codes are for the manufactures, dipping a coil in a tank of hot water doesn't change anything. altering an approved design does and only if it is stamped. [ not every tank you buy is stamped look at the air compressor you bought from HD] every one who has modified a propane tank for storage has violated the stamp that the tank was produced under. technically it would need to be re-certified for the application. History lesson -- these codes came about when steam boilers exploded on a regular bases killing lots of people, a standard was required so that pressure vessels design was consistent. back then every thing was riveted and used stay bolts, then weld designs came along and being new technology it had it's problems for awhile also. not sure on current code but it used to be anything less then 6 inch diameter was not considered a pressure vessel and did not require a stamp & pipe over 6 inch installed in an approved matter didn't either.