Two designs are presented for the adaptation of a spray-column direct contact heat exchanger for producing hot water in commercial applications from combustion exhaust. The designs are developed utilizing a numerical simulation of two and three-phase spray-columns which allow variations of key parameters influencing total heat transfer and which accounts for mass transfer effects. Design analysis shows that both on-demand and thermal reservoir systems are possible and that such systems are substantially less expensive than the classical boiler/shell-and-tube designs.