Fireplaces are objects most commonly found in the Basement/Cellar, but they will appear in later chapters as well (except for The Womb/Utero). Two fireplaces will usually be present in the Shop, in the Secret Room (unless Greed or Super Greed is encountered inside), and always in a Devil Room.
Touching the flame will do 1/2 heart damage to the player, even if they possess a flight upgrade. In addition, enemies will receive considerable damage if they happen to touch the fire; Hoppers are the only exception: they will turn into F.Hoppers, which are immune to the fire and do more damage.
Fireplaces can be extinguished by shooting tears at them. This can cause them to drop a single heart or coin variant (a half-heart, full heart, soul heart, eternal heart, penny, nickel, or dime). If the fire gets destroyed by an explosion, then it does not drop anything, which can sometimes be a disadvantage.
In Wrath of the Lamb, there is a chance for a normal fireplace to be replaced with a Hellfireplace. These burn with a more intense orange and can sometimes shoot slow-travelling projectiles directly at the player, which also deal 1/2 heart of damage. If the player is killed by these projectiles, it will appear on the death note that Isaac (boss) killed them. They drop items at the same rate as regular fireplaces, but will spawn them in the diagonally opposite corner instead of the actual location. They may be a reference to the Old Man from The Legend of Zelda. He would make the fires next to him shoot projectiles at Link if attacked.