Arrived on site to perform diagnostic on 2 year old Lennox system. Homeowner tried to turn furnace on last night and no hot air was coming out of the vents. On arrival made sure gas shut offs were in the ON position. Cycled system for heating. Blower motor immediately kicked on, no heat. Found thermostat was not configured correctly. Reconfigured thermostat to correct settings. Cycled for heat. System cycled on and started heating. Performed full evaluation of system, system is heating correctly. Replaced filter. System is fully operational upon departure.