This widespread use of the vent free variety of fireplace has contributed to a whole host of
indoor air quality problems not present with the use of naturally drafted fireplaces.
However, none of the new methods have brought all of the necessary attributes together to reach the full function and reliability of a fireplace built the old way.
First, the
metal insert fireplace type: The
metal inserts do not attempt to approximate the masonry fireplace except in the shape of the firebox and in their utilization of a thin applique of
refractory brick for a lining. The firebox itself is made of a double layer of sheet
metal, and the
flue is a
double walled metal
flue suspended within a framed chimney chase. Weaknesses of this type include a tendency to
wear out or
burn out, and a tendency for the chimney itself to be susceptible to leakage because of the frame construction of the chimney chase itself. Also, the use of a metal flue contained within a wood framed chimney chase raises questions of the use of hot metal flue materials close to the wood structure with only a small
air space separating the two. Finally, the framed chimney chase itself is susceptible to damage by high wind, and is susceptible to leakage regardless of the exterior cladding used to sheath the chimney. Most often the exterior chimney cladding is either
clapboard siding,
stucco, or brick
veneer applied over wood or metal framing. The flue and
damper in this type fireplace are far less durable than those in a traditional clay flue lined fireplace and chimney.
Second are the proprietary masonry fireplace systems, including those made of various standard, lightweight, and aerated concrete materials. These fireplaces exceed the metal fireplaces in quality and authenticity by utilizing masonry units for the area of the firebox, and therefore are capable of storing heat, and providing a longer lasting structure to which can be added a thin-set
refractory lining. This group of fireplace systems commonly utilizes one of two chimney flue types, often interchangeably: a double lined metal flue identical to the metal fireplace, or a stacked flue made of proprietary masonry blocks glued or mortared together to form only the flue liner. In either case however, the chimney liner still passes through a wood framed chimney, or requires traditional masonry methods to form the chimney. In the case of framed chases, the same problems exist with a high susceptibility to leakage,
wind damage, and difficulty in maintenance regardless of the
veneer or cladding used on the framed chimney chase structure. So, these fireplace chimneys are still
hybrid systems that combine a masonry or metal flue liner within a framed chimney. Most often the exterior chimney cladding is either
clapboard siding,
stucco, or brick
veneer applied over wood or metal framing, leaving the chimney highly susceptible to deterioration and water intrusion regardless of the exterior cladding used on the chimney. While the flue and
damper in this type of fireplace begin to approach the durability and functionality of a traditional masonry fireplace and chimney, this type of fireplace and chimney still fails to complete the entire picture since it relies on a framed structure to provide the chase for the flue liner, and therefore does not offer the true masonry chimney
mass found in a masonry chimney of traditional construction.
Third are the vent free fireplaces. They provide a
flame proof
enclosure of metal or masonry, with no provision for the conveyance of flue gasses out of the structure. Their use is limited to specially designed gas log appliances that emit relatively low levels of
carbon monoxide.
Fourth are the mechanically assisted ventilated fireplaces which rely on a fan and duct to evacuate
combustion gasses, and often utilize far smaller flue diameters than naturally drafted fireplaces utilize. Ducting of exhaust may be either vertical or horizontal out of the structure.
This is the most versatile, traditional, and proven type of fireplace, and the most difficult and costly to replicate in all of its features.