Preformed fall service on 80% down flow furnace checked the filters they looked ok cleaned flame sensor checked capacitor it read 7.4 of 7.5 the evaporator was clean I saw no safety concerns the blower amps read 3.7 and induce draft motor read .5 of .8 the system is operating correctly at this time. Asked customer to inspect hot water tank and found a part of it missing about 7 inches so the hot water tank has been terminating into closet and allowing small amounts of CO into the dwelling also asked if they felt like there wasn’t enough hot water customer did express that it takes a long time for hot water to reach the back of the ho e I explained that was due to sediment in the tank reducing the amount of pressure and found the temperature and pressure valve discharge line not install but laying in the corner.the missing part in the flue is a safety concern and recommend completing the flue at minimum.