A fireplace insert increases efficiency by maximizing the heat output of burning wood or other fuel. Like a wood stove that is designed to fit inside a masonry fireplace’s firebox, an insert is designed to contain and radiate heat back into the room. It can increase the efficiency of your fireplace by up to 80 percent.

A traditional masonry fireplace may feel like it’s heating your home, but in most cases it’s only warming the immediate area. When a fire is burning, the fireplace is actually wasting energy and creating drafts as it sucks warmed room air up and out the chimney. And a traditional fireplace does not take full advantage of the fuel it burns. Wood-burning fireplaces only release 5 percent to 10 percent of the warmth they generate into the room. The rest is lost through the chimney flue.

A fireplace insert has its own firebox, which is surrounded by a cast-iron or steel shell. The fire is viewed through a window in the tightly sealed door on the front. As the firebox gets hot, the metal shell contains the heat and circulates it back into the room, often with the help of a blower. A decorative metal facing seals the fireplace opening around the stove.

When purchasing a fireplace insert, look beyond appearance. Be sure to get one that’s properly sized for your needs. It’s important to get one that’ll fit the size of your fireplace and produce the right amount of heat. Measure the depth, width, and height of your fireplace so you’ll know a given insert will fit. Then consider the heat output. If an insert doesn’t have enough heat output, it won’t heat effectively. But if it pumps out too much heat, it may drive you out of the room.

An insert’s heat output range is measured in BTUs (British thermal units) per hour. (For more information, see What Is a BTU?) Inserts are designed to deliver from 30,000 to 85,000 BTUs per hour or more. The larger the room or area you want to heat, the more BTUs per hour the insert must deliver.