Fireplace inserts are made out of steel or cast iron, explains the Chicago Tribune. These metal fireboxes are placed into compatible, pre-existing brick or masonry fireplaces, notes eFireplaceStore.
Fireplace inserts typically use wood, gas or pellets as a source of fuel, according to eFireplaceStore. Some inserts use electricity or coal instead, notes the Chicago Tribune. Pellet-burning fireplace inserts usually require electricity to power the internal auger conveyor that distributes the pellets into the fire, explains eFireplaceStore.
Gas and pellet fireplace inserts burn more efficiently and use a cleaner source of fuel than other inserts, explains the Environmental Protection Agency. Homeowners should look for EPA-certified fireplace inserts to find the cleanest, most efficient insert models for each kind of fuel source.