What makes E.S.F.B.A (Efficient Solid Fuel Burning Appliance) so much better? Mostly it is the baffles.
E.S.F.B.A has a gap in between the firebox and the baffle. The baffle sets in the center of the stove, leaving a gap between the baffle and the fire box. When you start a fire, the smoke, fire and heat are forced to the outside of the firebox, where it will transfer heat to the heat exchanger and eventually, turn the blower motor on and heat your space.
Also, the shape of the stove doesn't allow waste of heat. You can't heat up a square type stove evenly. The fire is closer to the sides of a “box” than to the corners. Think of inflating a balloon inside a square box and where will it touch first?
(from sandlaketree.com web site)