CSA/ANSI 3.21, Industrial gas-fired natural draft heaters for installation in non-hazardous and hazardous locations in oil and gas process applications, is arranged in two parts: a) CSA/ANSI 3.21, Part 1, covers industrial gas-fired natural draft heaters for installation in non- hazardous locations in oil and gas process applications; and b) CSA/ANSI 3.21, Part 2, covers industrial gas-fired natural draft heaters for installation in hazardous locations in oil and gas process applications. This Standard (Part 1) applies to newly produced industrial gas-fired natural draft heaters (see Clause 3, Definitions) utilizing new and unused parts and materials intended for indoor and outdoor installation in non-hazardous locations in oil and gas process applications with a maximum input rating up to 12 500 000 Btu/h (3750 kW). The requirements of this Standard (Part 2) apply only to heaters produced in accordance with CSA/ANSI 3.21, Part 1, and intended for use in Zone 2 [Class I, Div 2] Hazardous Locations.