Where residential compressed natural gas (CNG) fueling appliances are located indoors, a methane gas detector shall be located within ______ inches of the highest point of the room or space.

A methane gas detector must be positioned within 6 inches of the highest point of the room or space where residential compressed natural gas (CNG) fueling appliances are installed indoors because methane, being lighter than air, tends to rise. By placing the detector close to the highest point, it maximizes its sensitivity to any accumulating gas that may present a danger. This ensures prompt detection of potential leaks, which is crucial for safety in residential settings. Proper placement of the detector is essential to safeguard the environment and the occupants by quickly identifying hazardous gas levels before they become critical.