Went out to a boiler today (about a year old Lochinvar WHN155) to do a tune up and here is what I found on the combustion analyzer: CO 22ppm, CO2 1.2%, O2 19.2%. this was at full fire. at 20% fire, the CO2 does not even register, CO is less than 10 ppm, and O2 was 19.9-20.1%. boiler is condensing, neutralizer was full of water and the condensate line started running when I fired the thing. Venting is short, exhaust is about 20 effective feet, intake air is about 10. Natural gas, inlet gas pressure static is 8.2", 20% is 7.7" full fire is 5.5" so there is a substantial pressure drop on the gas but it should still be above minimum inlet pressure to my understanding. Gas pressure stays steady as a rock. unit is adjusted for high altitude (installed at about 6500 ft.) I tried to adjust the combustion, thinking that it just may not have been set properly on installation, and I could get the CO2 up to about 2% but the CO started climbing rapidly and once it got over 100ppm I decided to back it back down to where it was.
So, Gas piping may be an issue, this can be fixed fairly easily to abate the pressure drop, but my feeling is that it is not the problem given that it does not drop below 5.5" and is steady. Other than that, I cannot find anything at all wrong with the system. not short cycling, burner chamber was clean as a whistle, no errors logged, nothing out of the ordinary except the combustion.
Btw, the analyzer was calibrated a couple months ago, and I have used it on several other jobs and am fairly confident that it is working correctly, but I will never rule anything out.