Original heating trade vintage catalog for C. A. DUNHAM COMPANY, was located in Chicago, IL. Manufacturers of Differential Heating, Concealed Radiators and Unit Heaters. Design of the heater with Relation to the Efficiency of the System, Influence of the Two-Speed Motor on Selection of Heater Sizes, Electric Motor Data, Type V and Type M Heaters, Vacuum Heating System. Disbound from a large 1931 industrial heating catalog, this unique old nine page technical brochure would display wonderfully matted and framed. A marvelous addition to your retro heating equipment library. Pages are detached and may contain a small amount of residual glue on bound edge. Slight damp stain on outer edge.