Preventative maintenence inspection on three Lennox high efficiency furnaces. Replaced both 16x25x1 air filters. 20x25x4 air filter in good condition. Visual inspection of burners. Flush condensation traps and drain lines. Clean flame sensors and checked ignitors. Pvc flue vents in good condition and no sign of leaks. 0PPM CO detected during testing. Humidifier pads in good condition. Test ran all furnaces in heating mode. All are operating properly and within temperature rise specifications, and blower and inducer motor specifications. Systems are operating properly, efficiently and ready for Winter.