I've got a 200K NG Raypak coming tomorrow. There is already gas line running from the meter to a manual shut off valve (which is capped) from a prior heater. My question is can I use flexible gas line to finish the run between the shut off valve and the heater. Probably only be about 2 feet. Also, does there need to be a rigid (iron) nipple coming directly off the heater if I do actually get to use flexible piping? Thanks for all the help.