Before people start comments about rocket mass heaters, PLEASE DO YOUR RESEARCH. Here is a web site for an educated answer: https://www.richsoil.com/rocket-stove-mass-heater.jsp    Erica and Ernie Wisner are without a doubt two of the most knowledgeable people around when it comes to thermal mass and rocket stove mass heaters. There is really no safe practical way to modify a conventional wood stove to add thermal mass - best solution is a wood stove design that incorporates thermal mass. I have NO interest with these individuals, I just believe in the correct information being available for all.

mad science brain