Rocket mass design…need feedback

Hi , you’all.

I am interested in building a Rocket stove outside my house on the slab of my walkout basement, and then run the horizontal exhaust pipes through the foundation wall into the basement into a mass.

I was thinking I’d build the style of rocket stove where the riser goes into a 55 gallon barrel before the exhaust exits horizontally at the base.

The stove pipe would run inside to the thermal mass then loop back and run back outside and up past the barrel… whose heat would help encourage draw / air flow.

I was thinking of just building framing and pouring a concrete thermal mass , but I’ve read some say that it isn’t good long term with the heat. Also, a large cement bench would be difficult to remove.

I’d like to use materials easily obtained at the hardware store, and make something that isn’t impossible to later remove and structure that is accessible to repairs.

So, I was thinking of doing cinderblock structure / framing around the exhaust pipe loop and then filling in the space with sand. I’ve seen people say that sand isn’t the most efficient, but  for my purposes it doesn’t have to be perfect. I like this idea because it could be heated immediately since it’s not wet.

But I’m also concerned about using something like sand instead of something like cob or cement because sand doesn’t provide an airtight seal.  I feel like I could end up having some smoke or carbon monoxide seeping out.

My design is built on a number of things I would like to do

1. Id like to use the rocket stove barrel to help boil maple sap outside.
2. I have a design from online where the barrel is removable and was thinking I could eventually do a barrel with a water pipe coil going around it to heat water for a rocket stove hot tub.
3. Id like to have a radiant mass heater In my house.
4. Id like to be able to change my mind and remove it if I need to… might sell the house some day and someone else might not like my esthetic.


Ok. That’s my idea.

Plain old dirt will work better than sand, especially if you tamp it well. To seal the joints of your exhaust tubing, you could use aluminium tape (the "real" duct tape) and this should mitigate any CO leakage concerns.

Take care in designing a coil and loop system to heat water. You want the system to be at atmospheric pressure, vented at the top and with no way for a steam bubble to form anywhere in the line.

Overall, this sounds like a cool plan and great stacking of functions. Keep us posted as you start the build.

Hey Phil,

Thank you for your reply.

Reading through the forums I’ve seen many people comment that sand isn’t good. BUT I’ve also seen many links to thermal conductivity charts that all place dry sand at similar conductivity as dry clay (which is basically cured cob right? And cob seems to be the most popular go to for RMH.

Could you give me any additional reasoning behind why sand is not good when based on the numbers it seems to be close or comparable to clay?

Thank you

I think it's a function of the grain sizes, entrained air, and how much direct contact there is between particles. Clean sand is limited somewhat, because even though the quartz grains individually have good thermal mass performance, they don't have good conductivity from one to the next. Cob (and rammed earth) have better conductivity on this basis.

Right,

That’s what I read in other threads: air.

I just wanted to be sure. My experience with the beach is… that dry sand can be really really hot just from the light of the sun….but as I sit here and think about it, the sand down a few inches is much cooler….so what you say about transferring the heat between grains makes sense.

Thanks

A typical cob mix is mostly sand, with clay percentages as low as 5-10%. That clay binder is the secret sauce, because it fills a lot of the voids and conducts heat that would otherwise not move far thanks to the air. We have lots of black sand beaches here, and although the top is like a griddle on a sunny day, you only need to dig down a little ways to have something you can stand on.

Phil,

Since I have your attention, let me ask another question.

What would you say I should do my mass with?
I’ve seen Lincoln 60 fire clay and sand tossed around (one ‘rocket scientist’ said 3:1 sand to clay. Is that a good ratio?). Others say you can cheapen the expense of the clay by adding concrete rubble or rock.

Would adding rock be a bad idea? In my head large rock in the mass would be ok, but it seems like an excess of pea rock or gravel might propagate cracks or hurt binding 🤷🏼‍♂️

Rock is great. A sand/clay mix at a 3:1 ratio and anything from pea gravel up to some nice melon-sized lunkers will do a fine job as mass. Cracking can be alleviated by adding some straw to the cob and if you want a next level binder, put some cow or horse manure in the blend. If your local subsoil has a decent clay fraction, you could just use that. A little more labour, savings on the cost and transport.

For a mass if you have containing walls, pretty much any kind of dirt as long as it is not fluffy organic material or pure sand will work fine as a filler between gravel and stones in the mass. The more clay up to 20-30% the stronger it will be.

