Related to this 'http://feedback.dayzgame.com/view.php?id=21134 ' issue as part of this problem could be having the fireplace fall through the ground.
Also related to this 'http://feedback.dayzgame.com/view.php?id=18533 ' issue as using that bug will render the Fireplace permanently not safe to ignite, even if the Fireplace is in a safe location.
Images for other reporters: http://imgur.com/2xUz2Fx,IhzE9jx,gZ9YAHP#2