What is a common issue that can cause an HVAC unit to freeze up?

Low refrigerant levels are a common issue that can lead to an HVAC unit freezing up. Refrigerant is the substance that circulates through the HVAC system, absorbing heat from the indoor air and allowing the unit to provide cooling. When there are low refrigerant levels due to leaks or insufficient charging, the system cannot effectively absorb heat. This insufficient heat transfer leads to the evaporator coil dropping in temperature significantly, causing moisture in the air to freeze on the coil. As frost builds up, it can obstruct airflow and further reduce the system's efficiency, resulting in a complete freeze-up.

While issues like high airflow, dirty filters, and improper insulation can also impact system performance, they generally tend to influence airflow and efficiency rather than directly cause freezing by altering the refrigerant cycle. For instance, dirty filters can reduce airflow, which may lead to freezing, but the fundamental cause of freezing in this context is often traced back to refrigerant levels.