Am I really saving energy by running just the AC fan?

I live in D.C. I find I can keep my house quite cool on most days by simply closing up the house by mid-morning and running the fan function of my central air, without using AC at all. But the fan is then running all day.

Is this still more energy efficient than using the AC, and having the fan/AC kick on and off as needed?

Question received via e-mail to
Green Energy Efficient Homes

Answer from Green Energy Efficient Homes

You will definitely use less energy by running the AC fan all day without using any cooling, rather than using the AC intermittently through the day.

Let’s look at the central AC energy use for a 3 ton air conditioner with an SEER rating of 14.5, which is the new minimum required standard for an ENERGY STAR qualified central air conditioner as of January 2009.

Assuming the air conditioner runs 8 hours a day, 125 days a year, its thermal output is 30 million BTUs. At SEER 14.5, that means it will consume 2069 kilowatt hours per year. At $0.10 per kwh that would cost you about $207 in energy costs.

The inside blower on a 3 ton air conditioner is typically 1/2 horsepower. Multiply horsepower by 746 to get watts, so your fan consumes 373 watts on full-power mode.

If you run a 746-watt fan 24 hours a day during that same 125 days, your usage is:

746 watts x 24 hours x 125 days / 1000 watts per kilowatt = 1,119 kilowatt hours, at $0.10 per kwh makes $111.90.

I’m not sure if your fan is running at full power when it’s only in fan mode; you may actually be able to set fan only mode to a lower speed, and therefore lower energy consumption, from the AC unit or furnace unit (depending on whether the fan is part of the furnace or part of a separate AC). On my furnace I can choose several different speeds for the indoor blower fan. Also my fan is a DC fan so it is more energy efficient.

Of course, if your current air conditioner is not a recent ENERGY STAR model, the fan will probably not use any more electricity than stated above, but the air conditioning unit will probably use 10-40% more than my AC calculations suggest. If your AC is really old, like mine, and doesn’t cool very well, you could be using double the energy of a new ENERGY STAR model. (I haven’t upgraded my air conditioner because, like you, we have figured out ways to live without air conditioning in Toronto, and almost never run it – see my Summer energy saving tips for more ideas on that). Also look over my entire Energy efficient cooling section for ideas on saving energy on your cooling bill.

Congratulations on living without AC in DC – I know DC can get pretty muggy in the summer. And yes, you are definitely saving energy by just using the indoor fan continuously rather than the AC intermittently.

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