Abstract: A fluid heater includes a housing, a burner unit disposed in a bottom portion of the housing for burning combustible fuel, a combustion chamber disposed within the housing where the fuel is burned and a heat exchanger disposed substantially within the housing over the combustion chamber. The heat exchanger absorbs heat generated from burning the fuel and conducts the heat to a fluid to be heated. The heat exchanger includes a pair of spaced, parallel, stainless steel tubesheets with a plurality of tubes running therebetween and sealingly received within mating apertures in each of the tubesheets. A plastic front header and a plastic rear header are removably attached to the tubesheets distal to said tubes. The apertures in the tubesheets preferably have forged flanges for increasing the surface contact area with the heat exchanger tubes. The heat exchanger is corrosion resistant due to the combination of corrosion-resistant tubesheets, tubes and headers.