Our Answer:

While it depends on the fireplace, most refractory panels used in manufactured fireplaces are largely decorative. There is usually a metal chassis behind the panels that seals the entire firebox. Cracks up to 1/8 inch in diameter are acceptable, but I recommend replacing the liner as soon as you are able, as it will usually begin to crumble when the crack becomes any larger.

From Section Page: Fireplace Panels and Liners

ID: fireplace-panels-and-liners