the high o2 number could also be due to a cracked heat exchanger i haven't seen many gas units run a 6% o2 most of the time it is between 8-11%. what size is the flue pipe, could be oversized. I would change the furnace to a direct vent 90% save money on gas a do away with the chimney. How old is the heater anyway??
Originally posted by 7X
No, those numbers are not ok.
Oxygen number is huge. Should be down somewhere around 6% to 9%. Stack temp is low. Underfiring is contributing to the high O2 number, but other factors can figure in there - like the vent configuration. If the furnace is sized right, it should be fired right. We still don't know what the temp rise is. Another thought on short cycling may be the anticipator or cycle rate setting on the stat.