In a substation building, the main CO2 gas or inert gas header line is typically positioned above the HVAC duct. This arrangement helps ensure that any potential leaks from the gas header line do not interfere with the HVAC system and allows for proper ventilation and safety measures. For specific standards or specifications, refer to industry guidelines such as those provided by the National Fire Protection Association (NFPA) or the International Building Code (IBC), which often include detailed requirements for the installation and placement of gas systems in relation to HVAC systems. Additionally, consulting with engineering standards specific to your region or the manufacturer's recommendations can provide further guidance.