Every once in a while you get a call from a large facility that has an issue keeping the common areas warm at ground level. The recommendation of adding fans when they are complaining about being cold has raised an eyebrow more than once….
How To Fight Air Stratification in Conditioned Facilities
The key to controlling stratification is to find a way to get the heated air at the upper levels of the space to drop down and mix with the cooler air at lower levels.
Among the more difficult buildings to properly heat and cool are those with high ceilings. During the heating season, the warm air rises towards the typically unoccupied areas near the ceiling, while colder air settles towards the floor where most building occupants are located. To maintain temperatures at the lower levels where most thermostats are located, the heating system must supply additional heat which rises towards the ceiling.
During the cooling season, the process is reversed with cool air settling to the floor. The higher one goes in the space, the warmer the temperature. If the space has a mezzanine level, maintaining temperatures at that level may require that ground level spaces be overcooled.
This process of the air being layered by temperature is known as thermal stratification. In a typical building, the temperature rise is approximately 0.5 degrees F per foot in height above the floor. The overheating and overcooling that is required to overcome stratification results in comfort issues and energy waste. Estimates of the annual energy savings that can be achieved if the effects of stratification can be reduced range between 15 and 20 percent.
The higher the ceiling of the conditioned space, the greater the potential for stratification. But ceiling height is not the only factor that impacts stratification. Heat generated within the space by occupants or equipment contributes to stratification as does solar gain. Stratification is more pronounced in buildings where the building envelope, particularly the envelope near the ceiling, is in poor condition, resulting in high heat losses due to conduction and exfiltration.
The key to controlling stratification is to find a way to get the heated air at the upper levels of the space to drop down and mix with the cooler air at lower levels. There are two major ways to do this: axial fans and high-volume, low-speed fans.
Axial fans are self-contained units that, when installed near the ceiling, can direct air towards the floor. They are small units that can be used in applications with ceiling heights up to 60 feet. They operate at a speed that, while high enough to move the air from higher levels to lower levels, is low enough to allow thorough mixing of the air from the ceiling with air located at the floor without causing irritating drafts. They are low in energy use and do not generate significant noise. A typical application requires roughly one fan per 1,000 square feet of floor space.
High-volume, low-speed fans in contrast are large units, typically eight to 14 feet in diameter. Also mounted near the ceiling, the units decrease stratification by churning the air rather than by forcing a stream of air downwards. They are best suited for use in open spaces with ceiling heights between 15 and 30 feet. Like axial fans, they are low in energy use and noise generation.
One consideration when installing high-volume, low-speed fans is that it may be necessary to relocate existing light fixtures. If lights are located directly above the fans, the rotating blades can produce an annoying strobe effect.
There are two basic types of control systems for both the axial and high-volume, low-speed fans: preventive and reactive. With preventive controls, the fans operate continuously to prevent the development of thermal stratification. Reactive controls measure the temperature at the ceiling and at the floor, turning the fan on when a preset temperature difference develops between the two. Both systems are effective in reducing stratification.
Fans can reduce thermal stratification in practically all facilities that have high ceilings. Properly designed systems can reduce the floor to ceiling temperature differential by more than 80 percent, providing both energy savings and improved comfort.
Fans for destratification are particularly effective in some special use applications. For example, the fans can help control condensation in areas with high humidity levels, such as swimming pools or gyms. Similarly, reducing thermal stratification can reduce fogging of windows during the cooling season and frosting of windows during the heating season.
Another special use application where fans can improve both comfort and energy efficiency is facilities like warehouses that have large door openings. Every time the door is opened, large quantities of conditioned air are lost to the outside. When the door is closed, it takes time for the HVAC system to restore the space to the desired temperature. Destratification fans can reduce the time required.
Destratification fans cannot solve problems with buildings whose envelope is in poor condition, particularly when those poor conditions exist near the ceiling. High levels of heat loss near the ceiling set up thermally induced currents that result in the cooling of the air, causing it to fall. Destratification fans in this case would accelerate this cold air back towards the floor. The heat loss issues of the building envelope must be addressed first.
Similarly, buildings that have improperly sealed openings near the ceiling must have those issues corrected first. Openings, particularly when they are near the ceiling, result in air leaks from the building. This exfiltration is a particularly strong contributor to stratification. For example, if air is leaking out of the building at or near the high point of the conditioned space during the heating season, it creates a slight negative pressure. This negative pressure draws cold air into the space, usually at lower levels. This cold air lowers the temperature of the space at the level of the thermostat, so it calls for more heat. The heated air then rises in the space, leaks out, drawing in even more cold air.
Facility managers responsible for buildings with poorly designed or operating HVAC systems must address those system issues first before investing in destratification fans. Poorly operating systems may not be properly distributing the conditioned air throughout the space, resulting in some areas being warm while others are cold. This in turn contributes to stratification. Correct the HVAC issues first, then consider installing destratification fans.
Destratification fans should not be installed in unconditioned spaces. While it might seem beneficial to keep air moving in these spaces, destratification fans are not meant to serve as ventilation fans. Unconditioned spaces tend to be poorly insulated, if they are insulated at all. During the cooling season, the fans would simply move hot air from near the ceiling down to the floor. During the heating season, they would just increase the quantity of cold air reaching the floor.
