One common mistake we see from homeowners is setting the fan to the ON mode. Most thermostats have two settings for the fan – AUTO and ON – and some technicians even encourage their customers to put the fan in the ON position. The suggested benefits would be improved circulation and better filtration. We strongly recommend against using this feature and find it helpful to offer a thorough explanation of how the A/C and its fan work in order to discourage using the A/C fan this way.
Your air conditioning system is composed of two main parts: the outside unit, which pumps refrigerant, or coolant, through the system and the indoor unit which moves air across the cooling elements and into the home. During a normal A/C cycle, the system starts cooling and the fan starts circulating air at the same time. The fan circulates cooled air throughout the home until the temperature is lowered, then the thermostat tells the A/C to turn off and the fan should turn off, as well. This is exactly how the system works when the A/C fan is set to AUTO. But when the fan is in the ON mode, the fan keeps running continuously, even when the outdoor unit is off.
Here in Jacksonville and the greater Northeast Florida area, we rely on our A/C for doing two jobs: cooling and dehumidifying, and the latter is equally important. While the A/C is running it is acting as a dehumidifier, as moisture from the air is collecting on the cold A/C coil. This works just like a cold drink on a warm day… moisture collects on cold surfaces. The water collected drips off the cooling coil into a drain pan then runs to the outside of the home through the condensate drain. But it takes awhile for the moisture to make its way down the fins of the coil, into the pan, and down the drain. When the thermostat shuts the A/C off, there is still a lot of water on the coil and in the drain pan. How your fan is set to operate will make all the difference in how well your A/C system deals with this moisture. The water drains best after the indoor fan has stopped and gravity can do its job, allowing the water to drip down and away. If the indoor fan keeps running, it is difficult for that water on the coil to drip down into the pan because the air from the fan is pulling in the opposite direction.
In addition to the airflow preventing the water from draining, we have the temperature of the cooling coil rising because the outdoor unit is no longer pumping refrigerant. Very quickly, the temperature of the coil rises above dew point, rather than below, so moisture in your home’s air is no longer collecting on the coil. In fact, that moisture is now getting evaporated back into the air being circulated by the fan and reintroduced into the home. In essence, an indoor A/C fan becomes a humidifier as soon as the outdoor unit shuts off.
There are other reasons not to leave the fan in the on mode. It costs more electrically because you’re running the fan motor constantly. And it’s a less efficient fan than, for example, a ceiling fan which doesn’t have to push through filters, coils, ducts, and diffusers. Another reason to use the AUTO mode is that if you have leaky ductwork it will increase air leakage of the house, adding heat & humidity, which makes the home less comfortable.