Have a Lennox Conservator III, Model G16RQ3X-75-1.
Problem: With room thermostat in Fan Auto position, blower comes on automatically but no hot air from vents, changing thermostat from Fan Auto to ON (Manual) blower comes on and hot air coming from vents. Need some guidance as to where the problem is (inside the heater unit?, room thermostat?). Any help would be appreciated. Thank you.