Performed 1 system heating maintenance. Customer has his own 5" filters. Changed filter. Flushed drain line. Coil looks to be in good condition. System electrical components all operational. Attic insulation level low and below beams. Recommend blowing in at least 6" of insulation to achieve better heat retention. Recommend surge protector for furnace to protect electrical components from lightning strike or power surges. System operating properly at this time.