An air conditioner is often a workhorse in the home, and many people feel that they're unable to live without one. So, when your air conditioner isn't cooling, getting it to work again is usually a priority. There are many possible reasons for this, including poor airflow because you haven't performed required maintenance or a refrigerant leak, but sometimes, the problem isn't with the air conditioning system itself but with the fan. It may be running even though the air conditioner isn't.
The fan is usually programmed to keep running for a minute or two after the compressor shuts off to make sure all the cool air in the system is circulated, but it shouldn't keep running for more than about five minutes. If it does, there could be a problem with the thermostat, the fan limit switch, or a relay.
Video of the Day
Check Your Thermostat
If your air conditioner fan won't shut off, your first task should be to check the thermostat. It could be that you've accidentally knocked it during home cleaning and set the temperature too low or switched it from "auto" to "on." If it's the former, your system is constantly trying to achieve an impossibly low temperature in your home, and since it never gets low enough, the fan and the entire system will continue to run indefinitely. If it's the latter, the fan will run until you reset the switch to "off" or "auto."
Reset your thermostat to a few degrees above room temperature and put it on "auto." You should notice that the fan switches off fairly quickly. If the fan still won't shut off, your thermostat could be broken, or the wiring may be faulty. This is usually a job for an HVAC pro to diagnose or repair.
Limit Switch and Relay
Many central A/C units have a limit switch to control the fan. Its purpose is to start the fan before the compressor switches on and keep it running after the compressor switches off. This part is typically located in the air handler, and it often has two settings: "manual" and "auto." It may have accidentally been switched to the "manual" position, and if so, switching it back to "auto" should stop the fan. Sometimes, this switch just malfunctions, though, and although it's an inexpensive part, it takes a pro to determine if it's actually bad and to replace it if it is.
The wiring from the thermostat to the fan passes through a relay located in the air handler. This is another type of switch, and it's supposed to open and close in response to signals from the thermostat. If it gets stuck in the closed position, the fan won't switch off, and the relay has to be replaced.
The Entire System Runs Constantly
It may not be just the blower that's faulty; the entire system could be running and supplying warm air. This could be because you haven't been keeping up to date on maintenance tasks, like replacing the air filter and keeping the outdoor housing clear, and in extreme cases, the evaporator coils in the indoor unit may ice over, compounding the problem. If de-icing is required, switch off the system using the circuit breaker if necessary and let the ice melt before switching the system back on. Then, perform those maintenance tasks you've been neglecting.
A more serious problem is that the system is undersized, and if the outdoor temperature is hot, it may not be able to reach the target temperature. Unless you're willing to upgrade your air conditioner, you may have to turn off the system and wait out the heat wave.