There's nothing worse in the summertime than a dead air conditioner, but what about an air conditioning unit that runs for too long? An AC system that won't shut down wastes energy, makes your home uncomfortably cold, and can even wear out components more quickly. While it might be less common than a system that doesn't run, it's still a severe problem.
If you're huddling under blankets while it's 90 degrees outside, it may be due to one of these three common causes.
1. Bad Thermostat Sensor or Wiring
Your HVAC equipment doesn't have much of a mind of its own. Instead, it waits for your thermostat to call for heating or cooling. Your thermostat, in turn, uses a temperature sensor to compare the ambient air to your setpoint. When the temperature in the room goes above the setpoint, it lets the air conditioner know that it needs to turn on.
A failing thermostat may continue to call for cold air even when it's already freezing in your house. First, check the temperature on the display. If it's showing a high number while you're shivering, the temperature sensor may be dirty or failing. Other possibilities include internal problems for more complicated programmable thermostats or an issue with the Y-wire.
2. Welded Contactor
The contactor is a relay that converts the low-voltage signal calling for cooling into a high-voltage signal to run the compressor. Contactors can fail for numerous reasons, including poor initial connection quality. Weak connections at the contactor can lead to arcing, damaging the contacts, or even melting parts of the contactor housing.
In some cases, this damage can cause the contactor to weld shut. When this occurs, the compressor on the unit will run continuously. If turning your thermostat up or switching it off doesn't stop the air conditioner from running, you may have a problem with your contactor. The contactor is a high-voltage component that can be hazardous, so it's best to contact a professional for further diagnosis.
3. Blower Problems
Another possibility is that the compressor is shutting off, but the fan continues to run. This situation can produce humid air from the vents since your evaporator won't have time to drain moisture while the fan is off. You can check by going to your condenser unit outside and listening to the compressor. If air is coming from your vents and the compressor isn't running, your blower may be the culprit.
Before calling for help, check your thermostat to confirm that you have the fan set to "auto" and not "on." If your thermostat settings are correct, there may be an issue with the wiring from the thermostat or a problem with the switch that signals the blower the shutdown. In either case, it's time to call in an HVAC professional to get AC system repairs.