Furnace

A forced-air furnace for heating a space, the furnace comprising:

• a housing having a top, bottom, front, rear, and opposite sides;
• a firebox in the housing having a combustion chamber adapted for receiving and burning fuel to produce heated products of combustion;
• a combustion air delivery system for delivering combustion air to the combustion chamber, the combustion air delivery system including:
• a primary combustion air passage including a primary combustion air outlet in the combustion chamber for delivering primary combustion air to the combustion chamber at a primary combustion air flow rate,
• a secondary combustion air passage including a secondary combustion air outlet positioned above the primary combustion air outlet in the combustion chamber for delivering secondary combustion air to the combustion chamber at a secondary combustion air flow rate, and
• a valve system fluidly isolated from the first combustion air passage and in fluid communication with the secondary combustion air passage, said valve system being configured for changing the secondary combustion air flow rate delivered to the combustion chamber in response to a change in combustion chamber temperature without changing the primary combustion air flow rate delivered to the combustion chamber;
• and
• a heat transfer device above the firebox including a post-combustion plenum having an inlet in fluid communication with the combustion chamber for receiving products of combustion therefrom and an exit for permitting products of combustion to exit the post-combustion plenum, the post-combustion plenum having a front side, a rear side opposite the front side, a top side, and a length extending between the front side and the rear side, wherein the heat transfer device includes heat transfer passaging adapted for transporting air to be heated by the post-combustion plenum, the heat transfer passaging including:
• a first passage portion extending lengthwise relative to the post-combustion plenum, extending adjacent to the post-combustion plenum and configured for directing air in a direction toward the front side of the post-combustion plenum, said first passage portion being partially defined by a first wall;
• and
• a second passage portion downstream from the first passage portion extending lengthwise relative to the post-combustion plenum defining a flow path extending adjacent the post-combustion plenum and configured for directing air in a direction toward the rear side of the post-combustion plenum, said second passage portion being partially defined by a second wall comprising at least a portion of the top side of the post-combustion plenum.