I plan to install a freestanding Martin fireplace heater in my livingroom. It's BTUS are 24K---40K Natural gas. The 3/4 supply line enters my basement and transitions into a 1/2" within about 3" foot. The heater will be installed upstairs with a total run of 21". I would like to use 1/2" or 3/8" black iron pipe but It would be MUCH simplier to make this run with 3/8" copper. Would this be acceptable?