern has prompted VAV minima to be typically set at 20-50% of maximum.

RP 1515 evaluated occupants’ thermal comfort and air quality satisfaction in operating buildings under both conventional and reduced minimum VAV flow setpoints, and measured the air diffusion performance index and air change effectiveness for typical diffuser types in the laboratory.  The hypotheses were that lowered flow operation would not significantly reduce comfort or air quality, and that HVAC energy savings would be substantial. The hypotheses were almost entirely confirmed for both warm and cool seasons.  But beyond this, the reduction of excess airflow during low-load periods caused occupants’ cold discomfort in the warm season to be halved, a surprising improvement.  It appears that today’s widespread overcooling of buildings can be corrected without risk of discomfort by lowering conventional VAV minimum flow setpoints.

There is great energy-saving potential in reducing variable air volume (VAV) box minimum airflow setpoints.

In the past, setpoints have been maintained at high levels because of three concerns: 1) low flows might cause the occupants draft discomfort from insufficient mixing of diffuser discharge air, 2) inability of VAV boxes to control at low flows, and 3) poor air quality resulting from a combination of poor control and insufficient diffuser mixing. It is worth examining these concerns to see whether they are justified. The controller accuracy and stability have recently been addressed by RP 1353, in which VAV boxes were found to control well at very low flow levels. The diffuser mixing issue and impact on comfort are addressed in this research project, RP 1515.

RP 1515 is a combined field and laboratory study, in which occupants’ thermal comfort and air quality satisfaction is evaluated in the field under reduced minimum VAV flow rate setpoints, and the mixing performance of diffusers is measured in the laboratory. The laboratory portion was performed with co-funding from Price Industries. Additional co-funding from the California Energy Commission’s PIER program allowed us to quantify the HVAC energy savings resulting from the reduced flows in the field study buildings.