A natural gas line is typically considered a part of the plumbing system rather than the HVAC (Heating, Ventilation, and Air Conditioning) system. The plumbing system in a building is responsible for the supply and distribution of water, as well as the removal of wastewater and gases. This includes the piping infrastructure for water supply, drainage, and gas lines.
While the HVAC system may utilize natural gas as a fuel source for heating, such as in furnaces or boilers, the gas line itself is considered a separate component and falls under the purview of plumbing. The gas line brings natural gas from the utility source or gas meter to the HVAC appliances or other gas-powered equipment within the building.
It's worth noting that the installation, repair, and maintenance of gas lines should be carried out by licensed professionals who specialize in plumbing and gas fitting. They have the expertise and knowledge to ensure the safe and efficient operation of the gas line system, adhering to local building codes and regulations.