Complete heating tune up. Change filter, check safety and operation. Find flue pipe not up to code, find heating system operating with little high CO level of 42 PPM but is it ok. Find minor ware on ignition and flame sensor and need to keep check. Overall, heating system operating normal. System in crawlspace and duct system not seal well, suggest duct sealing and APCO air purifier to improve air quality. Find evaporator coil baldy rusted and dirty.