Traditional wood-burning fireplaces are masonry structures with fireboxes and chimneys built from brick or stone. A newer type of wood-burning fireplace, called a low-mass fireplace, is prefabricated at a manufacturer's facility and transported to the construction site. Low-mass fireplaces are generally less expensive than traditional built-in-place fireplaces and may be more energy efficient as well.
Wood-burning fireplaces are generally valued for their aesthetics, but they create more pollution than gas-burning fireplaces and may be more expensive to operate and maintain. Open hearth wood-burning fireplaces are also not as energy efficient as those with an insert.
Gas fireplaces burn natural gas instead of wood, and thus provide cleaner, more efficient heat. Some gas fireplaces are constructed much like traditional wood-burning fireplaces, with a firebox that is vented through a chimney. Other types of gas-fireplace designs allow for more flexible choices of the fireplace's location because they're designed so that they can be located near walls or other potentially combustible structures. Some freestanding fireplaces may be located away from walls. Direct-vent or B-vent fireplaces can be vented directly through an exterior wall without the need for a chimney.
Gas fireplaces typically provide more control over the size of the flame and the unit's heat output. Some units are equipped with thermostats to regulate their output and remote controls to make their operation easy and convenient. But gas fireplaces don't offer the charm and the aesthetics of a wood-burning fireplace.
Fireplace inserts are prefabricated units made to be installed within the firebox of an existing fireplace, and they can be used to increase the efficiency of an older fireplace without requiring the installation of a new chimney or vent. When inserts are installed, a new metal liner is installed inside the existing masonry chimney.
Inserts may be either wood-burning or gas-burning; wood-burning inserts are typically designed like efficient wood stoves rather than traditional wood-burning fireplaces. They are more easily installed than a site-built fireplace, but they must meet local building code requirements when installed.
Ventless gas fireplaces do not vent their combustion gases through a chimney or wall vent. Instead, they draw combustion air directly from the room in which they're installed, and they exhaust combustion gases directly back into the room. This design allows for extreme flexibility in the placement of the fireplace, and the units are extremely efficient. Because of the concerns over their safety and their effects on the health of home occupants, the use of ventless fireplaces is restricted or prohibited in some states, counties or building jurisdictions.