Furnace

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

• a housing having a top, a bottom, a front, a 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, the combustion chamber having a front face adjacent the front of the housing, a rear face opposite the front face, a top face above said rear face, a bottom face below said front face and said rear face, opposite side faces extending from the top face to the bottom face and from the front face to the rear face, and an exit adjacent the front face permitting heated products of combustion to exit the combustion chamber;
• 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, and
• 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 heat transfer device above the firebox including:
• a post-combustion plenum having an inlet at an upstream end in fluid communication with the combustion chamber exit for receiving heated products of combustion from the combustion chamber and an outlet at a downstream end for permitting products of combustion to exit the post-combustion plenum, the post-combustion plenum having a thermally conductive top wall extending from the upstream end to the downstream end, and opposite thermally conductive side walls extending downward from the top wall and from the upstream end to the downstream end, and
• heat transfer passaging extending outside the post-combustion plenum adapted for transporting fluid past the post-combustion plenum, said heat transfer passaging including:
• first passage portions extending adjacent the opposite thermally conductive side walls of the post-combustion plenum, thereby heating fluid passing through the first passage portions, and
• a second passage portion downstream from the first passage portions extending adjacent the thermally conductive top wall of the post-combustion plenum, thereby heating the fluid passing through the second passage portion;
• 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.