RMH with a floor mass?

I just discovered rocket stoves and rocket mass heaters, and am in love with this idea. My wife and I are making plans to get a bit of land up North, and setup a cabin to eventually be our retirement home. It will serve as a vacation spot in the mean time. We are looking at ways to do this efficiently, and as sustainable as possible, so it will last generations, and remain inexpensive and low impact on the environment.

Anway, I was thinking of the annual temps in Northern AZ, and how the winters are cold and snowy, and the summers can still be pretty hot. Instead of a big bench for the heating mass, what about an entire floor? If the structure is one big room, say a 20x20 cabin, would it work effectively to build a Cob type floor? I'm envisioning running a network of the exhaust tubes across the entire floor space, then covering with Cob or something similar that will hold up to walking on it etc.

My thought is, that way, it heats up the entire place in the winter, with the whole big room getting radiant heat. Then during the summer, that nice cold clay floor should keep the whole place cool. Is that kind of configuration for an RMH possible and as efficient as the big bench design?

Welcome to permies. com Gyan!

I think it will be very efficient with the floors idea. Floors are big, and they will have a greater thermal mass than just a plain bench. With a greater thermal mass, that means you'll retain the heat longer. Here are some of Paul Wheaton's videos from his YouTube channel that I think you might enjoy (I set the playlist to the greenhouse video which does a similar thing to what you are describing):

Check this thread.

https://permies.com/t/33247/rocket-stoves/Milk-mass

Most excellent, thank you.

So then, is a concrete slab a good thermal mass for this situation? I wasn't sure about the strength of a cob flooring.

Gyan Gearson : Do not plan on using any Concrete products within the Entire Rocket Mass Heaters RMH's Rocket Burner, i.e the Feed Tube, Burn Tunnel, or Heat Riser,
or the RMH's Base, or the Floor under them, and also the First 4 - 5 feet of the start of the horizontal Exhaust Pipes run through the Thermal Mass, It will not take the
heat ! The 'Lime' found in all Portland types of Cement/Concrete Found in these areas will fail at temperatures well below the temperatures commonly found there !

This is where I strongly Recommend that you goto - -> rocketstoves.com to Download your PDF Copy(s) of the Brand New 3rd Edition of Evans' and Jackson's
Great Book 'Rocket Mass Heaters'

With ~100,000~ RMHs built World Wide, Most have been built following "The Book '', and 95% of all the First time builds ( that worked ) were constructed by D.I.Y
types much like yourself after reading this very complete Booklet. After You have read "The Book " yourself, you will be able to come here to Permies.com and
confidently talk to your Fellow Members, using the same terms to describe the Shape, Size, and Orientation of the RMHs parts as they relate to themselves and the
Whole Build !

I can freely promise you that your study of the book will save you large amounts of time, money, and frustration, over attempting to D.I.Y. a Complete RMH Build
without this valuable resource. I can even promise that even after you have completed your first RMH that this book will still have more to teach you !

For the good of the Crafts ! Think like Fire, Flow like a Gas, Don't be a Marshmallow! As always your comments/questions are solicited and are welcome ! Big AL

Alan, have you tried concrete for the mass?

Concrete can cope with 400 celcius before it degrades. If the exhaust of your rocket after the barrel is above 400C°, it's not running right. It's no good for the core if pure portland is used. But proof of using portland mixes in crucibles and forges is about on the internet.


Gyan, for added safety, you could make the transition area from barrel to mass out of metal even better stainless steel, joining to a pipe. And pour concrete all over that. Concrete if it goes over 400C° flakes off, and if you have metal, it might flake off, but it will not fall blocking the exhaust. It is good practice to make a cleanout at the transition area too, so you can clean fly ash accumulating there.

Remember, if you don't want to loose your heat, you need to insulate bellow and on the perimeter.

Concrete releases the heat a smidge faster than cob. IIRC it doesn't take as many BTU per kilo, or volume than cob. But it doesn't insulate like cob does. So, they're prety much hand in hand thermal mass wise. It's far less green tho.

Increasing the capabilities of cob and concrete is easy thought, add stones to the mass. Concrete mortar has gravell in it naturaly, while cob hasn't. Add your stones near the pipes first, but the more you put in there, the better. I think 70% stones, 30% concrete mortar is doable. Plus, you can do your floor in the same time, using stone slabs of some kind. But the same goes with cob. The heavier the stone is, the better it will hold heat.

Another nice trick, if you don't mind kneeling to light your fire, is to set the core below floor height (still insulated for sure!) with the mouth of the feed tube at floor height, like that, you can brush the dust directly into the pit it forms Very handy in a woodworking workshop.

Hello,
If you plan to use your floor as a thermal mass be sure to insulate the inside mass from the outside soil (all four sides and the bottom ). Avoid this step and loose heat that would otherwise be warming your space.

We have a sub-floor RMH in our greenhouse.

Satamax Antone wrote:Another nice trick, if you don't mind kneeling to light your fire, is to set the core below floor height (still insulated for sure!) with the mouth of the feed tube at floor height, like that, you can brush the dust directly into the pit it forms Very handy in a woodworking workshop.

I will do a floor mass, and I have planned to set the core below floor level as you say.
Or else the horizontal exhaust pipe would have to go down.

I have even planned to open the floor a little bit more, so that I have room for my legs and can sit down on the floor near the fire.
I still think that LOOKING at a fire is a PLEASURE we have become genetically meant for!!!

I just can imagine to have the orange light of the fire at floor level, hmmmm!
And a mirror above?

And I have found pumice stone not far from home, to insulate under everything.

If a RMH is not meant to warm up birds, let's not warm up moles!!!