Chimney Damper: Throat vs. Top-Sealing, and How to Use It

A chimney damper is a plate or door that closes off the flue when you're not using the fireplace. Open, it lets smoke out; closed, it keeps heated or cooled air from escaping up the chimney and keeps drafts, rain and animals out of the room. There are two kinds: a throat damper just above the firebox, and a top-sealing damper at the top of the chimney, operated by a cable.

Throat dampers

Most older masonry fireplaces have a cast iron throat damper in the narrow opening above the firebox. You open it with a lever, a rotating handle or a chain.

• Pros: simple and already there.
• Cons: metal-on-metal seal leaks air even when closed; the plate warps and rusts over time, and the hinge can seize.

Top-sealing dampers

A top-sealing damper mounts on the top of the flue with a spring-loaded lid and a rubber or silicone gasket. A stainless cable runs down the flue to a handle in the firebox.

• Pros: seals much tighter, so it saves more energy and stops cold air from sinking down the flue. Doubles as a rain cap and animal guard when closed.
• Cons: costs more to install; the cable and gasket need occasional replacement.

Top-sealers are a common upgrade when a throat damper is rusted or missing.

Is my damper open or closed?

• Look up into the firebox with a flashlight. If you see the flue or daylight, it's open.
• Feel for a draft. An open damper on a cold day pulls air strongly; you can also hold a lit incense stick or match near the opening (carefully) and watch the smoke.
• On a throat damper, the handle position varies by model: try it and look.

Always open the damper before lighting a fire. A closed damper sends smoke and carbon monoxide into the room. Leave it open until the fire is completely out and the ashes are cold, which can take many hours.

Gas logs: the damper must stay open

If you have decorative gas logs in a wood-burning fireplace, the damper must be permanently fixed open or removed. That's required by the National Fuel Gas Code (NFPA 54) and the log set's installation instructions. A damper clamp holds it open, so no one can close it by accident. Combustion gases from the logs need a guaranteed path out. Sealed direct-vent gas fireplaces work differently and don't use a damper.

Signs a damper needs attention

• It won't open or close, or the handle spins freely
• Daylight or a cold draft when it's closed
• Rust flakes falling into the firebox
• A smell in summer (a leaky damper lets chimney odors in; see smell from chimney)

Repair or replace?

A sticking throat damper can sometimes be freed with a wire brush and high-temperature lubricant. A warped or rusted-through plate is usually not worth fixing; installing a top-sealing damper is the common solution. Either way, have the chimney inspected and swept at the same time. For a full picture of how a traditional fireplace works, see masonry fireplace.