Thermostat set to heat
Heating
Repair. The furnace quit. Or it runs all night and the house never gets warm. That is the call nobody plans for, and it tends to come in January.
Replacement. The furnace is old, the repair bills keep coming, and you would rather put that money into a new one. Tell me what you have and how old it is, and I will take it from there.
Tune up. Nothing is wrong yet. You want the furnace looked over before the first cold night so it does not turn into a repair call later. That is a fall tune up.