Lennox G24M Gas furnace and a Hunter 44110 Digital Programmable Thermostat that I installed when I moved into this place (replaced an old rotary thermostat)... Worked fine for AC, but now this is my first winter in this house.. The heat absolutely refuses to get the room temp up to 75.. What should I check?

Your t stat is probably not calibrated right in the installer config check manual I believe it can be changed by 5 degrees far.

your t stat is probably not calibrated right in the installer config check manual i believe it can be changed by 5 degrees far.

Do you mean the SPAN setting? Mine is set at 3, which keeps the heater running the longest?