This firebox is made to house a gas-fired unvented burner which must be purchased separately. When installed this firebox has a 36 inch front opening. This firebox is designed with ceramic fiber firebrick that looks wonderful and helps to radiate heat into the room as well as a radiant design with heat-circulating air ducts. This item can be installed with non-combustible material up to the opening of the fireplace for true masonry. The framing dimensions for this item are 41-1/4 in. W x 37-3/4 in. H x 17-3/4 in. D. Note: This product is for use only with a listed gas-fired unvented decorative room heater not to exceed 40,000 BTU/hr.