Fireplace retrofit construction

A method for constructing a firebox retrofit system for a fireplace comprising a chimney, a flue, a lined rear wall, a pair of lined sidewalls, and a lined floor, the method comprising:

• a) providing a kit for assembling a retrofit firebox assembly frame, wherein the retrofit firebox assembly frame comprises:
• i) a first sidewall frame assembly comprising a plurality of vertically extending tube members, with the tube members being spaced apart;
• ii) a second sidewall frame assembly comprising a plurality of vertically extending tube members, with the tube members being spaced apart;
• iii) a back wall metal plenum member comprising a metal sheet having a lower vertical portion, a lower angled portion extending forward into the firebox, a central vertical portion positioned forward of the plane of the lower vertical portion, an upper angled portion extending rearward from the plane of the central vertical portion, and an upper vertical portion positioned rearward of the plane of the central vertical portion, wherein the lower central angled portion, the central vertical portion, and the upper central angled portion combine to form a smooth, symmetrical bulge in the back wall extending into the firebox;
• b) providing solid masonry units sized and adapted to build:
• i) a floor in the retrofit firebox assembly,
• ii) a first sidewall on the first sidewall frame,
• iii) a second sidewall on the second sidewall frame, and
• iv) a back wall adjacent the back wall metal plenum member, but spaced apart from the back wall metal plenum member to provide an air gap between the back wall masonry units and the back wall metal plenum member;
• c) providing an air duct adapted to receive air from the room in front of and below the firebox, and to pass that air rearward into the air gap between the back wall metal plenum member and the solid masonry units positioned thereon;
• d) assembling the retrofit firebox assembly frame;
• e) mounting the solid masonry units on the assembled frame to construct a firebox retrofit system comprising:
• i) a firebox comprising:
• a) a solid floor;
• b) side walls extending upward from the floor to a flue;
• c) a rear wall having:
• i) a lower vertical portion,
• ii) a lower angled portion extending forward into the firebox and spaced slightly apart from the lower vertical portion to provide a space for air to flow into the firebox between the lower vertical portion and the lower angled portion,
• iii) a central vertical portion positioned forward of the plane of the lower vertical portion and spaced slightly apart from the lower angled portion to provide a space for air to flow into the firebox between the lower angled portion and the central vertical portion,
• iv) an upper angled portion extending rearward from the plane of the central vertical portion, and spaced slightly apart from the central vertical portion to provide a space for air to flow into the firebox between the central vertical portion and upper angled portion, and
• v) an upper vertical portion positioned rearward of the plane of the central vertical portion, and optionally spaced slightly apart from the upper central angled portion to optionally provide a space for air to flow into the firebox between the upper vertical portion and upper angled portion,
• wherein the lower central angled portion, the central vertical portion, and the upper central angled portion combine to form a smooth, symmetrical bulge in the back wall extending into the firebox;
• d) one or more air ducts effective to route air from the room to the space between the lower vertical portion and the lower angled portion, and from the room to the space between the lower angled portion and the central vertical portion;
• e) an open front;
• and
• f) an open top effective for directing air and combustion gasses from the firebox to a flue.