Just like fires, the smelter and coal kiln need ventilation above them (a place for smoke to go) otherwise they become "smothered" and will flicker on and off.

I usually build my main base underneath Elders altar, which means i cannot vent the smoke out so I build a false ceiling to allow the smoke to rise up and eventually dissapate safely in the space above.

https://steamcommunity.com/sharedfiles/filedetails/?id=2657677489

https://steamcommunity.com/sharedfiles/filedetails/?id=2434836833