An indirect gas-fired condensing furnace assembly and method includes aprimary heatexchanger, a secondary heat exchanger, and a tertiary heat exchanger. Thesecondary heatexchanger assembly may be an intermediate single-pass tubular heat exchangesection madefrom a corrosion-resistant material. The tertiary heat exchanger assembly maybe a single-passtubular heat exchanger section with a corrosion resistant material. Thetertiary heat exchangerassembly may include a plurality of fins. The primary heat exchanger assemblymay include aplurality of aligned tubes wherein each tube includes a first straightportion, an intermediateportion, and a second straight portion such that the primary heat exchangertubes surround thesecondary and the tertiary heat exchanger assemblies such that the airflow isconfigured totraverse the primary heat exchanger assembly, the secondary heat exchangerassembly, and thetertiary heat exchanger assembly in various directions such that a risk ofcondensation freezingwithin the secondary and tertiary heat exchange assemblies is reduced.
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