Short answer: cold air usually means the furnace is not making heat, or the heat it makes is not reaching you. Start with the thermostat set to Heat and above room temperature, a filter that is not completely clogged, and a breaker that has not tripped. Beyond that, the usual causes are a failed ignitor, a dirty flame sensor, a gas valve or control fault, or, on an electric furnace, a failed element or sequencer. A blower can run perfectly while no heat is being made, which is why cold air alone does not tell you which part failed.
Work through it in order: thermostat call for heat, ignition, flame sensing, gas or electrical supply, safety switches, then airflow. That order matters, because a thermostat fault and a control-board fault look identical from the living room. If you smell gas, skip the checks, leave the house, and call your gas utility or 911.
Related reading: furnace repair in Springfield, furnace cost guide, common furnace problems. Call (615) 436-9171.
