I'm not a big fan of using rain water, simply because it has usually run down dirty, petroleum-based, roofing shingles, thru dirty gutters, and simply put, has been exposed to the elements, maybe TOO much (rain, is, after all, nature's way of cleaning the air) ... rotting leaves, etc., all contribute their own ingredients to the mix, some good, some not so good ... if you have a collection system that keeps it clean, I guess you would be OK ... basically, rain water is 'naturally' distilled water, which then picks up contaminants as it falls ('washing' our air for us, absorbing contaminants) ... of course, Mother Nature drinks it, so I guess it can't be TOO bad (but, rumor has it, she will drink 'anything') ... :smokin: