If you want to get rid of a specific fireplace entity via <DNP, then you don't want to enter the number of the entity on the entity list, but rather you want to enter the event number of the specific entity you're modifying. For TCS commands that address a specific entity in the room such as <DNP, <CNP, and <ANP, you always want to have the first parameter be the command be the event number of the entity.
If you want to delete all instances of a fireplace, then you'd do what you were doing, except use <DNA instead of <DNP. <DNA0038 will delete all instances of the fireplace entity that are in the room. If there's only one fireplace entity in the place where you're using this, then <DNA0038 will work just as fine, although I wouldn't recommend getting into the habit of this, because it's just better to have the script only address a specific entity if that's all you're dealing with.
EDIT: Aaaaaaand I got ninja'd by four posts.