I first heard about rocket mass heaters a couple of days of ago and the first thing I thought was I don't want cob inside my house. What's cleaner and has more heat capacity? WATER! I think it's a great idea. It would also be advantageous to use your water thermal mass as a hot water source / potable water reservoir. Rust was also something I thought about and stainless steel is the obvious solution. Stainless steel 55 gallon barrels are very expensive (> $700). As an alternative what about stainless steel beer kegs (~$150 for 15.5 gallons). It would probably be easier to find used kegs as they are way more common that 55 gallon stainless steel drums. Beer kegs are designed to be under a certain amount of pressure. You could have a check value to release anything over your desired water pressure rating keeping enough pressure for hot water outlet. Also have a manual pressure release valve inside for humidifier. You may consider a manual pump as well to have water output if you aren't heating much during the summer.
