p><strong>The most common problem when a furnace blows cold air, and first thing to check, is that the thermostat is set properly. </strong>Make sure the temperature is high enough to activate it, and that the thermostat is set to ‘heat’ and the fan set to ‘auto’. If that doesn’t’ fix the problem, the next likely cause would be the filter.</p> <p>Dust and dirt restricts airflow, so if the filter is clogged, it will on one hand slow the movement of air enough that whatever hot air ‘might’ have gotten there at a normal air pressure has simply cooled by the time it arrived. Also, preventing that air flow will cause the heat exchanger in the furnace to overheat and shut off too quickly. Try those two solutions and let us know if you notice any difference.</p