Performed Furnace precision tune up; clean and tune up furnace; clean burners, test ignition components, adjust gas level, test safety circuit, check indoor motors, checked all electrical components, tested indoor air flow, checked heating temperatures, and tested overall furnace operation. Found that gas piping is accumulating rust as well as gas valve under fresh air intake to furnace. This is caused by hot humid air condensing in colder basements in the summer time. Would recommend installing the fresh air vent drain kit to avoid any further damages to gas piping and or gas valve. Main flame safety was testing below manufacture specifications. I pulled flame sensor and cleaned it and retested sensor. Cleaned flame safety was testing well above manufacture specifications at that point in time. Customer did have replacement filters, however checked current filter and it was clean. Static pressure was low so I left filter in. Overall, furnace is in great working condition, with n