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Can Running the HVAC Fan Continuously Save on Energy Costs? Part 1 of 3

The following question was submitted to our website recently and we thought that perhaps others might be interested in the answer:

"Hello, I have a question, how much more electric do you use with the fan On vs Auto? I leave my fan On to keep the air circulating. I've heard everything from the AC pulls in outside air, which I know it doesn't, unless a door is open... to it costs just as much to run the fan On as it does when the AC is also running. I thought the fan was a low power unit, like 12volt."

Great question! To answer it, I am going to assume we are talking about a central air system with the blower motor (fan) located inside the home. These are usually closed loop systems that circulate conditioned air (hot or cold) throughout the home using wall, ceiling, or floor vents. There are larger vents, usually one per floor, that collect air and return it to the fan to be re-conditioned and then distributed througout the home again. There is a filter in the loop, usually located in the air handler where the fan is located. The filter cleans the air before it is re-distributed.

So the fan moves the air throughout the home with the help of the duct system. There are two types of fan. One runs on AC voltage and the other runs on DC voltage. You are more likely to come across the AC type, so let's use that as the example. A typical AC fan motor uses about 500 watts when it's running. We are not going to worry about efficiencies or power factors for this simple example, although they do impact the numbers slightly. So, if you run the fan continuously for a month (there are 730 hours in a month), you would use 500 watts x 730 hours = 365,000 watt hours. This is usually converted to kilowatt hours by dividing by 1000. So the fan uses 365 kWh every month when used continuously.

What's the cost? Well, the average cost of electricity in the US is 12 cents/ kWh. So it would cost about $44/mo to run the fan continuously.

In the Auto mode the fan only comes on when the heating or AC comes on. The cost of running the fan intermittently will be less, but depends on the on/off ratio. If we assume 12 hours on and 12 hours off each day, the cost is reduced by 50%, or about $22/mo.

If you do decide to run the fan continuously, there are a few important things you'll need to consider first. We'll discuss those in part 2.