Patent ID: 8021143

Claim:
A method of operating a duct-type furnace, comprising the steps of: a) providing a plurality of burners arranged in at least two groups, each group having its own gas control valve connected to a source of combustible gas; b) providing a plurality of heat exchange tubes associated with said burners, said heat exchange tubes having inlets and outlets; c) providing a collector communicating with said tube outlets and divided into first and second collector sections; d) communicating said collector sections concurrently with an inlet to an induced draft blower; e) firing one group of burners while deactivating the burners of the other group in order to provide a substantially 50% output for said furnace; and, f) modulating the gas output to said one group of burners to provide a 4:1 turndown ratio for said one group of burners thereby providing an 8:1 turndown ratio for the overall furnace.