The answer to 'why is my furnace blowing cold air' is that there could be several common causes, such as a problem with the thermostat, a clogged air filter, or issues with the furnace itself. Some key symptoms include the furnace running continuously but not heating the air, or the air coming out feeling cool instead of warm.
To troubleshoot, first check that the thermostat is set to 'heat' and the temperature is set higher than the current room temperature. If that's not the issue, replace the air filter if it's dirty. If the filter is clean, the problem could be with the furnace's heating element, blower motor, or other internal components. In those cases, it's best to call a professional HVAC technician to diagnose and repair the issue.