Abstract: Modular discharge structure for closed gas appliances having an inner end and an outer end, comprising: an outer tubular casing; an inner gas exhaust pipe, being placed inside the casing to define an annular supply line for combustion air therewith; a combustion air supply hood, forming a connection between the outside air and the annular supply line, and a combustion gas exhaust hood, forming a connection between the outside air and the gas exhaust pipe, and further comprising retaining means for axially keeping together, in series and in a mechanical and releasable manner, the casing, the combustion gas exhaust hood and the combustion air supply hood on the gas exhaust pipe.