One of the pieces of advice you often see regarding central air systems is that the fan needs to be on the AUTO setting. This setting links the fan to the thermostat; when the air conditioner compressor turns on, the fan will turn on, and when the compressor turns off, the fan will turn off.
For some people, that advice may seem meaningless. If it's hot, they want air circulating to combat the summer heat. But constantly running the fan that's part of the central system can make the interior of a home feel worse because the humidity goes up.
At first, you might think this is because the fan is now pushing hot, humid air into each room. But if the air conditioner cooled the rest of the house properly, that air wouldn't become very hot and humid so quickly. The answer lies in the fact that the air pushed around by the fan still runs through the entire air conditioning system — but without the benefit of a cold condenser coil.
The Fan Can Add Moisture Back Into the Air
When the air conditioner itself is on, the air passes over a cold coil that allows moisture to condense out of the air. The cooled air then leaves the coil area and goes back into the house without that moisture. This is why air conditioning dries out humid air.
However, when the air conditioner is not on, that coil warms up. If the fan is on, the air will still run past the coil, only now the coil is warmer and causing moisture to re-evaporate back into the air. That newly moistened air is pushed through the system by the fan and pushed back into the house. Remember, the fan is the part of the HVAC system that actually moves the air. Turning off the cooling part of the system won't stop air movement through the system; it will just prevent that air from cooling.
So, if you leave the fan on all the time, it's just going to push air past the warmed-up coil and bring humidity back into the house. If you want a fan on when the air conditioner itself isn't cooling, use a ceiling fan or table fan. Those don't push air anywhere near that warmed-up coil, so they do not make the house more humid.
If you've noticed your home becoming excessively humid even when the fan is on AUTO, or if you notice other problems with your heating or cooling system, contact an air conditioning and heating repair service. An inspection can reveal the cause, and the company's technicians can repair the system before the problem grows out of control.