where to find a design for a low cost, long duration heater?

[steveh2112]

hi, i've been watching countless videos on the tube about rocket mass heaters but i still have any questions. it seems the devil is in the details with these things and i worry if i design my own, i'll just waste time and money, so i'm hoping to find an actual design i can build. here's my thoughts.

1) its for a 8x12 well insulated greenhouse so it doesn't need to be very hot but i would like to fire it just once a day if possible, so lots of mass
2) i want to save money and i'm not great at metal work but i can do brick work and general construction, so i'd prefer a fire brick combustion chamber
3) i have plenty of stones and dirt in my yard i'd like to use for the mass part if possible, maybe cement the stones into a bench and fill with dirt? would that work?
4) does the barrel need to be exposed metal? can it be put inside a box made of something like cement board to help slow down heat release?
5) does the insulated riser inside the barrel need its own air supply for secondary combustion?
6) what diameter flu pipe should i use in the mass bench and does it need to go out and back with a U turn at the end? (i'm thinking  the bench will be about 8' long, 2-3' high and wide, although i could make it U shaped and wrapping around the entire perimeter of the greenhouse)
7) i assume air intake should be from outside air right to reduce cold air suction into the greenhouse. i'm think i'd like a J style wood inlet on the outside of the greenhouse so i can fuel it up without opening the greenhouse door and i don't want to bend down to  add wood if possible.
8) should there be a venturi at the end of the primary burn chamber where it transitions into the vertical riser?
9) should the heat riser be 1.5 times the length of the primary burn chamber (i saw that in a video)

i'm thinking something like this naturalhomes.org/permahome/rocket-mass-heater-basics.htm
but the A at 45 degrees and poking out of the greenhouse north side wall through an insulated, fire proof opening.
in that picture is looks like the heat mass surrounds the burn chamber and base of the barrel too, its that all ok?

i realize this is a big list of requirement so i will probably not find an off the self design, but if there are any experts our there that might sketch something for me with dimensions, i'd really appreciate it.

thanks

[nacho]

I do not know about J tube designs, but if you want to build a batch box rocket, you have a lot of designs in Pete's website batchrocket.eu BBR are different, you feed the fire twice a day, big clean fire, and you decide the dimensions you have to heat, and it is proportional to the size of the box.
you can use stones, dirt, everything you have in mind to do the mass. But is very important to spend some money in refractory bricks to make the box and the riser, and isolating them with ceramic blanket

[steveh2112]

thanks, that's a great website

[masonryrocketstove]

Jan 7, 2024 7:30:53 GMT -8 steveh2112 said:

hi, i've been watching countless videos on the tube about rocket mass heaters but i still have any questions. it seems the devil is in the details with these things and i worry if i design my own, i'll just waste time and money, so i'm hoping to find an actual design i can build. here's my thoughts.

1) its for a 8x12 well insulated greenhouse so it doesn't need to be very hot but i would like to fire it just once a day if possible, so lots of mass

The larger the mass, the hotter you’ll need to fire it, because the heat distributes through the mass. General rule of thumb is More mass = more required heat for the same mass surface temperature. With a JTube, you can build a larger core to charge a large mass quickly, or you can build a smaller core and fire it for a longer duration to charge a large thermal mass. The latter option is usually cheaper, but it does mean you have to stay at the stove and continue feeding fuel for longer each day. Tradeoff: pay a higher daily cost of more of your time? Or pay one-time higher material cost and earn back more available time each day for doing other things?

2) i want to save money and i'm not great at metal work but i can do brick work and general construction, so i'd prefer a fire brick combustion chamber

You definitely need a firebrick combustion chamber (the J-Tube). Steel burn tunnels and risers spall and burn out pretty quickly, especially once you insulate around them to get the temp up high enough for clean combustion and efficient heat transfer. The steel bell over the riser shouldn’t require any welding. The firebrick is one place I wouldn’t advise anybody to skimp on. If you buy good firebrick for tunnel and riser the first time, you’ll save money because you won’t have to keep replacing low-duty brick that crumbles each season or two.

3) i have plenty of stones and dirt in my yard i'd like to use for the mass part if possible, maybe cement the stones into a bench and fill with dirt? would that work?

Portland Cement might work if it’s far enough downstream of the combustion unit. Lime mortar and Portland cement don’t hold up well to high heat, condensation, or big swings in temp while thermal cycling up and down. Using material like cob or clay-sand mortar and brick or stone allows the mass to flex a bit with heat expansion, without cracking.
Some natural rock works better than others. Basalt and granite are good cheap / free in a lot of places. Limestone isn’t great. heat capacity and thermal conductivity are the main things.

4) does the barrel need to be exposed metal?

Yes. Top and at least partway down the sides.

can it be put inside a box made of something like cement board to help slow down heat release?

No. The steel barrel design is pretty important to allow rapid heat exchange right at that spot above the riser to keep from burning through the steel, and that shock-cooling transition from barrel top down the sides is what drives the draft through the mass before the gasses have made it all the way through to the chimney end, (before the chimney can contribute to draft.) otherwise the combustion unit will choke and smoke back on cold startups.

5) does the insulated riser inside the barrel need its own air supply for secondary combustion?

all combustion air comes down the feed tunnel with the fuel. If built to the specs of the design’s ratios, a JTube air intake is self-regulating.

6) what diameter flu pipe should i use in the mass bench and does it need to go out and back with a U turn at the end? (i'm thinking  the bench will be about 8' long, 2-3' high and wide, although i could make it U shaped and wrapping around the entire perimeter of the greenhouse)
7) i assume air intake should be from outside air right to reduce cold air suction into the greenhouse. i'm think i'd like a J style wood inlet on the outside of the greenhouse so i can fuel it up without opening the greenhouse door and i don't want to bend down to  add wood if possible.
8) should there be a venturi at the end of the primary burn chamber where it transitions into the vertical riser?
9) should the heat riser be 1.5 times the length of the primary burn chamber (i saw that in a video)

i'm thinking something like this naturalhomes.org/permahome/rocket-mass-heater-basics.htm
but the A at 45 degrees and poking out of the greenhouse north side wall through an insulated, fire proof opening.
in that picture is looks like the heat mass surrounds the burn chamber and base of the barrel too, its that all ok?

i realize this is a big list of requirement so i will probably not find an off the self design, but if there are any experts our there that might sketch something for me with dimensions, i'd really appreciate it.

thanks

For the rest of these questions, I’m gonna refer you to a really good YouTube instructional video posted not long back. I think you’ll get most if not all of your questions answered there. The guy does a great job explaining how the components work and why. And the one he builds is done on the cheap, too. Once you see that video, I think you’ll understand why a lot of the other questions might apply to batch boxes, but aren’r something you need to worry about for the simpler, easier-to-build JTube design.

donkey32.proboards.com/post/39191

After seeing that video, you should be able to sketch out a design for a JTube that fits your space and your needs. If you have specific questions, I’m sure people will be glad to help out.

..But you might be waiting a long time if you’re asking someone to make a design sketch for you ..when you’re the only one who knows your own heating needs, shape/space limitations, chimney placement options, available materials, budget, construction / fabrication skills, tools, etc.. all that kind of design parameter stuff will be specific to your build. None of us know those things for you.