Vicky Broadus co-authored this article.
Fans in Winter? Your Accountant Will Thank You.
Winter brings with it costly heating bills, and what you may not realize is a lot of your money is being wasted on heat you never feel — the heat that’s up at the ceiling.
That’s due to a phenomenon called stratification. Stratification can be summed up in three words: hot air rises. When heat is introduced into a space, it forms temperature layers, or strata. The warmest air is at the ceiling; the coolest is at the floor where people work and live. This creates obvious problems in terms of both comfort and expense.
And when you consider heating amounts to more than 35 percent of a commercial building’s annual energy consumption, according to the Commercial Buildings Energy Consumption Survey, those problems quickly cause budget pressures.
Problems of Stratification
Technically speaking, heated air from a forced-air system is 5 percent to 10 percent lighter than air at desirable room temperatures (65°–75°F) and naturally rises to the highest point in a space, where temperatures can reach 85°–125°F depending on structure and ceiling height. The thermostat, typically at occupant level, registers the temperature of the cooler, denser air, meaning the heating system is forced to over-heat the ceiling in an effort to reach the desired air temperature at occupant level.
Because most working and living environments rely on some kind of introduced heat when temperatures drop, stratification requires that we either pay for unbridled energy usage or find ways to undo the phenomenon. Obviously, the first option is less desirable.
To better appreciate the kind of thermal stratification that affects facilities and energy consumption, there’s no better place to go than an aviation hangar.
The American Airlines maintenance hangar in Charlotte, North Carolina, had a severe case of heat stratification because of its 100-foot tall ceilings and 400-foot wide bay doors. Eight heaters mounted at wing-height sent hot air straight to the ceiling, where temperatures could get so high they’d cause electrical equipment to malfunction. Meanwhile, mechanics on the ground were forced to work in bulky jackets due to the cold temperatures at floor level. Because of the stratified air, comfort was non-existent despite tens of thousands of dollars in monthly heating costs.
Stratification is a serious problem not only in hangars but in all facilities with tall ceilings, from offices to gyms to factories. The related energy expenses can be especially burdensome on organizations that rely heavily on donations to pay their bills. For example, at the First Baptist Church of Greater Cleveland, located in an area known for its cold winters, stratification contributed to heating bills up to $20,000 a month. The furnace strained to maintain a steady 74°F to keep the church’s pipe organ in working order — but that hot air instead rose and became trapped at the peak of the 55-foot-tall sanctuary. The financial burden was overwhelming.
Benefits of Destratification
Destratification is the creation of a uniform temperature throughout a space. In other words, it’s the undoing of the natural stratification, or layering, that occurs when warmer air is introduced by a heating system. In many cases, the most efficient way to achieve destratification in large buildings is through the use of large-diameter overhead fans. The problems at both American Airlines and the First Baptist Church of Greater Cleveland were remedied with large-diameter high-volume, low-speed (HVLS) overhead fans, which are able to move large amounts of air using very little energy.
The ability of an HVLS fan to effectively destratify is based on three main factors: the jet of air produced, the volume of air moved and the lack of draft created at low speeds. Unlike small ceiling fans, HVLS fans can create a slow-moving column of air that reaches all the way to the floor. In tall spaces, small fans cannot efficiently mix the entire volume, resulting in air that remains stratified. The volume of air moved by the fan is critical for complete destratification; the fan must turn over all the air in the space at least once per hour to homogenize the temperature.
To avoid drafts, which can feel chilly even when warm air is being moved, HVLS fans should be slowed to 10 to 30 percent of their maximum rotations per minute (RPM) in the forward direction — not reversed. Running in the forward direction moves a large volume of air to the floor without creating a draft (measured as air velocity of ~30 fpm or less at occupant level). Contrary to conventional wisdom, it has been shown that reversing a fan at higher speeds requires more energy and increases the rate of heat loss through the roof and is ineffective for bringing hot air down.
Destratifying a space can lower heating costs as much as 30 percent because of the simple fact that the stratified, warm air is pushed down to the thermostat level, meaning the heating system no longer has to work as hard to maintain the set temperature.
The architectural firm of McCool Carlson Green designed Machetanz Elementary in Wasilla, Alaska — the state’s first LEED-certified school — with an open layout. At the heart of the school is an 1,800-square-foot, two-story multipurpose room. But with an open floor plan and frigid outside temperatures come mechanical concerns, as HVAC systems work harder to try to combat heat stratification. In the winter months, the thermostat setting is reduced to 55oF overnight. According to school officials, the addition of one 12-foot diameter fan has resulted in a significant reduction in the amount of time needed to warm the building to 68oF before students arrive each day. While most schools in the district can take three to four hours to achieve desired temperatures, Machetanz takes just 15 to 20 minutes through the even distribution of heat.
In the design phase, architects projected a building performance of 18,500 MBtu/year. Designers then made a few modifications (orientation, daylighting and controls, among others), revising the performance goal to an impressive 8,700 MBtu/year. After the first full year of occupancy, architects discovered the building performance was 4,300 MBtu/year, representing over $200,000 in annual savings.
At another airplane hangar, the savings from destratification by HVLS fans was even more impressive. Before the installation of five 24-foot diameter fans at British Airways’ Hangar 6 at London’s Gatwick Airport, the difference in temperature between the floor and the mezzanine during heating season was close to 20oF. After installation, not only did the fans create a more comfortable and efficient workspace, but British Airways was able to cut energy consumption in the hangar by more than $114,000 during the first four months of the fans’ operation.
And back at the First Baptist Church of Greater Cleveland, a single 12-foot fan pushes down the warm air at the top of the 55-foot sanctuary ceiling, keeping the congregants and the pipe organ warm and saving thousands of dollars on heat every winter.
Regardless of the type of facility, tall ceilings and cold weather lead to high heating bills and uncomfortable occupants. HVLS fans provide energy savings and improved comfort all winter.
Vicky Broadus is a writer for Big Ass Fans, which produces, sells and installs industrial and commercial fans for large spaces. Christian Taber is a senior research engineer at Big Ass Fans, working with the business development team to identify new research opportunities. Taber is an ASHRAE-certified high-performance building design professional, a building energy modeling professional, a certified energy manager, and a committee member of ASHRAE Standards 90.1 and 189.1.