Rust Inside a Prefab Fireplace Firebox

Why the Steel Rusts at All

A factory-built fireplace is a steel box lined with refractory panels, under a metal chimney in a framed chase. Its steel is protected only as long as it stays dry. The makers know it: the Majestic MPL11 manual says its chimney cap or housing is there to keep rain and debris out, and warns that a poorly installed or poorly placed cap can let wind-blown rain into the chimney. Superior even specifies galvanized sheet for the ember protector under its fireplace, to prevent corrosion.

So rust inside the firebox is a message about water. Before cleaning or painting anything, find where the rust is, because each place has its own source.

• Damper blade, smoke shelf and top of the firebox: water coming down the flue.
• Back and side walls, streaks from the top corners: water running down outside the flue.
• Floor and grate area: water or wet ash sitting on the bottom.
• Everywhere at once, with warping: heat damage as well as water.

Rust on the Damper and the Top of the Firebox

The damper and smoke shelf sit directly under the flue, so rain that gets past the cap lands on them first. Look for a damaged or missing termination cap, a cap knocked crooked by a storm, or one that sits where wind drives rain sideways into it. The Superior Pro Series manual notes that where side-driven rain is a problem, an anti-draft shield can be added to its round terminations.

Rust here, with a rusty puddle on the panels after a storm, means the fix starts at the top of the chimney, not inside the fireplace.

Rust Down the Back and Side Walls

Streaks running down the walls from the top corners, or rust on the outside of the casing, point to water getting into the chase and running down outside the flue. The openings to look for are at the roof: a rusted chase cover, a storm collar that has lost its seal, or flashing that has lifted. The Majestic manual has the installer seal the flashing to the chimney with mastic and seat the storm collar in it, and warns that a poor installation can let rain water into the home; Superior caulks the storm collar to the pipe.

This is the path that also wets the chase framing, so it is worth acting on quickly. Our page on how long a chase cover lasts shows how to read the chase top from the ground.

Rust on the Firebox Floor and Grate

Rust on the floor and under the grate comes from water collecting at the bottom, from either of the paths above, or from ash left sitting damp in the firebox for months. Fuel matters too. The Regency insert manual lists salt drift wood among the fuels never to burn, and the DuraTech chimney instructions say driftwood and chemically treated wood are corrosive to the appliance, stovepipe and chimney.

Keep the firebox swept of old ash when the fireplace is idle for long periods, and burn only clean, dry wood.

Rust Inside an Insert

A fireplace insert is a steel box too, and the same reading applies. Rust on its top and baffle points to water down the liner; the DuraLiner instructions state flatly that moisture must not enter the liner system, which is the cap's job. Rust with warping is different: the Regency i2450-2 manual says overfiring can cause warping and premature steel corrosion. Our guide on how long an insert lasts covers the parts that can be replaced.

When Rust Means the Unit Is Done

Surface rust on a sound panel or damper is a warning; rust that has eaten through is the end. The Superior Pro Series parts list marks the firebox bottom, the firebox surround and the firebox top assembly as not field replaceable, and the manual says never to use substitute materials. When one of those has holes, the fireplace cannot be patched back inside its listing. A damper blade, grate or chimney section that has rusted out can usually be replaced with the maker's listed part.

Tracing the Rust From the Hearth and the Yard

With the fireplace cold, shine a flashlight up past the damper after a heavy rain and look for drips or wet streaks; note which wall they run down. From the yard, look at the cap, the storm collar and the chase cover for rust, tilt or gaps. Do not scrape or paint inside the firebox before the leak is found, and do not climb onto the roof; the roof end of this job belongs to a crew with fall protection.

Finding the Leak

On a free inspection we trace the water from the rust upward: firebox, damper, flue, firestops, chase and cap. Our prefab fireplace repair crew then fixes the source first and the damage second, and writes both up in one estimate.

Tracing water takes an experienced crew and some patience, and both come from a local, family-owned company that is Proudly American. Our Proudly American crew, family-run and Spring Hill based, will show you where the water came in before any part is replaced.