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Hello, so far it sounds like the controls have the fan running way to slowly, so that the furnace is operating in its low stage only... that could be a thermostat wiring problem, or wrong thermostat, or fan speed control dip switches not set correctly.
An accomplished Lennox factory trained tech with 20 years or more experience could most likely diagnose and correct the problem in less than an hour. Its a bit technical for you to attempt yourself.
If you can tell me which direction you are from the center of the nearest city I will see if I can find a highly qualified Lennox dealer for you.