Just removed our old heater

[Penney H. & Mike E.]

Your gas line is probably 3/8" copper line with a 3/8" flare nut on the end where it was connected to the heater. You need to plug this nut with a 3/8" flare plug. You need two crescent wrenches to do this. It is also helpful to put some propane rated thread lubricant on the threads of the 3/8" flare plug. This will make a good seal without having to over tighten the connection, which could possibly crack the flare nut. Feel free to ask questions.
Mike, the propane technician.