Rocket mass heater on concrete basement floor and other building practice questions.

I'm concerned about the heat from the core damaging my basement floor. I am using manufactured refractory brick(thin kind) and plan to put a layer of pavers or something under it. Are there options, materials or practices I can implement that will put my mind at ease?

I plan to lay the mass directly against the floor up to the wall so the walls themselves might act as a mass. Is this a good/bad idea with a conventionally constructed basement foundation. It will be in the corner. Would it be better to leave a gap between the mass and foundation wall?

I also plan to run the chimney out the basement window that is above where I plan to put the bell. I've had to adjust my plans(in my head) and move the bell to the left so I can angle the insulated, double-walled stove pipe through a thimble to be installed in the basement window and not block the bedroom window above.

How far to the side of the window do I need to make the chimney? It will need to run all the way from the basement to the roof. I think I'm wrong about this but is there any concern that a too-tall chimney will stall if the flue gas gets too cold as it rises? I feel like the heat from below will do fine pushing the cooled air through the rain cap- even if it does get down to ambient temps, but want to check with the experts. I got only the one section of the double walled stove pipe.

Should I use kaowool insulation around the heat riser even though I'm using refractory brick? should I also use kaowool as an expansion joint between the cob and and core? I saw a guy doing this on youtube and don't know if it's good, bad or indifferent but it seems to make sense to me.

I have an idea to place rebar or other metal against the horizontal flues and into the mass so that maybe the heat will spread through the mass faster, like heat fins on a radiator. I also don't know if this an okay idea.

My only goal with this project is to keep the expensive, natural gas, central forced air furnace from running. The gas company seems to double the price every year and I'm becoming sick of it. Save money and stop using fossil fuels as much.

Thank you!

Hi Sy;
Let me try to answer some of your questions.
To protect your concrete floor you have options.
First, you can use 2x4s and create a 3.5" box below your RMH.
Fill that box with a mix of perlite & clay, this will insulate your floor.
Another option is to lay clay bricks flat on your floor and sit a double layer of cement board on top.
You would still want a thinner layer of cob to protect the cement board from deterioration.
This method allows warm air to circulate under your core, although it does create a dust bunny breeding ground...

Next, leave at least a one-inch air gap to the outside wall, you can not allow your RMH to attempt to heat the ground (wall).... the ground wins every time!

Next, once outdoors your chimney must be fully insulated, all the way to the top, (very expensive).
Any non-insulated pipe outdoors will be one giant headache trying to keep a draw going.
Your view is the only question about how close to your window you can be.

Yes, wrap your riser with kaowool.

Adding rebar will not be of any great benefit at all.
You started out mentioning a bell, but now it sounds like you are thinking of building a piped mass.
Two completely different designs.
You should also plan on installing a bypass in your system to assist with cold start-ups.


No need to wrap your core if it is cob-covered.

Thank you for responding!

I think I will try the clay/perlite mix under the burn chamber. It seems like the most accessible option for me. I would prefer to avoid the dust bunnies if I can. I have 2 dogs, 5 cats and 3 bunny-bunnies in a ~950 square foot, 2 floor house. I'll have to add some safety features for them. maybe a convection shroud and chicken wire dome over the stove fan?

"Next, leave at least a one-inch air gap to the outside wall, you can not allow your RMH to attempt to heat the ground (wall).... the ground wins every time!"

This was my first instinct (1ft) but I have started second guessing myself with the project coming up so soon. Is laying the mass directly on the floor okay? I had the idea to lift it slightly with semi loose bricks and flat pavers to prevent heat loss to the floor.

"You started out mentioning a bell, but now it sounds like you are thinking of building a piped mass.
Two completely different designs."

I think I am confused at this point. The attached images are pretty close to where my inspiration comes from or have features I want to incorporate into my design.

I have 2, 55 gallon drums set aside for this project. 1 is for what I'm calling the bell and the other will be cut in half to make the manifold, headspace permitting. I would be willing to shorten the whole thing and make the manifold and bell a 2-in-one type deal if I need to but it's ~8 feet from the floor to the ceiling (floor joists above) and I plan to use a metal sheet above as a heat shield, if necessary. Also a stove fan but I doubt that will help with safety.

I know that the fewer bends, the better the flow. but I intend to make a "sectional couch" similar to the one in the picture. I think I see some design flaws there. It looks to me like the mass pipes are angled incorrectly(or it's the camera angle). And I will bring the flue all the way back to the bell so it can pick up some more heat just to help the draft.

I want the mass to be big enough to lay flat or sit on( ...well, both. I want to sit multiple people. I'm 6'2 being generous for the design's sake) with the feed tube towards the user so I can sit comfortably while working it and watch tv or something.

"You should also plan on installing a bypass in your system to assist with cold start-ups."

I had actually scrapped this from my design for simplicity. My next idea was to put a clean out under the window and burn some newspaper in there. Now, having typed it out, I feel like it's a dumb idea. I made a prototype in my chicken coop with... tactically acquired... cinder blocks from the neighbors yard I cleaned up and a $15, 55 gallon drum. All the flue work is cobbled together from half cinders, pavers and mud to seal it. I'm pretty proud of it considering the lack of resources I had at the time.  

My concern is that it will be forgotten and left open for a full burn cycle; as what Paul Wheaton described when talking about his pebble style rocket mass heater of science. I tried to think up an arm/lever with a feed tube cover on the end. When you place the cover over the feed tube that is what opens the bypass. and when you push it to the side, that closes the bypass. I mean fully divert from one to the other.  

Are there pre-made bypass parts or will I need to fabricate my own?

Is sealing the flues with aluminum tape a go or no-go?