The invention relates to the technical field of gas water heaters, in particular to a condensing heat exchanger. A condensing heat exchanger shell comprises a housing and a fume hood. The housing is connected with the fume hood. A lower fume guide cavity and a heat exchanging cavity are formed in the housing and arranged in parallel. A heat exchanging body is arranged in the heat exchanging cavity. An upper fume guide cavity and a flow guide portion arranged in the upper fume guide cavity are formed in the fume hood. The upper fume guide cavity corresponds to the lower fume guide cavity. The flow guide portion is arranged in a fume channel in the heat exchanging cavity. Under the effects of the structures, fume can be guided to flow downwards from the top, it is guaranteed that the flow direction of fume is consistent with that of condensate water during heat exchanging, and liquid film is prevented from being formed on heat exchanging surfaces of the heat exchanging body. In addition,water flow channels of a heat exchanging body assembly are matched with water grooves of an upper water sealing cover and a lower water sealing cover correspondingly, water flow flows in the heat exchanging body, a water path structure of a condenser is simplified, and no welding technology is needed.