My son called me and said his gas logs were smoking the painted celing. He stated that nearly all the celing rafters in his living room, were shadowed with discoloration. He said there was black discoloration on the central unit filters. There is no sign of visible smoke or smell. He has used the logs for about 6 years. I told him to make sure flame was not touching the logs. He said he checked and it was ok.
That's about the only thing I can think of, what could be other causes?
I had an unvented nat gas heater that created black smoke/dirt on upper walls. The cause was that I ripped up a small section of carpet 30 ft away from it that kicked up regular dust but also nasty rubber dust particulate from the underlying pad. Where I'm going with this is that even for the self aware it is easy to stir up something in the home that pollutes the air so the ventless flame may produce nasty smoke or worse fumes. Don't know if that helps or not!