It's been hot here in the Atlanta area lately. Just three weeks ago we were wearing jackets with low temperatures in the thirties Fahrenheit, and now we're getting up above our summer design temperature every day. That means that a lot of air conditioners are running as everyone tries to keep cool. One common question I get from homeowners is: Should I put the fan in the on position? Most thermostats have two settings for the fan - auto and on - and some well-meaning HVAC techs tell their customers to put the fan in the on the position to distribute the cool air in the house better. Let me back up a bit here. For purposes of this discussion, your air conditioner has two components: the part that cools and the part that moves the air. You adjust the part the cools by changing the temperature setting on your thermostat. You can affect the air movement by setting the fan to auto or on. When the fan is in the auto mode, the air conditioner moves air only when the cooling part is running.

When the thermostat senses that the house has reached the setpoint, it shuts off both the cooling and the fan. When the fan's in the on mode, the fan keeps running continuously, even when the cooling part is turned off. One of those settings is better than the other. The photo below shows how my thermostat is set, and that's how I recommend you set yours, too. The number one reason you don't want the fan to run continuously is that your air conditioner is probably oversized. Since your AC is responsible for doing two jobs, cooling and dehumidifying, moisture from the air condenses on the cold evaporator coil when it runs. An oversized AC will shut off after a short run time and leave a lot of water sitting on the coil. By running the fan after the coil warms up, a lot of that water will evaporate and be put right back into your home. So, putting the fan in the on position compounds the error of having an oversized cooling system. Your house will cool down quickly without dehumidifying well.

Running the fan continuously makes the dehumidification worse and your home less comfortable. Update (9/8/14): See my actual data on how setting the fan to the on position raises the humidity. Even in a dry climate where dehumidification doesn't matter because there's little moisture in the air, there are other reasons not to leave the fan in the on mode. First, it costs more because you've got that fan using about 300 watts while it's running, and you're probably not getting much benefit from it. Good HVAC design in new homes will obviate the need for this if you're running the fan because of air distribution problems. [See addendum below regarding dry climate fan operation.] Another reason to use the auto mode is that if you have leaky ducts outside the building envelope, it can add to the air leakage of the house. Depending on the type of duct leakage, you may end up with higher infiltration (air leaking in) or higher exfiltration (air leaking out). In either case, you add to your cooling load and make the home less comfortable.

Here's another really good recommendation, too. ac and heater unitsIf it's just too hot out, go find yourself a nice waterfall to cool off in, like I did this weekend.external ac to dc power supply Addendum (6/2/13): If you're in a dry climate, running the air conditioner for a few minutes after the compressor shuts off can help cool the home more efficiently. in wall ac heating unitsYou don't want to run the fan continuously 24/7, though, because that will use more energy than is necessary for the extra cooling and have the duct leakage problems mentioned above if ducts are outside the enclosure. The way to do this would be to install John Proctor's Western Cooling Control, which Martin Holladay wrote about at Green Building Advisor. Do Programmable Thermostats Save Energy?

People love their fans. They love to set them on timers, to schedule them to come on in the afternoons, to circulate air to keep the house from getting stuffy. We felt the full force of that love last week when we released Advanced Fan Control, a feature that can do all of the above. Customers have already started using it. Many used to set Nest’s fan to Always On to bring cool air upstairs in summer or just keep air fresh, then they’d go to bed or leave the house and forget to turn it off. Now homeowners can schedule the fan to run from 6-11 p.m. or 8-10 a.m. and forget about it. It’s almost like having an air conditioner. Advanced Fan Control is proving to be one of our most popular new features—the love for fans is ubiquitous. But so is the misinformation. With all this talk about fans, we’re hearing from more and more customers who’ve been told it’s energy efficient to keep the fan on all the time, 24 hours a day. Whenever the topic is brought up, it’s a debate. Leaving the fan on seems to make sense, after all, especially in summer.

If the fan can keep things from getting stuffy, you’ll use your energy-hog of an air conditioner less, and less AC will always save energy. Turns out, it could cost you hundreds of dollars a year. While using the fan instead of the air conditioner really is energy efficient, leaving it on all the time usually isn’t. The average air conditioner has a 500 watt fan, which can use about as much energy over the course of a year as a refrigerator. The costs of running it 24/7 quickly add up. Newer systems have fans that use less energy, but keeping them on all day can cause other problems. In the summer, the ducts that run through your attic or close to your walls can heat up, filling with warm air. Without the AC to cool it down, the fan just blows hot air into your house. You end up running the AC more, not less, to counteract all that extra heat. Hot ducts can be a problem even if you don’t have the fan running all the time—you may be cool and comfortable in your bedroom, then when you turn on the fan on for the night, you fill the room with hot, stale air.

The fan can giveth and taketh away. That’s why Advanced Fan Control gives you a few different ways to control the fan, so you can find what works for your home. For those who want to circulate the air, we added an option that lets you run the fan for 15, 30 or 45 minutes every hour. You get the comfort benefits of a fan blowing all the time, but significantly reduce the fan’s actual runtime. For Nesters who want the fan on for a particular period of time - while they’re asleep, for example, or just in the afternoon when everyone comes home—we’ve created a simple scheduler and a timer. You can turn on the fan exactly when it’s most efficient—when it replaces air conditioning—and it’ll automatically turn off when you tell it to. The fan deserves all the love it gets—it can make a huge difference in your comfort and your energy bill. But as with all things wonderful and beloved—chocolate, Christmas, chocolate—you can get too much of a good thing. This post is part of our HVAC 101 series about how to make the most of your heating and cooling system.