"ON" or "AUTO" – the importance of the fan setting in Charleston, SC

As a moisture control expert in Charleston, one common problem that I run into is the fan setting on the air conditioner’s thermostat. Virtually all modern thermostats have two settings for the fan - Auto and ON. The layperson may think it only natural that you would want the fan set to ON to keep air moving and better distribute the conditioned air throughout the home. But there are some dangers…

For the purposes of this article, let’s break the air conditioner into 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 the thermostat. And you control how the fan functions by choosing either ON or AUTO.

When the fan is set to AUTO, the air conditioner moves air only when the cooling part is running. When the thermostat senses that the house has reached the set temperature, it shuts off both the cooling part and the fan. When the fan is set to ON, the fan runs continuously, even when the cooling part is turns off.

For the Charleston, South Carolina area, it is my opinion that, for the majority of circumstances, the better choice is the AUTO setting.

Air conditioning accomplishes two things – it cools and it dehumidifies. Although dehumidification is only a secondary benefit of the cooling cycle, we obviously want to maximize that feature in our warm, humid environment. Moisture from the air condenses on the cold evaporator coil when the air conditioner runs, and leaves water sitting on the coil and in the drain pan when the cooling cycle ends. By running the fan continuously after the cooling cycle ends and the coil warms up, a lot of that water will re-evaporate into the air stream and be put right back into your home. Therefore, running the fan continuously will reduce the amount of moisture being removed and make the home less comfortable.

The above situation is exacerbated if the air conditioner is oversized. An oversized air conditioner will shut off after a short run time, removing very little moisture from the air but leaving a lot of water sitting on the coil. With the fan running continuously, the house will cool down quickly without dehumidifying well, if at all, and all of that moisture on the coil will go right back into the air making the home less comfortable and more suitable for mold development.

There is also the real potential for bringing in excessive amounts of unwanted air into the home. All ductwork leaks to some degree, and if the ducts are located in an unconditioned space such as a crawl space or attic, that’s the air that will be coming into the home – 24 hours a day, 365 days a year. In Charleston, during the warmer months, that air contains a lot of heat and humidity. In addition to the heat adding to your power bill, the moisture brought in can overwhelm the air conditioner and result in high humidity, discomfort and the onset of mold in the home.

There are other reasons not to leave the fan set to ON, but hopefully the above will at least make you think twice and consult a professional before doing so.