A pellet burner includes: a burner body having a combustion chamber and an open top; a feed opening at a sidewall of the burner body; a through port under the burner body; and a grate structure above the through port and including two tiers of supports both secured to the burner body. Supports in the first tier are lower than the adjacent supports in the second tier. The second tier of supports are staggered with the first tier. A passage is between the two tiers of supports. The passage is narrower than a diameter D of the pellet fuel. A spacing between two adjacent supports in the second tier is greater than D but smaller than 2D. The feed opening is above the grate structure by no less than 3D. The pellet fuel drops from the feed opening. Air entering via the through port disperses the burnt pellet fuel ash.

A dual-fuel fireplace apparatus is described. In one example, an apparatus includes an enclosure having a door disposed thereon and a wood burning compartment disposed within the enclosure. The wood burning compartment has an insulated housing configured to support temperatures associated with burning of wood within the wood burning compartment. A gas burning compartment is also disposed within the enclosure and has a gas burner disposed therein. A mechanism including a motor is configured to cause movement between a first mode in which the wood burning compartment is viewable through the door and a second mode in which the gas burning compartment is viewable through the door.

A furnace including a combustion chamber for burning fuel can have increased fuel burning efficiency, increased heating efficiency, and decreased harmful emissions of combustion byproducts. A combustion air delivery system delivers primary and secondary combustion air to the combustion chamber. Primary and secondary combustion air may be delivered at amounts that increase burning efficiency. An amount of secondary combustion air can be controlled by a valve system. A heat transfer device efficiently transfers heat from products of combustion for heating an enclosed space.

A duel-fuel fireplace apparatus is described. In one example, an apparatus includes an enclosure having a door disposed thereon and a wood burning compartment disposed within the enclosure. The wood burning compartment has an insulated housing configured to support temperatures associated with burning of wood within the wood burning compartment. A gas burning compartment is also disposed within the enclosure and has a gas burner disposed therein. A mechanism including a motor is configured to cause movement between a first mode in which the wood burning compartment is viewable through the door and a second mode in which the gas burning compartment is viewable through the door.

A furnace including a combustion chamber for burning fuel can have increased fuel burning efficiency, increased heating efficiency, and decreased harmful emissions of combustion byproducts. A combustion air delivery system delivers primary and secondary combustion air to the combustion chamber. Primary and secondary combustion air may be delivered at amounts that increase burning efficiency. An amount of secondary combustion air can be controlled by a valve system. A heat transfer device efficiently transfers heat from products of combustion for heating an enclosed space.

A duel-fuel fireplace apparatus is described. In one example, an apparatus includes an enclosure having a door disposed thereon and a wood burning compartment disposed within the enclosure. The wood burning compartment has an insulated housing configured to support temperatures associated with burning of wood within the wood burning compartment. A gas burning compartment is also disposed within the enclosure and has a gas burner disposed therein. A mechanism including a motor is configured to cause movement between a first mode in which the wood burning compartment is viewable through the door and a second mode in which the gas burning compartment is viewable through the door.

A furnace including a combustion chamber for burning fuel can have increased fuel burning efficiency, increased heating efficiency, and decreased harmful emissions of combustion byproducts. A combustion air delivery system delivers primary and secondary combustion air to the combustion chamber. Primary and secondary combustion air may be delivered at amounts that increase burning efficiency. An amount of secondary combustion air can be controlled by a valve system. A heat transfer device efficiently transfers heat from products of combustion for heating an enclosed space.

A duel-fuel fireplace apparatus is described. In one example, an apparatus includes an enclosure having a door disposed thereon and a wood burning compartment disposed within the enclosure. The wood burning compartment has an insulated housing configured to support temperatures associated with burning of wood within the wood burning compartment. A gas burning compartment is also disposed within the enclosure and has a gas burner disposed therein. A mechanism including a motor is configured to cause movement between a first mode in which the wood burning compartment is viewable through the door and a second mode in which the gas burning compartment is viewable through the door.

A furnace including a combustion chamber for burning fuel can have increased fuel burning efficiency, increased heating efficiency, and decreased harmful emissions of combustion byproducts. A combustion air delivery system delivers primary and secondary combustion air to the combustion chamber. Primary and secondary combustion air may be delivered at amounts that increase burning efficiency. An amount of secondary combustion air can be controlled by a valve system. A heat transfer device efficiently transfers heat from products of combustion for heating an enclosed space.

A furnace including a combustion chamber for burning fuel can have increased fuel burning efficiency, increased heating efficiency, and decreased harmful emissions of combustion byproducts. A combustion air delivery system delivers primary and secondary combustion air to the combustion chamber. Primary and secondary combustion air may be delivered at amounts that increase burning efficiency. An amount of secondary combustion air can be controlled by a valve system. A heat transfer device efficiently transfers heat from products of combustion for heating an enclosed space.