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28 Aug 2009

Can Running the HVAC Fan Continuously Save Energy? (Part 1 of 3)

Question

“What’s more energy efficient – ON or AUTO?

I leave my fan on to keep air circulating but I’ve heard lots of different answers, including ‘The AC pulls in air from the outside,’ but I don’t think that happens unless a door is open. The other answer I’ve heard is that ‘It costs just as much to run the fan on as it does when the AC is also running,’ but I thought fans require low power so that doesn’t seem to make sense to me, either.

Answer

Let’s start by making sure we’re on the same page with regards to definitions:

  • AUTO mode = Fan is on only when heat or AC is triggered by the thermostat setting
  • ON mode = Fan is on all the time

Let’s also assume we’re talking about a central air system with a blower motor (fan) located inside the home. These systems are usually “closed loop systems,” which circulate conditioned air (heat or AC) throughout the home via ductwork and vents in the floor, walls and ceilings. There typically also are larger vents, usually one per floor, which collect and return air back to the fan to be reconditioned and distributed throughout the home. The system usually includes a filter in the loop, typically in the air handler where the fan is located; the filter cleans air before it is recycled.

There are two types of fans – AC and DC voltage. AC fans are more prevalent, so let’s use that as our example; let’s also ignore efficiency and power factors because they’re immaterial.

Warning, math required!

A typical AC fan motor uses about 500 watts when it’s running. Therefore, if you run the fan continuously for a 30-day month (720 hours), you would use 360,000 watt hours (720 x 500) or 360 kilowatt hours (kWh). Therefore, the fan uses about 360 kWh per month in the ON mode. The average kWh in the US costs about $0.12 so it costs about $43 per month, or about $520 per year, to run a fan continuously.

At this point, it should be pretty obvious that AUTO costs less than ON but let’s math it out to its logical conclusion.  If we assume an average of 10 hours on and 14 hours off per day, energy costs are reduced by 60% to about $18 per month or $215 per year.

Bottom line – If you run the fan in AUTO mode, you’ll save about $300 per year.

If you do want to run the fan continuously, there are a few important things you’ll need to consider…

…see Part Deux of Three!