Why is my furnace blowing cold air after I changed the filter?

There could be a few reasons why your furnace is blowing cold air after changing the filter. One possibility is that the filter was not installed correctly or is the wrong size, causing restricted airflow. Another reason could be a problem with the thermostat settings or a malfunction in the furnace itself. It is recommended to check the filter installation, thermostat settings, and if the issue persists, contact a professional HVAC technician for further inspection and repair.