I'm a chimney mason and sweep in coastal North Carolina, where
I do restoration work on fireplaces and chimneys in historic
structures. At the other end of the spectrum, I tear out a lot
of old prefab wood-burning fireplaces every year and replace
them with new ones.
A prefab fireplace is more like a kitchen range or other
household appliance than a masonry chimney or fireplace, which
can last for generations if it's maintained and repaired as
needed. Prefab fireplaces have a definite and limited service
life and are designed to be used up and periodically replaced.
As a mason, I can't get enthusiastic about them. But they're a
high-profit item for builders, so they're often installed in
new houses. That popularity, combined with their limited life
expectancy, provides me with a steady supply of work.
Anatomy of a Prefab
The rules for installing UL-listed prefab fireplace systems
are entirely separate from those that regulate masonry
fireplaces. Each prefab fireplace system has its own
distinctive installation instructions. Specific information,
such as clearances to combustibles and chimney terminations,
varies from one manufacturer to the next, and it's up to the
individual installer to follow the directions in the manual
provided with the unit.
In simple terms, prefab fireplaces are basically double-walled
metal boxes lined with cast refractory panels that are usually
made to look like firebrick (see Figure 1).
Figure 1.Prefab fireplaces can be installed in an
alcove in an interior wall or in an exterior chase. This chased
installation includes a ceiling-level firestop, which is
required by most building codes. The firestop itself and the
exterior walls below are insulated with fiberglass batts. The
inner wall of the chase and the upper portions of the three
outer walls above the firestop should be left
Some models have blowers to circulate air between the inner
and outer walls, while others rely on convection. (Even
blower-equipped models shouldn't be seen as heating devices,
though; like traditional masonry fireplaces, they typically let
much more heat escape up the flue than they add to the living
space.) Many wood-burning fireplaces are designed to accept
atmospherically vented ornamental gas logs as an aftermarket
add-on. This is a popular modification, even though a fireplace
that's been converted to gas can no longer be used with
Prefab fireplaces are vented with a double-wall pipe that
exits from the top of the unit. The inner wall of the flue is
stainless steel, while the outer wall is typically galvanized
or painted. The entire assembly is often enclosed in a
wood-framed chase built against an exterior wall. The chase
itself may be supported by cantilevered joists or rest directly
on a bumpout in the foundation. Prefabs can also be installed
in a framed alcove in an inside wall.
How fireplaces wear out.
Most prefab fireplaces come with warranties that run from 20 to
30 years, although they often contain disclaimers or exclusions
to limit coverage of damage related to environmental
conditions. In our coastal service area, where prefabs are
under constant attack from salt-laden air, we regularly replace
fireplaces that are between 15 and 30 years old. Units located
right on the coast fail sooner than those that are farther
The way a fireplace is used also affects its life expectancy.
A fireplace that sees a lot of hot fires won't last as long as
one that is used only occasionally. Chimney fires are
especially damaging; they can quickly destroy the stainless
flue liner. Widespread rust caused by water that's leaked
through a rusted-out chase cover, bad flashing, or a missing
termination cap is another common discovery (Figure 2).
Customers sometimes tell us that they'd heard water dripping
but thought it was insignificant because they didn't see any
water in the house.
Figure 2.Most failed or failing prefabs are
discovered in the course of a routine chimney cleaning, not
because the homeowner realizes something is wrong. A missing
termination cap (top) or leaking chase cover can quickly cause
internal rust. This sort of damage is often evident around the
damper at the top of the firebox (middle) or behind the louvers
of a circulating fireplace (bottom).
We often encounter fireplaces so far gone that we can easily
poke a hole through the metal with a cleaning brush or even a
finger. At that point, the only practical solution is to
replace the entire unit.
Out With the Old
When you consider the limited service life of typical prefab
fireplaces, it's surprising how little thought goes into their
eventual replacement. Every case is different, but our goal is
always to remove the old unit with as little disturbance as
possible to the interior finish.
Cut to the chase. If the
front of the fireplace is faced with brick, tile, or some other
finish that was applied after the unit was originally
installed, we prefer to come in from behind so we don't have to
tear the fireplace surround apart and rebuild it when we're
done. That often means cutting through the outside wall of the
chase (Figure 3). If the fireplace is built into an inside
wall, it's usually possible to cut away the drywall and framing
on the other side of the wall.
Figure 3.Removing a fireplace from the back
eliminates the need to dismantle and later rebuild the existing
interior finish. An opening in the exterior chase provides the
working space necessary to disconnect the flue and remove the
fastenings that hold the old fireplace to the framing (top).
The unit can then be pushed back into the chase and out the
access hole (bottom).