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Replacing Factory-Built Fireplaces, continued

Out the front. If the trim around the front of the fireplace is easy to remove, we just pop it off, cut or remove the fastenings that hold the unit to the framing, and pull it out of the opening (Figure 4). If there's no easy way to remove the trim and no good access from the back, we sometimes resort to cutting the fireplace apart with a recip saw and removing it piece by piece.

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Figure 4.The simple three-board facing around this circulating fireplace was easily removed without damage, making it possible to slide it forward into the living space for removal.

Removing the flue. When a fireplace is installed in an exterior chase, the flue sits directly on top of the unit and runs straight up through the chase cover. If the flue is less than 28 feet long, there's usually no need to anchor it to the framing anywhere else, making it easy to remove by pulling it straight up.

In interior applications, the old flue can be harder to get at. Concealed indoor flues often need to be routed around beams or other framing obstructions, which requires the use of offset elbows provided by the manufacturer (Figure 5). The offset elbows are sold in pairs, usually in angles of 15 or 30 degrees. The manufacturer will specify how many offsets can be in a chimney (typically two pair) and how far they can be spaced.

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Figure 5.When flue offsets are used to route the vent stack around obstructions, the attached metal straps are nailed or screwed to the framing to support the weight of the chimney above. Offsets in an existing flue slated for removal are easy to deal with if they're located in an attic or other accessible area, but if they're enclosed in a finished wall, it's sometimes necessary to cut additional holes to reach them.

In With the New

We encourage our customers to think carefully about what they want before they choose a replacement fireplace. Do they want an efficient heating appliance, or do they just want to look at a fire a few times a year? Do they really want to deal with wood, or would they be happier with the convenience of gas? It's a frustrating experience to install a wood-burning fireplace only to have the customer retrofit it with a set of aftermarket gas logs as soon as we finish. Manufactured gas fireplaces are much more efficient than such conversions, and not that much more expensive.

Clearances and standoffs. Although prefab fireplaces are sometimes called "zero-clearance fireplaces," that is a misnomer. The installation manual that comes with each unit will specify the required clearances to combustibles, which must be followed to the letter. Most fireplaces also have attached triangular "standoffs" that physically prevent the top of the unit — the hottest part during use — from coming too close to the framing. Those should never be flattened or removed to allow a fireplace to fit an existing opening (although we often remove older units that have been modified in that way).

The firebox of a typical modern prefab is 36 or 42 inches wide, and the unit requires a 46- or 52-inch rough opening. The older models we replace generally required somewhat larger rough openings for a given firebox size. That's helpful, because it means we can usually install a fireplace the same size as the original without having to cut away surrounding combustibles.

Prefab fireplaces are designed to sit directly on the combustible floor, but that makes me a little nervous. I usually put down a piece of cement board to act as a fire-retardant base. It costs just a few dollars, and it helps me sleep better at night.

Flanges and strapping. Most prefab fireplaces are designed to be fastened to the framing with nails driven through a mounting flange on either side of the front face. If we've removed the existing trim or fireplace surround and have access to the front of the unit, we'll fasten it to the framing with deck screws, which are later concealed by the trim. In cases where the replacement unit is smaller than the original, we often have a local sheet-metal shop fashion additional trim from black 24-gauge sheet steel (Figure 6).

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Figure 6.Gaps between a smaller replacement fireplace and the existing interior finish (top) are closed with a custom sheet-metal surround screwed to the front of the fireplace (middle). Existing assembly screws on the fireplace are backed out enough to allow the notched flange of the surround to slip into place before being tightened down with a nut driver (bottom). Where needed, extended trim is added to the sides of the fireplace as well.

If we removed the existing fireplace from the back, we use a different method to secure the new unit: After attaching the sheet-metal surround to the front of the fireplace, we screw two lengths of plumber's strapping to the framing, wrap them around the back of the firebox, and join the ends with a 1/4-inch bolt (Figure 7). This acts as a turnbuckle to pull the unit forward so the metal surround butts against the existing trim.

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Figure 7.

When a new fireplace is inserted from behind, it's not always possible to fasten the nailing flanges provided by the manufacturer to the framing. An alternative method is to fasten metal plumber's strapping to the framing with deck screws and pass it behind the back of the fireplace (left). A 1/4-inch bolt slipped through holes in the strapping acts as a turnbuckle to pull the unit forward so it rests snugly against the trim (right).

Chases and Roof Penetrations

Today's chimneys typically "snap-lock" together without tools. Once they are assembled, it's difficult to disassemble them without damage. A 2-inch clearance is usually required between the pipe itself and surrounding combustibles, including insulation. It's especially important to make sure that the ventilation holes at the bottom of the chimney aren't obstructed by insulation, because that can reduce the flow of cooling air between the stainless-steel liner and the outer wall and create a fire hazard.

Lost and found. It's not uncommon to find rot and other damage in exterior chases, usually as a result of leakage at the chase cover (Figure 8). We also find quite a few beer cans, hardware, and tools left behind by previous workers and lots of miscellaneous debris.

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Figure 8.The stamped one-piece chase covers provided by many fireplace manufacturers are a common source of leaks (left). The author's preferred cover, fabricated by a local sheet-metal shop, has a soldered collar that extends several inches above the cover itself (right). A conical storm collar is caulked to the flue with silicone. The cover is sized to extend an inch beyond the chase on all sides to prevent water that drips from the edges from running down the outside of the chase and rotting the siding.

Repairing or rebuilding a damaged chase is a simple carpentry project. We will do this ourselves if the customer requests it, but we prefer not to. Our service area is glutted with low-cost handymen, and we don't want to compete with them. We also don't want to be known as a home repair company, preferring instead to focus on chimneys and masonry.

Through the roof. Whether the fireplace is mounted in an interior wall or an exterior chase, a fire-stop spacer and an insulation shield should be installed at the level of the attic floor. If the chimney passes through an unused, inaccessible attic space, it can be left exposed. If it passes through an accessible attic used for storage, it should be framed in and protected from impact or contact with people and items around it. The pipe penetration through the roof is weatherproofed with a flashing kit provided by the manufacturer or with a rooftop chase (Figure 9).

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Figure 9.In applications where the flue exits through the roof, the penetration can be enclosed in a site-built or manufactured rooftop chase or left exposed and weatherproofed with a standard flashing kit, shown here.

Bob Priesingis the president of Havelock's Chimney Sweep in Havelock, N.C.