Zero-clearance fireplaces are portable units made of sheet metal. They are purchased in kits that must be assembled in the home. This kind of fireplace does not require the venting of smoke through a conventional chimney. Instead, it vents smoke and gas through an exhaust duct that travels through the wall to the outside. The zero-clearance fireplace can be placed in any room and moved about the room as well.
The zero-clearance fireplace can be set up in homes without chimneys. "Zero-clearance" means there is no need for clearance between the fireplace and surrounding walls, shelves and other structural fixtures. There is no need for a hearth or apron in front of the firebox, nor is a masonry foundation used. The fireplace can be placed anywhere in a room, although areas of heavy traffic should be avoided. Even while a cozy fire is roaring in the firebox, the exterior of the fireplace remains cool to the touch.
The zero-clearance fireplace has a built-in heat exchanger. Smoke from the fire is sent through an exhaust duct or vent to the outside instead of through a tall chimney. A blower unit recirculates warm air into the room where it is being used. These fireplaces are used with sealed doors in which air is prevented from passing directly into the firebox by a gasket seal. Some of the units allow the owner to operate a valve that controls the movement of air into the firebox, which in turn controls the burning rate inside and the amount of hot air released.
The zero-clearance fireplace can also draw warm air out of a room and out the ventilation system, thus keeping the room cool in the same way as a ventilation fan. This kind of fireplace is used with its doors open so that air can move freely into and through the firebox.
While a typical masonry fireplace built into a wall may burn at only 15 percent efficiency, zero-clearance fireplaces burn at up to 70 percent efficiency, meaning 70 percent of the material placed within them goes through complete combustion. As a result, the firebox develops much greater heat and burns without the heavy smoke and fumes often developed by the conventional fireplace.