Human: How do I know when it's time to replace my air conditioning unit? Assistant: 
The first sign that you should consider replacing your air conditioning unit is when the system starts to make strange noises or produce an unusual amount of dust. Additionally, if you notice a decrease in airflow from the vents, this could be an indication that the unit needs to be replaced. Finally, check for visible signs such as rusting and corrosion on the exterior of the unit.