Buddies and I built a Hottub in 2014 for $75 including building a RMH on a skid. Used cinder blocks, commercial dust collector pipe, copper piping, sand, a barrel, Roxul insulation and a free Hottub with some minor leaks and a bad pump. It was a little redneckish but it heated a Hottub full of 10C water to 40C in 7 hours. Here's a few pics. I believe we measured over 700F inside the rockets with an infrared thermometer. Not sure how accurate it is but all I know is that thing got hot! Definitely want to build a RMH for the garage. I have video too but can't post it here unfortunately.
