The answer to 'why is my HVAC system blowing cold air' is that there could be several common causes, such as a problem with the thermostat, a refrigerant leak, or a malfunctioning compressor. Some key symptoms include the system continuously running, the air coming out of the vents feeling too cold, and the system not effectively heating your home.
To troubleshoot, you can first check the thermostat settings to ensure it's set to 'heat' mode and the temperature is set higher than the current room temperature. If that's not the issue, the problem could be a refrigerant leak, which would require a professional HVAC technician to diagnose and repair. In some cases, the compressor or other internal components may have failed, also requiring professional service.