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I first encountered challenges of thermal stratification when working with wineries. Winery barrel warehouses tend to lack good air mixing, and the thermostats—positioned at human height near the ground—read an allowable 60°F while the air at the upper barrel racks could reach temperatures of 70°F or more, which can be problematic for wine quality. In outdoor wine tanks, I discovered surprising vertical temperature gradients, where the upper sections of wine in the tanks were far warmer than the thermostat probe at the bottom of the tank.
We find a similar phenomenon in the overhead heating systems of commercial buildings. When supply air is heated and discharged through ceiling diffusers, the hot air will not naturally fall to the level of the occupants. Instead, it must rely on its discharge velocity, the speed and direction at which it leaves the diffuser, to mix with the cooler air below.
Occupant Comfort and Energy Use
If the temperature of the supply air is too high, the discharge air velocity cannot overcome the density difference between the hot and cold air. Mixing worsens, and the hot supply air “short-circuits” to the ceiling exhaust grilles, without reaching the occupied space. The thermostats then call for more heating because hot air is not delivered to the occupant level. In many control sequences, this leads the system to make the air even hotter and dump higher and higher volumes of that air, resulting in wasted energy and compromised thermal comfort.
HVAC Control System Problems
We saw an example of extreme discharge temperature in a building with 270 variable air volume (VAV) boxes, many serving zones with 12-foot-high ceilings. The image below shows a BMS photo of one of its VAV box supplying air in heating mode. Our team took this photo while conducting a functional performance test of the air distribution system for a new construction project. Note the discharge temperature of 116°F.
BMS Screenshot of VAV Box with 116°F Discharge Air Temperature
To avoid thermal stratification, common guidance is to limit the supply air temperature within 15°F to 20°F of the zone air temperature—that is, the air temperature at occupant level. The thermostat at this zone reported a temperature of about 70°F, meaning the supply air temperature should have been at no more than 85°F or 90°F.
ASHRAE Standard 90.1-2019 recognizes the risk of thermal stratification and calls for limiting overhead supply air temperatures to 20°F above space temperature setpoint for zones that have both supply and return/exhaust air openings higher than 6 feet above the floor. Addendum H of ASHRAE Guideline 36-2018 also recognizes the issue and suggests limiting the discharge air temperature setpoint to a maximum temperature differential of 20°F above the space temperature setpoint.
The Cause and Solution
With 270 VAV boxes, and many serving zones with 12-foot ceilings, the VAV discharge air temperature setpoint had been programmed to reset between 91°F and 105°F. Frequently the air reached higher temperatures, such as the 116°F reading shown in the photo.
The heating system was susceptible to severe short circuiting between supply and exhaust, and we called out this issue to the owner. Resolution is still underway and will require reprogramming the setpoints for all VAV boxes.
Designing around Thermal Stratification for Comfort, Cost and Product Integrity
Whether our wine, energy use, or thermal comfort is at stake, it is worthwhile to recognize the non-uniformity of temperature distributions and adjust the design and control of our systems accordingly. If you have questions about thermal stratification or other undesired HVAC&R phenomena at your facility, please reach out at any time.
Acknowledgement: I wish to thank Lyn Gomes, who first brought the relationship of VAV discharge air temperature and thermal stratification to my attention.