Abstract: A heater for use as a fireplace insert, but which may be modified to function as a free standing heater. The heater includes an inner firebox having inner bottom, top, rear and opposite side walls interconnected along adjacent marginal edges to form a combustion chamber therein. An outer housing is provided for snug reception within a fireplace and the housing includes outer bottom, top, rear, front and opposite side walls. The firebox is received within the housing with at least major portions of the inner bottom, top, rear and opposite side walls spaced inwardly from opposing portions of the outer bottom, top, rear and opposite side walls and the outer front wall closes the front of the firebox. Circulating air inlet structure opens through the front wall into the space between a first set of opposing surfaces of corresponding inner and outer walls and air outlet structure opens through the front wall from the space between a second set of opposing surfaces of corresponding inner and outer walls.