Hi Everyone. I reeeeally want to build a rocket mass heater for my living room. It's a wood floor, with typical wood joists and a basement below. (Massachusetts)  I want to install the rocket mass heater in my existing fireplace chimney, but I'm concerned that I'll need (lots of) additional support installed in the basement to carry the weight of a RMH above it.

Has anyone installed a RMH in this condition before? What materials did you use to keep the weight load "light"?

Thank you!

https://www.amazon.com/Rocket-Heater-Builders-Guide-Step/dp/0865718237/ref=sr_1_1?ie=UTF8&qid=1517331374&sr=8-1&keywords=rocket+mass+heater

Erica and Ernie's book on rocket mass heaters has a section on building on wooden floors. Basically the thing needs extra support as any mass you have is going to be heavy. Lightweight materials tend to insulate rather than store heat. Buy the book and find out what kind of extra joist support you will need. Good luck

If you take the mass out, you seriously limit your ability to store and slowly release the heat.

The core can be made lightweight, and skinned with just metal, as seen in Matt Walker's designs using low density fiber board.

However, heat storage requires mass.  

Is the existing chimney/fireplace brick/masonry?

If it is, you could possibly use IT as a bell efficiently, provided that you insulate the exterior side, so that you ONLY lose heat from that bell into your structure...

You would also want to cap that bell inside the chimney so that the heated bell area of masonry does not extend vertically past the insulated portion of your living space, and REMOVE any insulation in walls that may be on the interior, heated space side of the chimney (above a mantle, or in a second story.

  This idea may or may not be applicable to your situation, but could allow you to use the existing mass and support structure of an existing masonry chimney if you have one.

  Would need more details as to how much of the mass is exposed to the interior heated space and other info to determine whether and how such a setup would work, and whether it would be worth it, given your chimney's design and location within the structure.