What common issue can cause an HVAC system to blow hot air?

A malfunctioning thermostat is a common issue that can cause an HVAC system to blow hot air. The thermostat is the control center for the heating and cooling system, dictating when the HVAC unit should operate. If the thermostat is malfunctioning—whether due to an incorrect setting, a faulty sensor, or wiring issues—it may fail to communicate the desired temperature to the HVAC system. As a result, it can incorrectly signal the system to heat the space instead of cooling it, leading to the unexpected blowing of hot air from the vents.

In other options, while a dirty air filter can restrict airflow and cause heating issues, it typically does not directly cause the system to blow hot air unless it affects the heating cycle. A refrigerant leak is related to cooling issues, resulting in reduced cooling efficiency, and improper duct insulation generally concerns energy loss rather than temperature control. Thus, a malfunctioning thermostat is the most direct cause of hot air blowing from the system.