A furnace may blow cool air for a short time during startup or after the burner shuts off, but air that stays cold usually points to a failed ignition sequence, dirty flame sensor, blocked filter, open limit switch, or a thermostat or control issue. Confirm the thermostat is calling for heat and check the filter. Do not bypass a safety switch or open a sealed combustion compartment. If the blower runs continuously, note whether the burners light at all and tell the technician. A diagnosis is especially important for high-efficiency furnaces because intake, exhaust, condensate, and pressure-sensing problems can look like a failed furnace.
What to ask next
The right next step depends on the system, the urgency, and the building. Keep the model number, symptoms, age, and any previous service notes nearby when you request an estimate. A professional should explain the diagnosis or sizing basis in plain language.
When to bring in a professional
Use the callback form to describe the issue, or review HVAC services to find the closest starting point. For urgent loss of heat, see what to do in a cold snap.