Performed preventative maintenance for a navien tankless boiler/hot water heater. Model #ncb/240/130h(ng), serial #2164s2151406131. I powered the system down and removed the access panels. Checked all electrical connections to ensure that they are secure and in good condition. Inspected the expansion tank and copper piping for leaks or corrosion. All the ball valves and auto fill are in good condition. I powered the system back up and called for heat. I took a combustion analysis test and received the following readings, o2 is at 8.9% and the co levels are 25ppm. I called for heat at all four zones to ensure that they are operating properly. I observed the system operating properly and shut down once it reached temperature. The system is operating properly at this time. No Charge FWI