Modular heat exchanger with sections interconnected by connectors

A heat exchanger, comprising a series of interconnected modules, each module comprising at least part of a water duct and part of a flue duct, wherein at least the parts of the water ducts of successive modules are interconnected, forming at least two continuous water ducts through and/or along a number of said modules, wherein water duct parts of two adjacent modules are interconnected by a connector inserted into an opening in at least one of the modules forming a sealing connection, wherein the modules each further comprise a flue inlet and a flue outlet, and wherein the part of the flue duct of each module connects said flue inlet with said flue outlet, wherein an inlet for the part of the water duct in said module is closer to the flue outlet than to the flue inlet and wherein an outlet for the part of the water duct in said module is closer to the flue inlet than to the flue outlet.