Properly applied, destratification fans can improve comfort levels while reducing energy use. Typical energy savings from the use of the fans provide a simple payback of five years or less.
James Piper, PhD, PE, is a writer and consultant who has more than 35 years of experience in facilities management. He is a contributing editor for Building Operating Management.
During the cooling season, the process is reversed with cool air settling to the floor. The higher one goes in the space, the warmer the temperature. If the space has a mezzanine level, maintaining temperatures at that level may require that ground level spaces be overcooled.
This process of the air being layered by temperature is known as thermal stratification. In a typical building, the temperature rise is approximately 0.5 degrees F per foot in height above the floor. The overheating and overcooling that is required to overcome stratification results in comfort issues and energy waste. Estimates of the annual energy savings that can be achieved if the effects of stratification can be reduced range between 15 and 20 percent.
The higher the ceiling of the conditioned space, the greater the potential for stratification. But ceiling height is not the only factor that impacts stratification. Heat generated within the space by occupants or equipment contributes to stratification as does solar gain. Stratification is more pronounced in buildings where the building envelope, particularly the envelope near the ceiling, is in poor condition, resulting in high heat losses due to conduction and exfiltration.
Fighting stratification
The key to controlling stratification is to find a way to get the heated air at the upper levels of the space to drop down and mix with the cooler air at lower levels. There are two major ways to do this: axial fans and high-volume, low-speed fans.
Axial fans are self-contained units that, when installed near the ceiling, can direct air towards the floor. They are small units that can be used in applications with ceiling heights up to 60 feet. They operate at a speed that, while high enough to move the air from higher levels to lower levels, is low enough to allow thorough mixing of the air from the ceiling with air located at the floor without causing irritating drafts. They are low in energy use and do not generate significant noise. A typical application requires roughly one fan per 1,000 square feet of floor space.
High-volume, low-speed fans in contrast are large units, typically eight to 14 feet in diameter. Also mounted near the ceiling, the units decrease stratification by churning the air rather than by forcing a stream of air downwards. They are best suited for use in open spaces with ceiling heights between 15 and 30 feet. Like axial fans, they are low in energy use and noise generation.
One consideration when installing high-volume, low-speed fans is that it may be necessary to relocate existing light fixtures. If lights are located directly above the fans, the rotating blades can produce an annoying strobe effect.
There are two basic types of control systems for both the axial and high-volume, low-speed fans: preventive and reactive. With preventive controls, the fans operate continuously to prevent the development of thermal stratification. Reactive controls measure the temperature at the ceiling and at the floor, turning the fan on when a preset temperature difference develops between the two. Both systems are effective in reducing stratification.
Fans can reduce thermal stratification in practically all facilities that have high ceilings. Properly designed systems can reduce the floor to ceiling temperature differential by more than 80 percent, providing both energy savings and improved comfort.
Fans for destratification are particularly effective in some special use applications. For example, the fans can help control condensation in areas with high humidity levels, such as swimming pools or gyms. Similarly, reducing thermal stratification can reduce fogging of windows during the cooling season and frosting of windows during the heating season.
Another special use application where fans can improve both comfort and energy efficiency is facilities like warehouses that have large door openings. Every time the door is opened, large quantities of conditioned air are lost to the outside. When the door is closed, it takes time for the HVAC system to restore the space to the desired temperature. Destratification fans can reduce the time required.
Fan limitations
Destratification fans cannot solve problems with buildings whose envelope is in poor condition, particularly when those poor conditions exist near the ceiling. High levels of heat loss near the ceiling set up thermally induced currents that result in the cooling of the air, causing it to fall. Destratification fans in this case would accelerate this cold air back towards the floor. The heat loss issues of the building envelope must be addressed first.
Similarly, buildings that have improperly sealed openings near the ceiling must have those issues corrected first. Openings, particularly when they are near the ceiling, result in air leaks from the building. This exfiltration is a particularly strong contributor to stratification. For example, if air is leaking out of the building at or near the high point of the conditioned space during the heating season, it creates a slight negative pressure. This negative pressure draws cold air into the space, usually at lower levels. This cold air lowers the temperature of the space at the level of the thermostat, so it calls for more heat. The heated air then rises in the space, leaks out, drawing in even more cold air.
Facility managers responsible for buildings with poorly designed or operating HVAC systems must address those system issues first before investing in destratification fans. Poorly operating systems may not be properly distributing the conditioned air throughout the space, resulting in some areas being warm while others are cold. This in turn contributes to stratification. Correct the HVAC issues first, then consider installing destratification fans.
Destratification fans should not be installed in unconditioned spaces. While it might seem beneficial to keep air moving in these spaces, destratification fans are not meant to serve as ventilation fans. Unconditioned spaces tend to be poorly insulated, if they are insulated at all. During the cooling season, the fans would simply move hot air from near the ceiling down to the floor. During the heating season, they would just increase the quantity of cold air reaching the floor.
Properly applied, destratification fans can improve comfort levels while reducing energy use. Typical energy savings from the use of the fans provide a simple payback of five years or less.
James Piper, PhD, PE, is a writer and consultant who has more than 35 years of experience in facilities management. He is a contributing editor for Building Operating Management.
from https://commercialacrepair0.blogspot.com/2019/01/every-once-in-while-you-get-call-from.html
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