A furnace that blows warm then turns cold partway through a cycle is usually shutting down on a limit switch, most often because a dirty filter or closed vents have restricted airflow. A furnace that blows cold from the first second is more likely failing to ignite at all. Short cycling every few minutes, no response to the thermostat, or a unit that runs constantly without reaching temperature each point somewhere different. In older South Lebanon, OH homes with basement-mounted furnaces and retrofitted ductwork, restricted return air is behind a large share of winter callouts.
An HVAC professional will read the control board fault code first, then test the flame sensor, ignitor, pressure switch and inducer, check static pressure across the filter and coil, and inspect the heat exchanger. Pro HVAC connects homeowners in South Lebanon, OH with professionals who handle natural gas furnaces. Call (321) 463-9811.
