Montgomery's houses carry a median build year in the mid 1990s and Hillsborough's sits in the mid 1980s, according to Census Bureau estimates, right in the stretch when builders across Somerset County traded brick fireplaces for prefabricated fireboxes and a listed metal chimney running up through a framed wood chase. The switch changed how the chimney gets swept. A brush built for mortar joints can dent or scratch the thin steel pipe inside a factory-built system, so the tools, the pace and the checkpoints differ from what a brick flue in an older Somerville or Raritan house calls for.
What sits behind the drywall
A factory-built fireplace vents through a round metal pipe, usually double-wall or triple-wall with an insulated air space between layers, running from the firebox collar up through a framed chase to a cap above the roofline. The chase is built like a closet around that pipe, closed in with drywall or sheathing, and it holds firestops where the pipe crosses each floor and the attic. Those firestops keep a chase fire from spreading floor to floor, and they are one reason a sweep works from the firebox up rather than tearing into the chase from outside. The pipe sections join with a twist-lock or screw connection, and each joint is a place where a brush can catch if it is the wrong size or style.
Brushing a metal flue without scoring it
The brush for a metal flue is a soft wire or poly bristle sized to match the pipe's listed diameter, not the stiff masonry brush used on a brick liner. Working the brush at an angle, or forcing one sized for a larger flue, can scratch the interior wall or loosen a seam, and a scratched wall gives creosote more surface to cling to on the next burn. The sweep works a section at a time, checking each joint by hand for looseness or gaps as the brush passes, and confirms the damper and firebox opening are protected from the debris falling down. On some systems the pipe can be dropped apart from inside the firebox for a closer look at a section that would otherwise be hidden behind the chase wall, and that gets reassembled and checked for a tight, square fit before the job is called finished.
What creosote looks like inside a metal pipe
Creosote in a metal flue tends to show as a dry, flaky soot on the walls after a season of well-seasoned wood and good draft. That flaky deposit brushes out easily and is the normal residue of wood burning. A denser, tar-like glaze is a different problem: it points to unseasoned or wet wood, a damper left mostly closed during the burn, or long, slow fires that never got the flue gases hot enough to keep rising cleanly. Glazed creosote does not always come off with a standard brush pass, and repeated glazing is a sign to change how the fireplace gets used, not only to schedule another cleaning. Either way, a factory-built system with any buildup gets a look at the joints underneath it, because a section that has separated even slightly gives smoke and heat a path into the chase framing that the pipe was built to prevent.
The cap, spark screen and chase top
At the top of the chase sits a metal pan, called the chase top or chase cover, that sheds rain away from the framing below, and the cap with its spark screen sits above the flue opening itself. A rusted chase top, a seam that has opened, or a cap held on with corroded screws lets water track down into the chase and onto the firestops and framing, sometimes for a season or two before a stain shows up on a ceiling below. The spark screen mesh is checked for rust-through and for a buildup of debris that can choke the draft even when the flue itself is clean. A sweep checking these points from the roof is looking for the same three things every time: is the cap secure and the right size for the flue, is the mesh open and sound, and is the chase top pan sealed at its seams and around the pipe.
When repair enters the picture
Chimney cleaning covers the soot and the inspection of the pipe, cap and chase top. Chimney repair covers what the inspection turns up: a corroded cap or spark screen swapped for one rated to the flue size, a chase top pan replaced or resealed where rust or an open seam has let water in, and a pipe section replaced where a joint has rusted through or separated instead of reseated. On a house from the 1980s or 1990s, a cap or chase top nearing the end of its service life is common enough that a sweep expects to find one that needs attention rather than treat it as unusual. Left alone, a leaking chase top does its damage slowly, inside a wall cavity nobody looks at until paint or drywall gives it away.
A yearly routine for a 1980s or 1990s house
The Consumer Product Safety Commission recommends a yearly inspection of home heating systems, including chimneys and flues, checking for blockages, corrosion and disconnections, along with working carbon monoxide alarms on every floor. For a factory-built fireplace in Montgomery, Hillsborough or elsewhere in Somerset County, that yearly visit means a brush sized to the pipe, a check of every joint from the firebox up, and a look at the cap, the spark screen and the chase top from the roof. Between visits, burning seasoned wood and keeping the damper open enough for a strong draft keeps creosote in its flaky, easy form instead of the glazed kind that calls for extra work.