A small amount of graphite grease should be applied sparingly and only occasionally to the spindle of the doorknob, twice a year is usually sufficient.
Ashes from the fire have a gritty texture and over time this can cause wear on the doorknob spindle. Use a tiny amount of grease to lubricate the groove in the first thread of the spindle which can be applied with a matchstick or cocktail stick.
Please note: only a small amount is needed, if too much is used it will melt and dribble down causing an unsightly stain, be sure to remove any excess before relighting the unit.