There is rust on my damper and the metal in the firebox

Likely causes, most common first

Rust needs water and time, and these are the ways water reaches a damper and firebox, likeliest first.

1. Most common

   An uncapped flue taking rain straight down

   The sign: A wet smoke shelf after rain, orange streaks running down from the damper frame, and a bare flue tile open to the sky from the yard.

   The fix: A stainless cap with a screen keeps rain, leaves and animals out, and the rust stops growing once the metal dries. Chimney Cap Installation, from $299

2. Common

   A cracked or porous crown soaking the top of the chimney

   The sign: Rust that keeps coming with a cap in place, white deposits or damp brick where the chimney passes the attic, and water only after heavy rain.

   The fix: The crown is repaired or recast with a drip edge so water sheds off the brick instead of soaking through it into the flue. Chimney Crown Repair, from $349

3. Common

   Condensation in a flue that is too big or no longer used

   The sign: Rust in a fireplace nobody lights, damp soot on a furnace cleanout door, or a chimney whose furnace moved to a wall vent years ago.

   The fix: A Level 2 camera run sizes the flue against what vents into it and shows the line where exhaust has been condensing. Chimney Inspection, from $129

4. Common

   The damper plate and frame have rusted past sealing

   The sign: A handle that grinds, flakes of rust falling when the plate moves, daylight at the edges with the damper shut, and a cold draft in the room.

   The fix: A seized throat damper is freed, or replaced by a top-sealing unit that doubles as a cap. Damper Repair and Replacement, from $149

5. Less common

   Wet ash left in the firebox or the cleanout

   The sign: Ash left in the firebox all summer, an ash dump door or basement cleanout rusted shut, and a sour, damp smell at the hearth.

   The fix: We empty the ash dump and cleanout, replace a rusted ash dump door or lintel, and repoint firebox joints that wet ash has eaten. Smoke Chamber and Firebox Repair, from $299

Two minutes of safe checks

These checks separate rain coming down the flue from a flue that sweats, which decides the fix.

• On a rainy day, shine a flashlight onto the smoke shelf behind the open damper: standing water or wet, shiny soot there means rain is coming straight down.
• Stand back on the sidewalk and look at the chimney top: a bare clay tile or an open metal pipe with nothing over it is a flue with no cap.
• Rub the rust with a dry paper towel: loose orange powder that keeps coming is active rust still fed by water, while dark, hard scale is old and stable.
• Open and close the damper once and watch the plate: it should seat flat without shedding flakes, and a grinding handle means the frame is pitted too.

Nothing on this list needs a ladder, the roof or a lit fire. Leave those to the visit.

Where the water on your damper is coming from

Often the water arrives from the top of the chimney: through a flue with no cap, or through a cracked crown that lets rain soak the brick around the flue.

A flue without a cap takes rain straight down to the smoke shelf, the ledge behind the damper that nobody sees. Water pools there against the damper frame, and the rust works forward onto the plate and down the firebox walls.

The other source is condensation. A flue sized for a big fireplace or an old furnace, now carrying little or nothing, stays cold, and moist air condenses on its walls and runs down. That rust appears in fireplaces nobody uses.

Why Portland fireboxes rust through the winter and idle flues rust all year

Portland fireboxes rust in winter because measurable rain falls on more than 150 days a year, about two thirds of it from November through March; idle flues rust all year because they sweat.

The 1940s and 1950s ranch houses of Hazelwood, Cully and Milwaukie often carried the furnace flue in the same chimney as the fireplace. When the furnace moved to a wall vent, the old flue was left with a water heater or nothing, and it has sweated ever since.

The result is a damper that rusts in a fireplace that has not seen a fire in a decade, and a basement cleanout door that will not open. The fix is a liner or a cap sized to what is left, which the camera run settles.

What fixing a rusted damper or firebox involves

The visit finds the water before it touches the metal, because new metal under the same leak rusts the same way.

1. Trace the water

   From the firebox we check the smoke shelf, damper frame and firebrick for wet lines, then look at the cap, crown and flashing from the roof and the flue from inside, with photos of where it comes in.

2. Stop it at the top

   A missing cap gets a stainless cap with a screen, and a cracked crown is sealed or recast with an overhang. Where a flue sweats because it is oversized or orphaned, a liner or a cap sized to the remaining appliance ends the condensation.

3. Deal with the metal

   A throat damper with solid metal left is freed, cleaned and refitted. One pitted through comes out, and a top-sealing damper on a cable often takes its place, sealing better and keeping rain out at the same time.

4. Firebox and cleanout

   A rusted ash dump door or lintel is replaced, the cleanout is emptied, and joints the wet ash has opened are repointed with refractory mortar. You see the posted price before it starts.

Related pages

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