This heater uses a high efficiency, three-pass, double helical coil design. This three pass helical coil configuration cools flue gasses significantly before they contact the external shell, eliminating the need for internal shell insulation which reduces long term maintenance issues associated with internally insulated heater shells. Standard models are available from 500,000 to 75MM Btu/hr in both vertical and horizontal configurations. Base efficiencies can exceed 88% (LHV Basis), and with an optional economizer can exceed 93% (LHV basis).