Getting down with heat

Air destratification can reduce heating costs in winter


Regardless of a building’s design, high or low ceilings, poor insulation, windows or not, comfort is certainly achievable. In the winter months many industrial facilities simply succumb to the cold, negatively effecting worker productivity and in turn costing a fortune for those with heating capacity.

The process of air destratification is not new, but the method of achieving a properly heated space has been simplified with the help of large diameter, low speed fan. These 6- to 24-foot diameter fans help regulate temperatures and curb energy consumption without incurring a draft.

Stratification occurs because hot air is less dense than cold air.  The air coming out of a forced air heater, or in the case of some manufacturing facilities, the heat produced from the equipment itself, is approximately 5% to 7% lighter than the air in the space and thus tends to rise to the ceiling. This results in a significant temperature difference in the space anywhere from approximately 10 to 30 F.

Warming in Wisconsin

Located in Superior, WI, the employees at the 45,000 sq. ft. Amsoil synthetic lubricant plant are well versed in cold working conditionings. Winters average 10-15 F within their manufacturing plant. Inside temperatures at floor level remained at 45-50 F while the temperature at the roof deck averaged 75-80 F. According to plant manager Jack Swonger, they needed a way to destratify the air in the space – bringing the warm air down to the occupant level.

After installing eight HVLS fans,  including 14- and 24-foot fans from the 30-ft high ceiling, the ceiling to floor temperature differential was brought down to less than 1 F.

According to Swonger, after installing the fans, which were manufactured by Big Ass Fans, typical winter thermostat readings now average 62-64 F. The fans “knocked our gas consumption for heating down about 35% right from the get-go,” said Swonger. He said the plant also saw a regulation in plant temperatures.

With ambient temperatures more consistent, his employees are more comfortable, Swonger said, and “the heaters hardly ever kick on anymore.”



A destratified space has only a slight temperature difference from floor to roof deck.  The extra fuel required to heat the air at the ceiling to an elevated temperature results in wasted energy as the increased ceiling temperature results in increased heat loss through the roof.  Savings accrue by slowly circulating this trapped heat down to the occupant/thermostat level before it’s able to escape from the building.

Even though the thermostat setpoint remains the same, the heating system does not have to work as hard to maintain the given setpoint. The energy savings achieved from reducing the amount of heat escaping through the roof is similar to turning the thermostat down three to five degrees.

It is becoming increasingly more common to use large diameter, low speed ceiling fans to destratify a space. 

The ability of any fan to effectively destratify the air in a space is based on three primary factors – the jet of air produced, the volume of air moved and the lack of draft created by the fan.

The fan must be able to create a slow moving jet of air that impacts the floor.  If the fan’s airflow cannot reach the floor, then it cannot mix the entire volume of the space resulting in air that remains stratified.  This is one of the primary reasons why small fans often cannot effectively destratify tall spaces. 

It is important to note that destratification is one of the few applications where the volume of air moved by the fan is critical; the fan must turn all the air in the space over at least once per hour.  If the airflow volume is not sufficient, the air will remain stratified.  An appropriately sized large diameter, low speed fan achieves this without creating a draft.

The fan must be able to create a jet that reaches the floor while moving a large enough volume of air without creating a draft (air velocity of ~40 fpm or less at occupant level).  This is why fans are typically operated at lower speeds during the winter.  Ideally the fan should generate uniform air flow to minimize fluctuations in air velocity.  The more blades the fan has the more consistent the airflow.


Auto Destratification

The ability to regulate energy consumption can be taken one step further with an intuitive automatic destratification system effectively measuring temperature variances between the occupant level and roof deck. Serving as a convenience to facilities managers, the remote temperature sensors are set to predetermined temperatures at the beginning of the season.

Doing this automatically adjusts the fan speed depending on the temperature differential both while the building is occupied and at night when vacant. Whereas time is money, facilities managers can set parameters on the system once during each season, no longer having to adjust speeds routinely during the day to accommodate employees.

For maximum performance in destratification applications, the fan should utilize airfoils designed to operate efficiently at low speeds.  In addition, the more uniform the airflow delivered by the fan, the faster the fan can spin without causing a draft. 

To destratify a space with the minimum amount of energy and the minimum number of fans, select a 10-bladed fan with an airfoil designed to be extremely efficient even at lower operating speeds. An important point to note is that most large diameter, low speed fans destratify the air using a different method than standard ceiling fans. 

If a fan is designed to operate efficiently even at very low velocities, slowing the fan will provide enough air movement to circulate the air in the space and reduce the stratification without reversing the fan or causing drafts.

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