Tall ceilings can be both a builder’s dream and an HVAC contractor’s nightmare. The nature of residential great rooms and lofts allows for an extreme variance in temperature - an unwelcome feature in terms of both occupant comfort and energy consumption. Heat stratification in the winter months, and stale air in warmer months, make a big difference in the usability of a space.

The introduction of air movement by means of large-diameter, low-speed fans can provide the necessary comfort, further optimize HVAC system effectiveness, and save money on both energy use and building supplies. And it’s not just for indoor environments; the comfort of outdoor spaces can be enhanced as well.

OPERATION

Year round, high ceilings can be problematic for homeowners in terms of comfort. Similar to traditional ceiling fans, large-diameter, low-speed fans (like the Isis®) increase air velocity, creating a more comfortable environment for occupants. This 8- to 10-foot-diameter fan uses little energy to move a goodly amount of air.

“Many homeowners are frustrated by the performance of their small ceiling fans,” said Casey Becker, residential market specialist for the company. “Running at full speed, they can only feel the air if standing directly underneath a 48- or 52-inch fan. With tall, 15- to 25-foot ceilings, the airflow is almost nonexistent.”

The effectiveness of a large-diameter fan lies in its ability to move large volumes of air slowly and gently, without the localized airflow of high-velocity fans. Used on their own, large-diameter fans can effectively circulate stagnant air. When paired with an appropriately installed HVAC system, they help distribute the conditioned air evenly.

In the company’s current residential line, a 10-foot clearance is required from floor to fan, which means that at least a 12-foot ceiling is desired.

SUMMERTIME

The relationship between large-diameter, low-speed fans and HVAC systems is a marriage of energy efficiency and comfort, especially in large residential environments. In warmer months, large fans can help improve personal comfort with an evaporative cooling effect; although the fans do not lower the air temperature in a space, the perceived effect can make a person feel 10°F cooler. As a result, homeowners can raise their thermostat settings without sacrificing comfort, further reducing cooling costs by as much as 15 percent annually.

In Winchester, Tenn., Clay Taylor’s cathedral ceilings rise 24 feet from the floor. The premise behind the overall design was to take advantage of the natural ventilation provided by numerous windows, allowing the Taylors to operate their HVAC system more sparingly.

Given the layout of the 1,000-square-foot main living space (which includes the kitchen plus living and dining rooms), they had originally specified four small, high-speed fans. In lieu of this design, one 8-foot-diameter Isis fan was installed at the peak, providing the needed airflow.

“If it stays below 80°, we don’t use air conditioning,” despite the lack of shading around the house, said Clay. “I try to keep the a/c at least 10° below the expected exterior temperature. When it gets into the 90s, I will set it to about 80°. The fan definitely makes a difference when it’s on.”

WINTER DESTRATIFICATION

Along with summer comfort, large-diameter fans are capable of destratifying the air in a space in the winter, reducing heat-energy consumption by as much as 30 percent. Because hot air naturally rises as cold air falls, the fans mix these two extremes, creating a more uniform temperature.

Contrary to the typical method of reversing the direction of a high-speed fan, large-diameter fans are slowed to 10-30 percent of their maximum rotations per minute, redirecting warm air from the ceiling to the occupant level. Smaller fans are run in reverse direction at very high speeds to pull air across the ceiling and down the walls, which results in a draft. Slowing down a large-diameter fan directs the warm air to where it is needed.

Given heat’s natural tendency to rise, lofts can become unbearably hot as the heating system works hard to maintain the given temperature at the occupant/thermostat level. The fuel required to heat the air in a large home results in wasted energy, because much of that heat collects at the ceiling level. Savings accrue by slowly circulating this trapped heat down to the occupants.

Even though the thermostat’s set point remains the same, the heating system does not need to work as hard to maintain the given set point.

Large-diameter, low-speed fans with gearless, direct-drive motors do not create audible mechanical noise. As a result, the large-diameter fans offer a noise level of less than 35 dBa at maximum fan speed, making them more suitable for residential environments.

OUTDOOR LIVING

This quieter operation carries over to the outdoors as well. In very hot climates, where a little air movement can make a huge difference in comfort levels, the breeze created by these fans complements the attractiveness of outdoor living spaces.

Mickey Redwine designed an elaborate boathouse and cabana on his property in Lago Vista, Texas. He originally installed a dozen high-speed regular and misting fans to keep his guests comfortable. Intertwining outdoor dining and sitting areas, Redwine eventually changed out all of the original fans and replaced them with three anodized aluminum fans, wet rated for outdoor use.

“There is a huge difference between having the three fans, which is quieter, and puts out probably 10 times the air volume of the others combined,” he said. “You can’t even hear them run. If it’s making any noise, it’s not noticeable.”

In any given season, in any given environment, large-diameter, low-speed fans can provide the homeowner more versatility than HVAC alone. Whether it’s a renovation project or new construction, it can pay for contractors to research these types of options.

Publication date: 03/21/2011