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. ac unit miamiSince your AC is responsible for doing two jobs, cooling and dehumidifying, moisture from the air condenses on the cold evaporator coil when it runs. hvac unitAn oversized AC will shut off after a short run time and leave a lot of water sitting on the coil. window air conditioning units cost

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. If it's just too hot out, go find yourself a nice waterfall to cool off in, like I did this weekend. 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. You 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. This Thermostat Setting Can Cost You Money and Make You Sick How NOT to Use Your Heat Pump Thermostat

Do Programmable Thermostats Save Energy? NOTE: Comments are moderated. Your comment will not appear below until approved. heating & cooling distribution,How do you use the fan setting on an air conditioner? Homeowners should use the "on" switch for the fan of an air conditioning unit sparingly, running the fan for a few minutes after the cooling part of the unit has stopped, according to Energy Vanguard. Using the "auto" option for the fan is a better choice. How do you turn the fan of a Lennox air conditioner on high? How do you remove a frozen air conditioner filter? How do you diagnose air-conditioner problems? Most air conditioner units have buttons labeled "fan" or "auto." Some air conditioner technicians recommend putting the air conditioner's fan setting permanently to "on" to distribute cool air around the home evenly, but this strategy can backfire and add more humidity to the home, according to Energy Vanguard. Instead, homeowners should choose the "auto" option, or if homeowners choose to run the fan, they should do it for just a few minutes after the cooling part of the unit shuts off.

Air conditioning units have two main components, one that cools the air, and another that distributes the cooled air around the house. As the air is cooled, it also has moisture stripped from the air that condenses on the cold evaporator coil. An air conditioning unit that shuts off the cooling part after a short run may have water sitting on the coil, and when the fan is run after the cooling part shuts off, the water on the coil may evaporate and go right back into the air, claims Energy Vanguard. When this water goes back into the air, it raises the humidity level and can make a home feel uncomfortable even after it has been cooled off by several degrees. Learn more about Heating & Cooling Do all attic fans have a humidity setting? Not all attic fans are outfitted with a humidity setting, which requires that the ventilation fan unit have an internal humidistat. Homeowners who have a s... What does a thermostat switch do? A thermostat switch is used to control the mode of a unit, which is the heating or cooling element, and the circulation fan on applicable units.