What a Block-Off Plate Above an Insert Does

Where the Plate Sits

Behind every insert's faceplate there is a gap: the old firebox is bigger than the insert, and the space around and above the insert's steel body is open to the damper throat. When an insert goes into a masonry fireplace, the old damper usually comes out. The DuraLiner instructions call it not needed and allow part of the smoke shelf to be cut back so the liner passes freely, while the Regency i2450-2 manual says a damper that will become hard to reach should be removed or fixed open.

That leaves a hole the size of the damper opening, with the liner running up through it. A block-off plate closes that hole. It is cut from sheet metal to the shape of the throat, with one opening for the liner, and fixed at the damper frame or the smoke shelf.

• Insert body in the firebox, faceplate against the fireplace face.
• Liner rising from the insert's collar through the throat.
• Block-off plate across the throat, cut close around the liner.
• Sealant along the plate's edges and around the liner hole.
• Open flue above, with the insulated liner continuing to the top.

Why an Open Throat Wastes Heat

An insert heats a room two ways: through its glass and steel face, and by warming the air in the cavity around its body. Without a plate, that warm cavity air has a short, open path straight into the old flue. It rises, cools and leaves, and room air is pulled in behind the faceplate to replace it.

With the plate in place, the cavity becomes a closed pocket, and more of the heat comes back into the room through the faceplate and the insert's own convection channels. The faceplate itself helps: the Regency manual fits insulation strips between the faceplates and the fireplace face to seal the opening. Our guide on an insert that does not heat the room covers the other places heat goes missing.

What the Plate Is Made Of

None of the manuals we opened for this page names a block-off plate or gives a thickness for one, so any figure you read for it comes from an installer's practice rather than a maker's rule. What the plate has to be is simple: metal that will not burn or soften next to a hot liner, cut cleanly, with no gaps around the edge.

• Sheet steel or stainless steel, cut to the throat with a hole for the liner.
• High-temperature sealant along the edges and around the liner.
• Fastened to the damper frame or the smoke shelf, without removing bricks or mortar.
• No combustible material anywhere on the plate or the throat.

Insulation on Top of the Plate

Some installers lay insulation on top of the plate to slow heat loss through the metal. If any goes there, it has to be the right kind. The DuraVent DuraTech instructions, written for where a chimney passes through a building, accept stone insulations such as mineral wool that melt above 2000 F and shed water, and they say fiberglass is not approved. The same caution applies anywhere near a liner.

The liner's own instructions limit what may touch it. DuraLiner says no insulation or other material beyond what it specifies may go around the liner or in its required air spaces. So any blanket on the plate stays clear of the liner unless it is the liner maker's own insulation.

What Has to Stay True Around the Plate

A block-off plate is a detail inside a larger job, and it must not undo the rules of that job. The insert's manual still calls for a continuous liner from the collar to the top of the chimney, and the plate does not replace that. The Regency manual also says the insert must not connect to a chimney serving another appliance, so the plate is never a way to share a flue.

The plate must be removable for a full inspection, and its sealant must hold at the temperatures the throat reaches. A plate that has slipped, rusted through or lost its seal stops doing its job without any sign in the room.

Checking for a Plate Yourself

With the insert cold, a flashlight and a mirror at the edge of the faceplate can sometimes show whether metal closes the throat above the insert. Do not pull the faceplate or the insert to look; the faceplate seal and the liner connection are easy to disturb and hard to set right. Smoke smell in the room, a cold draft from behind the faceplate or a slow-heating insert are reasons to have it checked.

On a fireplace insert installation we fit and seal the plate as part of the liner work, and on an existing insert we check it at the sweep. Ours is a local family company, and the experienced hands that fit plates are Proudly American ones.

Getting It Looked At

Ask for a free inspection and we will tell you whether a plate is there, whether it is sealed, and what fitting one would involve, with a written estimate. Spring Hill homeowners get the same Proudly American family crew from the first look to the finished job.