FSEC-PF-306-96

FSEC-PF-306-96

Patrick W. James, Jeffrey K. Sonne, Robin K. Vieira, Danny S. Parker, and Michael T. Anello

Florida Solar Energy Center (FSEC)

FSEC-PF-306-96

Human comfort studies have shown that people prefer higher temperatures when they are subjected to a breeze. Ceiling fans are often used to create air motion in residences. Simulation studies (including one given in this paper) have demonstrated that in Florida, using ceiling fans combined with raising a home’s temperature 2 F will generate about a 14% net savings in annual cooling energy use (subtracting out the ceiling fan energy and accounting for internally released heat). This savings drops to 2.6% with a 1 F increase in set point and to a negative 3.7% savings with only a 0.5 F increase in set point. If the thermostat is not adjusted at all for fan use, cooling energy use may increase by 15%. The sensitivity of the simulation to changes in the convective heat transfer coefficient between indoor air and indoor surfaces from ceiling fan operation is also presented. The sensitivity is inconsequential for insulated houses.

Figure 1. Ceiling fans give off heat and use electricity. Photo of an operating ceiling fan taken with a heat sensitive infrared camera. Color is proportional to temperature. The hot center of the motor housing can clearly be seen (105 F). A typical fan will use 50-150 Watts while operating at high speed.

This paper also presents metered results from an analysis of 400 Florida households that indicate no cooling energy savings due to ceiling fans. Homeowner-reported thermostat settings were the same for homeowners with and without ceiling fans and measured thermostat settings were not statistically different for a sample of homes in which the indoor air temperature was measured.

Human comfort studies have shown that people prefer higher temperatures when they are subjected to air motion. When cooling is desirable, (i.e. under summertime conditions), air movement across the skin can increase comfort and/or be used to reduce energy use if, because of the cooling effect, thermostat set points are also raised. Comfort research based on Fanger’s thermal comfort equation shows that at 60% relative humidity, 90% of people would be just as comfortable if the air dry bulb temperature were raised from 79.2 F to 82.5 F, if air speed was also increased from still air at 20 feet per minute to 150 feet per minute (fan on high). At 80% relative humidity, 90% of people would also be as comfortable if set points were raised from 77.9F to 81.8F while similarly increasing air speeds (based on Fanger 1982; McIntyre 1980).

Ceiling fans are often used to provide desirable air movement; they are very popular options in Florida residences. Because the fans have been shown to effectively increase comfort while potentially decreasing energy use, their installation and use has long been promoted by energy extension services, utilities and even through energy code compliance credit.

Ceiling fans save energy if their use also results in reduced air conditioner use. Fans can extend the natural ventilation season when the air conditioner is not in use and/or allow for higher thermostat set points. Calculations show that for the Florida climate, for every 1F a thermostat is increased, cooling energy use is decreased by almost 10%-15% (Fairey et al. 1986). However, ceiling fans use energy and produce additional sensible heat load during the cooling season. So for fans to be of value as an energy conservation strategy, the energy use reduction benefits provided by the fans must offset the load they incur; the savings from raised thermostat set points must outweigh the additional electrical load from the motors and heating of the air.

FSEC-PF-306-96

An experimental energy conservation study published in 1984 monitored energy use in 16 houses in which, as part of the study, ceiling fans were alternately used and then disabled over one week periods (Lawrence 1984). It was estimated that the test spanned 2/3 of the cooling season. The study showed an average air conditioning energy use savings (including fan energy use) for the weeks the fans were used of 188 kWh/yr or 3.7%. Some of the houses did, however, experience a substantial negative cooling energy use savings; savings ranged from -38.7% to +28.9%. No thermostat set point or other more detailed schedule information was provided in the report.

In summary, comfort studies indicate that wind speed can be traded for temperature and simulation results indicate significant energy savings from small increases in thermostat set points. However, field research is limited and available data show savings are minor or inconsistent. So the rest of this paper attempts to answer two questions- how much energy can ceiling fans potentially save in Florida residences and are the fans being used in a manner that realizes these saving?

Data from a simulation study, a home occupant survey and metered results from over 400 recently-constructed audited homes were used to evaluate energy savings of ceiling fans.

The simulation was conducted to show the relationship of residential cooling energy use to interior thermostat set points and fan use for three Florida cities- Jacksonville, Orlando and Miami. The house used in the simulation was specified to match typical Florida construction; its characteristics are shown in Table 1. The FSEC 3.0 (Florida Software for Environmental Computation 3.0) hourly computer simulation was used for the study (Kerestecioglu et al. 1992). FSEC 3.0 is an energy simulation software package that uses hourly weather data and actual building geometry to describe energy uses and loads. It can use either finite-element or transfer function methods to determine the building energy balance. For this model, an energy balance was performed using actual ceiling fan and cooling system data.

Table 1. Simulation House Description


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