The efficiencies of illustrative embodiments of vertical center flue type fuel-fired base water heaters are improved by installing in the water heaters variously configured tubular condensing type secondary heat exchangers of smaller diameters than the associated center flues. The efficiency increases 5 are achieved with components and manufacturing processes similar to those utilized in the base water heaters and secondary heat exchanger materials similar to those in the tank portions of the base water heaters. C)l (00 Coo 0o (NJ CC
