Abstract: An economizer system for preheating feed or return water in a high pressure steam or hot water smoke-tube boiler having a smoke-heated convection-effecting portion. The smoke tubes at the end of the smoke tube convection-effecting portion are surrounded by a jacket with staggered baffle plates penetrated by the smoke tubes so as to form a wavy or serpentine flow channel for the feed or return water in forced counterflow to the flue gas. The jacket may be encircled by a second jacket forming a chamber which may be filled with insulating material or form a preheater in the water supply system to the flow channel of the economizer.