The VariMax Once-Through Heater is a high-efficiency, industrial, indirect-fired gas heater used to heat process air streams without contaminating the air with the products of combustion. They are particularly useful for recirculating ovens, makeup air applications in cold environments and industrial processes with moderate temperature rises. The heaters are available in six sizes from 500 to 3,500 MBH, with a minimum 3:1 turndown ratio (exact turndown depends on condensing conditions in the heat exchanger) and are capable of handling airflows from 2,000 to 65,000 SCFM. They can produce air temperature rises from 20°F to 150°F, with a maximum outlet temperature of 600°F. Efficiencies are in the 70% - 88% range, depending on the flow volume, inlet temperature and firing rate. The heaters may be placed in series or in parallel as long as the temperature restrictions are not exceeded. They can serve as a stand-alone product or be integrated into a fully packaged system with fans, dampers, filters, and controls.