Somewhat like a traditional gas or wood fireplace insert, a pellet insert installs into a fireplace and burns wood pellets.
On top of the insert is a hopper which holds the pellet fuel. An electric motor drives a screw auger that feeds the pellets slowly into the fire. A fan blows air under the burning pellets so they burn hotly just like air blows underneath a blacksmithing fire. Another fan draws room air through a heat exchanger and back into the room.
Pellet inserts are fuel specific; that is, they are designed and intended to operate only on wood pellet fuel.
Although they use very little electricity, power is required for operation. The auger has to feed pellets into the burner, and air must be blown into the burning pellets. Without that air, burning pellets will simply smolder and produce very little heat.
Most pellet inserts can use a millivolt thermostat to regulate room temperature; some cycle between high & low heat output, others shut down entirely & re-light themselves when the thermostat calls for heat.
Pellet inserts use 4” stainless steel flexible vent up through the chimney. Be sure not to use lower temperature rated flexible gas vent; doing so both violates building code & can be unsafe.
Pellets are made of compressed sawdust, recycling surplus material from lumbering and furniture making operations. The sawdust is formed into pellets under many thousands of pounds of pressure. Pellets are typically sold in 40 lb bags, about the size of a bag of dog food. Pellets need to be stored in a dry area; if soaked by rain or snow they swell up back into sawdust and cannot used used in your pellet insert.
Premium Grade pellets meet industry standards for purity, size uniformity, and heat output per pound of pellet fuel. Premium Grade pellets burn cleaner, produce less ash, and are pure sawdust, while lesser grades may use resins or glues as binders.