If your home still has an older, “drop-in” fireplace insert, it may be doing more harm than good. A common legacy setup—often called a slammer—slides a metal insert into the masonry opening without a continuous, properly sized chimney liner. That shortcut might have felt convenient years ago, but today we know it can create real safety and performance problems.

What’s a “slammer” insert?

A slammer is an insert that vents through the existing masonry flue without a dedicated, listed liner connected from the appliance to the top of the chimney. Instead of controlled venting, it relies on gaps in the old flue. The result is weak draft, smoky burns, and accelerated creosote buildup.

Why older installs are dangerous Chimney-fire risk.

Poor draft and incomplete combustion produce excessive creosote—a highly flammable byproduct that can ignite inside the chimney. Also the unit and chimney flue can not be cleaned thoroughly without completing removing the unit. Sometimes this can be incredibly hard. Creosote often times end up building up on the unit and in hard to access smoke shelfs, especially when installed in metal fireboxes. All around it's an outdated system and a known problem and terribly inefficient compared to a properly vented modern stove.

Carbon monoxide exposure. Improper venting creates a pathway for CO to leak into living areas.

Wasted heat & higher fuel bills. Air leaks and poor sealing mean you burn more wood and get less heat.

Hidden masonry damage. Hot gases and acids can escape into gaps, eroding mortar and terracotta tiles over time.