Oofta
Legend
Something else to consider is that charred wood is fire resistant. So after the first cantrip or two, the wood is going to be harder to burn.
In addition, I don't see the whole door going up in flames, I see a fist-sized hole being (very) slowly formed. It would take hours to burn down most doors, and probably create a great deal of smoke. Even creating a bonfire on one side of a door would take a significant amount of time to burn through the door.
It would be an interesting experiment. Take a wooden log (oak or similar hardwood) apply a blow torch to it for a couple of seconds, take the flame away for a few seconds and apply again. Kind of a "spell caster's Myth Busters". Having started my share of fires, if the flame ever did start the door on fire I suspect it would go out in a very short order. You need continual heat for several minutes to get a log burning.
In addition, I don't see the whole door going up in flames, I see a fist-sized hole being (very) slowly formed. It would take hours to burn down most doors, and probably create a great deal of smoke. Even creating a bonfire on one side of a door would take a significant amount of time to burn through the door.
It would be an interesting experiment. Take a wooden log (oak or similar hardwood) apply a blow torch to it for a couple of seconds, take the flame away for a few seconds and apply again. Kind of a "spell caster's Myth Busters". Having started my share of fires, if the flame ever did start the door on fire I suspect it would go out in a very short order. You need continual heat for several minutes to get a log burning.