A heat exchanger core assembly includes a frame assembly and a plurality of core bundles positioned within the frame assembly. Each core bundle includes at least one row of tubing defining at least one end of the core bundle, an edge seal gasket to seal the core bundle from an adjacently placed core bundle, and an indoor face seal gasket to seal the end with respect to the frame assembly. A method of replacing a core bundle of a heat exchanger core assembly includes identifying a core bundle requiring replacement, removing the core bundle by accessing the core bundle and sliding the core bundle with respect to the remaining plurality of core bundles from the frame assembly, and inserting a replacement core bundle in a space defined by the removed core bundle.