Superior, Wis., is a city on the front lines when it comes to winter. Situated on Lake Superior and between two rivers, it’s buffeted by bitter-cold winds off the water that routinely send the wind-chill plummeting below 0°F.
Amsoil Inc., makers of motor oils and lubricants, needed a cost-effective way to keep employees comfortable while reducing energy use in its 450,000 sq.-ft. distribution center. Because warm air rises, an automated heating system led to temperatures at the ceiling up to 30°F warmer than those at the floor, and standard ceiling fans couldn’t circulate the heat. Needless to say, workers weren’t too happy spending the day in their overcoats.
The company’s predicament was hardly unusual. Keeping large buildings comfortable in winter while keeping heating bills within budget is an annual headache for managers. Heating accounts for nearly 50% of energy use in warehouses and almost 40% of energy use in all non-residential buildings, according to data from the U.S. Energy Information Administration. Unfortunately, much of that energy is wasted because of what’s known technically as thermal stratification.
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Anyone who works in a climate that requires heat to supplement the thermal power of the sun knows the effects of this phenomenon, even if they don’t recognize the term. And the higher the ceiling and poorer the insulation, the more noticeable the problem.
Thermal stratification is the product of a basic rule of physics: hot air rises. Air from a forced-air heating system is generally 5% to 10% lighter than the air already in a space. As the hotter air rises, it forms temperature layers, or strata. The hottest layer is on top, at the ceiling; the coldest is at floor level. Therein lies the problem.
Most thermostats are located 4 to 5 feet off the floor, so they register the temperature at that height. Because the hottest air always heads toward the ceiling and because doors of any warehouse or DC are always opening and closing, the thermostat is constantly sending the message to the heater that it needs more heat. Naturally, the more the heater runs, the higher the heating bills. And the taller the ceiling, the bigger the bill. In extreme cases, such as a large airplane hangar with 100-foot ceilings, temperatures can get so high up top that they cause electrical equipment to malfunction. That was the case at the American Airlines maintenance hangar in Charlotte, N.C. The stratified air resulted in tens of thousands of dollars in monthly heating costs.
Stratification not only strains budgets; it also contributes to occupants’ discomfort because the majority of employees, like those at Amsoil, work at or near ground-floor level, where the colder air stays. In the airplane hangar, mechanics on the ground worked in bulky jackets while the equipment sizzled at the ceiling.
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. Clearly, the first option is unacceptable.
It’s been shown that one of the most efficient ways to combat stratification is the installation of a simple, cost-effective technology: high-volume, low-speed (HVLS) ceiling fans. High-quality HVLS fans are able to destratify, or undo the natural temperature layering that occurs when heat is pumped in, by recirculating the air so that a nearly uniform temperature is created throughout a space, from floor to ceiling.
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 create a slow-moving jet of air that reaches all the way to the floor. 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% of their maximum rotations per minute (RPM) in the forward direction—not reversed. Running in the forward direction will move a large volume of air without creating a draft (measured as air velocity of ~30 fpm or less at occupant level). Contrary to conventional wisdom, it’s been shown that reversing a fan at higher speeds requires more energy and also increases the rate of heat loss through the roof.
Destratifying a space can significantly lower heating costs because of the simple fact that the warmer air is at thermostat level, meaning the furnace no longer has to work as hard to maintain the set temperature. Facilities of all sizes—from department store stockrooms to large warehouses to airplane hangars—have reported savings of up to 30% through the use of HVLS fans.

American Airlines installed four 24-foot fans at its maintenance hangar in Charlotte, N.C.

At the American Airlines hangar in Charlotte, four 24-ft (7.3-m) fans created a uniform temperature, allowing managers to lower the thermostat setpoint by 5°F (2.8°C) without affecting worker comfort. In fact, mechanics who had relied on bulky coats to stay warm in cold weather took them off as soon as the fans started running.
And at Amsoil in Wisconsin, the installation of six 24-ft (7.3-m) and two 14-ft (4.3-m) diameter HVLS fans gently recirculated heat down to employees’ level and created even temperatures throughout the 450,000 sq. ft. space. The temperature difference between the ceiling and floor dropped to just one degree, decreasing the company’s gas consumption by 35%. Employees felt the difference too—overall productivity increased because employees no longer devoted as much of their day to shivering.