1 system furnace repair and preventative maintenance check. Check and find heating system 21 year old Rheem system operating but very dirty, duct system not up to code and dirty, missing filter. Blower assembly dirty. Gas line not up to code. Find ac unit cage in, missing valve cap. Due to low load and low temperature, was not able to check proper charge but ac unit operating at this time. Suggest duct cleaning, suggest to pull/clean blower assembly, suggest to pull/clean evaporator coil in the attic, suggest to replace flame sensor, suggest to do combustion calibration for the heating system and suggest to upgrade the gas line.