Stacking functions is great, but I see a lot of issues with the ideas you posted. Having the whole combustion core outside will lose a large portion of the heat generated to the open air, especially if you use a barrel over the riser. That will waste maybe half of all your heat. Boiling maple sap on the barrel top will also be very inefficient; you need the fire blasting on the bottom of the whole sap pan to do any real quantity. A purpose-built rocket evaporator is not difficult if you can build a RMH.

Making rocket cores for a few individual functions will get you much better efficiency for each task, and a basic J-tube or L-tube is not hard or very expensive to make. You might be able to make a portable J-tube in a metal skin that could be moved from heater to hot tub. A hot tub would need to be mounted higher than the core to have thermosiphoning; if you have ground at first floor level very close to the basement floor level heater, it might be practical to use the same core for both from one spot.

Hey Glen,

I appreciate your feedback.

Certainly heat will be lost through the barrel. That’s the intention, both to provide warmth outside on the porch and to work my syrup. I don’t see it as a full waste.

In older posts you had said,
“Barrel top center temperatures can get as high as 1000 degrees F though 500-800 is a more usual maximum, and the sides can be up to several hundred, especially near the top.”.

That is significantly hotter than the 219 degrees F needed for syrup.  I do small quantities. I have about a half dozen mature forest grown trees I tap. Enough for family and personal use.  Based upon you saying barrel temps of 500+ and my smaller needs, do you still believe the heat wouldn’t be sufficient?

Beyond that I know the exhaust gases will still have plenty of heat. Why not move the gas in doors and capture it? You say half of heat is radiated through the barrel , and I’ve also read it said 2/3rds. So, maybe that is more than I want lost there. What if I  build masonry or cob halfway or more up the barrel to provide some mass / insulation / slowing of heat loss in the barrel? I think that would help keep more heat In the exhaust gas.

Ultimately, while any single purpose designed device may be more efficient at that single task, I like the idea of doing a single build well and using it for multiple tasks even it’s less efficient.

I have friends who heat their home using an outdoor wood burning furnace. They feed it with black locust that they copice. I like the idea of having my fires outside of my house. Feels safer to me (I had some second cousins die in a house fire when I was a kid).

Ultimately, for me, the fuel (wood) is free. I have an excess of brush I’ve been clearing. Dead Ash, grey dogwood and plumb thickets taking over fields, stands of black locust, fallen storm damaged trees. I’m doing some programs with the department of conservation and USDA to get rid of invasive species and re establish prairie habitat as well as forest management which involves removing trees.  

The cost is less of an issue for me than my time. I’d like to do one build well. So, that’s why I’m asking for the feedback. I appreciate your knowledge and advice and hope you’ll continue to push back on my ideas and help me develop a plan that will work best for my goals.

I’ve sat and thought on the advice I’ve been given and decided in a few major changes.

I was thinking I could make something that was only semi permanent for home heating, but seems like I really just need to commit to something more rigid and monolithic to be functional.

And then my other purposes, it will be simpler to make a standard rocket stove that could be transportable.

With that said, I looking for resources on batch box design. The masonry work is enough for me, and I don’t think I’m interested in wrestling with designing a matchbox and burn chamber, and I am open to buying something someone else is fabricated, and has already proven the design of.  Any suggestions?

If you want to do a batch box (a good idea for outdoors as it will not need to be tended as often), there is extensive information including exact construction details at batchrocket.eu. You can follow one of the published designs exactly.

For just a few trees' worth of sap, the top of a barrel or a portable rocket stove may work well enough.

I think if you must build your core outside, a masonry bell with thick insulation outside may be the best bet. That would moderate temperatures for your circulating fluid heating, and keep a source of warmth after the fire is out. The insulation would keep the mass from losing so much heat, and let you do less work to keep it hot (allowing that money or fuel source is not an issue). Also, building the mass against the basement wall in a convenient location would let some of the "waste" bell heat go to the house instead of the air.

I've been putting off building a rocket stove. One reason is that I want it portable and hadn't quite decided on materials or design.  I just saw the Lorena and have now decided on a design!  I also want it to be portable so I can move it over into the shade, to another location or into a building for protection.  I'm not sure whether it should be on a skiddable base or on a small (and strong) trailer.