The answer to 'HVAC system blowing cold air' is that there are several common causes, including a malfunctioning thermostat, a refrigerant leak, or a problem with the compressor or blower fan. Symptoms may include the system running continuously, the air vents blowing cold air even when the system is set to heat, or the system not reaching the desired temperature.
Some potential solutions include checking the thermostat settings, inspecting the air filters, and ensuring the outdoor unit is clear of debris. However, more complex issues like refrigerant leaks or compressor problems should be addressed by a professional HVAC technician. It's generally best to call for professional service if the problem persists or you're unsure of the cause.