Choosing a top damper or a throat damper for a flue that sits idle

Where each damper sits and how it closes

Of the two types of chimney dampers, a throat damper sits in the narrow passage just above the firebox, where smoke leaves the fireplace and enters the smoke chamber. It is a metal plate worked from the room with a handle or lever. BIA's fireplace design note describes the plate opening toward the back of the fireplace. The Florida Building Code requires masonry fireplaces to have a ferrous metal damper that can be worked from the room.

A top sealing damper moves the closure to the other end of the flue. Its frame is set on the top of the flue tile, often with the adhesive supplied with it, and a spring-loaded lid seals against a silicone rubber gasket. A stainless cable runs down the flue to a handle fixed to the firebox wall. Pulling the cable down and hooking the handle holds the lid shut, and releasing the handle lets the spring lift the lid open. The same code section allows a damper in the chimney as well as in the fireplace, as long as it is operable from the room.

What an open throat leaves exposed between fires

When a throat damper closes, everything above it stays open to the weather. Rain that gets past the cap, leaves, and animals looking for shelter all reach the top of the damper plate. The plate and its frame are ferrous metal, and wet metal rusts. Rust and debris can then keep the plate from seating, or from opening at all.

The seal itself is metal on metal, with no gasket. BIA's design note describes the damper's job as closing the fireplace opening to outside air. Top damper makers sell their products as throat damper replacements that help stop heated or air-conditioned air from escaping. A fireplace lit only on rare evenings spends almost all its time closed, so that seal carries most of the load.

The case for sealing the flue at the top

A top sealing damper changes both of those conditions. With the lid shut, the whole flue below it is closed off from rain and animals as well as from the room. BIA's design note says a damper at the chimney top may help reduce water getting into the flue. Manufacturers describe their top dampers as sealing out weather, insects, birds and other animals.

Several models also serve as the chimney cap, with a mesh screen around the lid, and some add spark protection. The gasket is what separates them from a throat damper. National Chimney describes lids that close on a silicone rubber gasket for a tight fit.

Where a top damper does not fit

A top damper is not right for every chimney. Check these limits before choosing one.

• Flues venting gas logs, which the damper maker's product notes say not to fit
• A flue top that is broken or uneven, which needs repair before a frame can seal on it
• Unusual flue sizes and shapes, which may need an adapter or a different model
• A chimney with several flues under one cap, where access and fit need a closer look
• Factory-built chimneys, where only parts the fireplace maker allows may be added
• A fireplace insert or stove with its own liner, which follows that appliance's instructions

Gas logs keep the throat damper open for good

Vented gas logs change the damper question. The Real Fyre vented gas log manual calls for a damper clamp that must be permanently installed, so the damper can never fully close. Where the clamp will not fit, the manual offers three alternatives. A permanent stop can be installed, the blade can come out, or the damper can be cut back to leave the required opening. The damper must be fully open whenever the logs burn.

National Chimney's product notes for its top dampers say not to install them on chimneys that vent gas logs. So a fireplace with vented gas logs normally keeps a throat damper that is clamped so it cannot fully close, or has its blade removed. The cap on top still does the job of keeping out rain and animals.

Living with the cable and handle

The cable is the part you touch. It hangs from the lid, runs down the flue and ends at a handle clipped to a bracket on the firebox wall. Before any fire, release the handle and confirm the lid has opened. Keep the lid up until nothing in the firebox is still glowing.

BIA notes that a chimney-top damper must be weighted or spring-loaded to open if its operating mechanism fails. That is a safety design, not a reason to ignore a frayed or broken cable. If the handle feels loose or the lid will not close, leave the fireplace unlit and have the cable looked at. Some models carry a chimney fire safety device, so ask whether yours does and what it needs after a fire.

When repairing the throat damper is enough

A throat damper that still opens, closes and seats reasonably well may only need a repair. Cleaning the plate, freeing a stiff hinge or replacing a worn handle can bring it back. If the chimney already has a sound cap and screen, and the fireplace is used as a wood fireplace, a repaired throat damper may be all you need.

Replacement becomes the question when the plate has rusted through, the frame is cracked, or the damper can no longer be reached or fitted. At that point you choose between a new throat damper and a top damper. Our chimney damper replacement visit compares both against your flue and your fuel.

What a damper estimate should name

Ask for the written estimate to answer these points before you approve the work.

• Which damper type and model is proposed, and why it suits this flue
• How it mounts, on the flue tile or on an adapter
• Whether the existing cap is removed, kept or replaced by the damper's own cap
• What happens to the old throat damper, left open, fixed open or removed
• Where the handle will sit on the firebox wall
• Any flue or top repair needed before the frame can seal

Next step for an idle flue

On a free inspection we check the damper you have, the top of the flue and the fuel you burn. If a top sealing damper fits, the estimate says so and lists the work. If a repair to the throat damper is enough, it says that instead. Either way, you approve the written estimate first. Any open chimney is covered at the end of each work day, and the record we hand over says which damper was repaired or fitted.

Sources

• National Chimney top-mounted damper products: Gasketed top dampers, cable and handle hardware, cap versions, adapters, their sale as throat damper replacements and the instruction not to use them on chimneys venting gas logs.
• BIA Technical Note 19, Residential Fireplace Design: The damper's job of closing the opening to outside air, throat and chimney-top damper locations, reduced water entry with a top damper and the open-on-failure design.
• 2023 Florida Building Code, Residential, chapter 10: A ferrous metal damper in the fireplace or chimney that can be worked from the room, and factory-built systems installed to their listing.
• Real Fyre GR47 vented gas log manual: A permanently installed damper clamp with vented gas logs and the alternatives when a clamp does not fit.