Abstract: A high turn down ratio furnace is attained by plural burners or burner stages having control valves operated by thermal sensors. Heat output control is by a variable pressure device for the fuel supplied to the burners and individual on-off valves for some of the burners. Burner arrangement and/or design and control enables one primary burner to ignite a secondary burner so that it can ignite other secondary burners, so that only one flame ignition device at the primary burner is necessary.

Abstract: The arrangement of the individual air dampers with respect to the heating coils in a vertical tube face and bypass air treatment unit such that the dampers each move laterally across the coils results in a substantial reduction in the number of moving parts required to control air flow around the coils. Selective proportioning of the air flow is accomplished by splitting each damper into two sections and permanently fixing one section with respect to the coil set while allowing the other section to slide across the face of the coil set in mating relationship with the fixed section.