Abstract: A hot-water-furnace supplemental heater (60) replaces a manifold (48) used with a normal hot-water-furnace (10) having multiple furnace heating tubes (22). The supplemental water heater has a housing with a false bottom (66) and a supplemental tube (67) extending between a false bottom opening and a top-wall opening of the housing. A sidewall (62) of the housing and the false bottom form a manifold cavity (72) which collects combustion materials exiting from top ends (52) of the furnace heating tubes and directs them into the supplemental heating tube for heating water in a supplemental water heater cavity (68) defined by the sidewall, the false bottom, a top wall (64) and the supplemental heating tube. In a system, the supplemental water heater is placed in series with a furnace water heater to preheat water passing into the furnace water heater.