Is our central air HVAC unit system more efficient with the fan in "on" or "auto"?

According to inspectapedia.com/aircond/AirFilters6.htm some systems are not rated for continuous fan operation. If such a system is set to "on" or continuous blower operations it is likely to cause premature motor failure. The advantages of continuous operation are (a) better air filtration (assuming your system has a very good filter system) and (b) more uniform air temperature in the home.

They also state: - quote - A downside of continuous blower fan operation is higher electrical costs. On the other hand, if we needed to reduce building dust and allergen levels and/or increase comfort levels, this approach might avoid wasting money on other attempts at air cleaning that are expensive and less effective. More careful study is needed to really evaluate the claim that continuous blower fan operation uses more overall energy.In some cases there may be a net energy savings, depending on how hard the heating or cooling system has to work to satisfy the thermostat (and occupants) during "on" cycles.

- end quote - The bottom line is that if your system is designed for continuous operation, has great filtration, and your setting is far from the ambient temperature outdoor (i.e. Very cold setting in summer or very hot setting in winter) you will probably do better to leave it "on." If your system is not designed for continuous operation, you tend to set the thermostat at reasonably low setting in the winter and high settings in the summer, you will likely save money on both electrical usage and motor replacement by setting it to "auto.

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