When the fireplace damper sticks, will not close or leaks air

Throat damper, top damper or a prefab unit's damper

Chimney damper problems look different depending on the type. A throat damper sits just above the firebox, a cast-iron or steel plate on pivots inside a frame built into the masonry. It is operated by a hanging poker handle, a lever, or a knob on the fireplace face that turns it through a gear. The residential volume of the Florida Building Code, in section R1001.7.1, calls for a masonry fireplace to have a ferrous metal damper, in the fireplace or in the chimney, set 8 inches or more above the opening's top edge and worked from inside the room.

A top-sealing damper sits at the top of the flue instead. It closes a lid down onto a gasket and is held open by a spring, with a cable running down the flue to a handle clipped inside the firebox. Factory-built fireplaces have their own damper at the base of the metal chimney, described in the unit's manual. The three fail differently, so the checks below start with the throat damper and then cover the other two.

If you are not sure which you have, a cable running down one side of the firebox and clipped to a bracket gives away a top-sealing damper.

Before you touch it

Work only on a cold fireplace. Ash can hold heat long after the flames are gone, so wait until you are sure the firebox is cool. Wear gloves and eye protection, because rust flakes and soot come down as soon as the plate moves, and look up from beside the opening rather than from directly under the plate.

If there is a gas log set in the fireplace, turn it off and leave it off while you check. A clamp or bolt holding the damper partly open in that fireplace may be deliberate: log sets built to ANSI Z21.60 come with instructions that require the damper to be fixed open to a minimum area. Do not remove it.

How to tell if the damper is open

Point a flashlight up past the lintel. A closed throat damper lies roughly flat across the throat and you see the underside of the plate. An open one stands up on its edge, and you see past it into the smoke chamber above. Some fireplaces have an offset flue or a deep smoke shelf, so you may not see daylight even with the plate wide open.

The handle position is a clue but not proof. A poker handle or rotary knob that has slipped off its linkage can sit in the open position while the plate stays shut, a confusing fault because everything looks right from the room. When the handle and the plate disagree, trust what the light shows you.

Damper won't open, or opens only partway

Rust scale is the first suspect. Water coming down an uncapped flue settles on the plate and its seat, and over time the scale locks the two together. A warped plate can also jam against its frame, and fallen mortar, brick or nesting material lying on top can hold it down.

Work the handle back and forth gently, a little further each time. A light tap on the plate with a wooden handle can break scale loose. Do not use a steel bar or a hammer, because cast iron cracks, and skip spray lubricants, since the plate sits in the heat path and oil left on it will smoke at the next fire.

If the plate will not free with gentle effort, stop there and do not light a fire. A plate that opens only partway shrinks a passage that R1001.6 requires to be no smaller than the flue, and smoke can back up into the room.

Damper won't close, or closes but leaks air

Close the damper and aim the light at the edges of the plate. A steady line of light or a draft you can feel around the edges means the plate is not seating. Heat warps thin plates, rusted pivots let a plate drop out of line, and scale built up on the seat holds it off the frame.

Remember that a cast-iron throat damper closes metal on metal and does not close airtight, even when it is in good shape. What you are looking for is a plate that no longer lands flat, not a perfect seal. A top-sealing damper that will not stay closed points to a stretched or broken cable, a cable clip that no longer holds, or a lid that no longer seats on its gasket, and those parts are at the top of the chimney, which is not a ladder job for a homeowner.

What rust, warping and a missing plate mean

Light surface rust on a damper that opens fully and closes flat is a watch item. Flakes of rust falling into the firebox each time the plate moves mean the plate is losing metal, and they also mean water is getting down the flue, so the next thing to look at is the top of the chimney and its cap.

A warped plate will keep leaking air and can jam in its frame, and straightening cast iron is not a fix. A missing plate leaves the fireplace with no way to close, which a masonry fireplace built under the current code is required to have. Plates were sometimes removed for an old insert or lost to rust, and in either case the frame, the throat and the pivots need checking before a replacement is chosen.

Wear shows up at the handle end too. A poker handle that has bent, or a rotary control whose gear has worn, turns stiffly or slips without moving the plate, and on some dampers those parts can be replaced separately.

When it becomes a repair job

Stop and call for a repair visit if the plate is stuck closed or partway, drops shut by itself, rattles loose in its frame, or has gone missing, or if the handle turns without moving anything. The same goes for smoke coming into the room with the damper set to open.

The repair options run from freeing and re-hanging the existing plate, through a new plate or control where one is still made, to replacing the whole frame or fitting a top-sealing damper instead. A written estimate should say which of those your damper needs and why.