a zero clearance fireplace of the type adapted to be installed against an outside wall of an interior space or room to be heated is provided with six walls which comprise four substantially vertical walls , a top wall and a bottom wall at least two of said vertical walls having airtight glass side walls connected to the frame structure of the fireplace . the bottom wall , the vertical side wall and the top wall are provided with plenum structures which are inner connected to form a heat exchanger . the rear wall is provided with a vertical inner plenum in which outside fresh air is introduced and conducted into a bottom inner plenum below the combustion chamber of the fireplace to provide fresh outside air to the gas burner system in the combustion chamber . the top wall of the fireplace over the combustion chamber is provided with a tapered shape which diverts the exhaust gases from the gas burner in the combustion chamber directly into an exhaust stack so as to generate a high velocity exhaust of the combustion gases which generates a high draft aspiration effect through the combustion chamber .