A Few Things I Have Noticed
My greenhouse is not yet in production, and so I have not been too aggressive about running my Rocket Mass Heater thus far. There is enough passive solar gain, and the evenings are still “warm” enough that it’s generally staying above freezing. Mostly, I will run the Heater if I am going to be working in the greenhouse on getting the next step taken for my aquaponics. There isn’t much sense is burning wood unless that energy is going to be put to good use.
However, even with my own limited run-time experience thus far, there are a few things I have noticed.
Moisture Is The Enemy
The first is that anything but bone dry wood costs you a lot of performance. I had read this before I started running my own Rocket Mass Heater, but I was surprised to see how much of a difference it actually made. On my own 210mm / 8.4in system, what I’m finding is that if I can “hear it hiss”, then it has cost me 56° C / 100° F or more of peak operating temperature.
Making steam is expensive, in terms of energy consumption. It takes 100J of energy to raise a gram water from freezing to boiling. It then takes another 100J to actually go from liquid water to steam. Then the steam goes back to draining heat out of the system at a rate of 1J per degree that the steam heats up. Since there is functionally no upper limit on how hot steam gets, as long as the water vapour is in the system then it is pulling heat away from the fire.
The good news is that a roaring rocket stove moves gasses from Feed Tube to Exhaust Pipe pretty quickly, so once the water is in steam form, it isn’t staying in your system too terribly long.
Blocks Don’t Burn Fast
Rocket Mass Heaters are also sensitive to the size of the feedstock, even once they have been running at full power for a while. Keeping in mind that you are trying to burn the wood gas, the reason begins to become apparent: surface area for venting the boiling gasses.
Take a stick of wood 45cm / 18in tall and 50mm / 2in square. For its mass, it has a working surface area of 950cm^2. If I split it in half down its length, then the total surface area of the two pieces goes up to 1400cm^2. Splitting those two sticks in half lengthwise again raises the total available surface area to 1850cm^2.
That extra surface area exposes more of the wood mass to the radiant and convective heat from the Burn Tunnel, allowing more of the wood mass to begin producing wood gas to drive the combustion process. With nearly double the surface area to work with for our given mass, we can guess we will see double the rate of flammable gas production.
There is no more or less total energy available. This an important fact to keep in mind. By splitting the wood into smaller pieces, what you are increasing is the rate at which the energy is released. It is this higher rate of release that yields higher temperatures on the Barrel Cover.
Larger pieces of wood take longer to burn, and more energy to get them started burning. If you are trying to quickly produce a lot of radiant heat from the Barrel Cover, burn smaller pieces.
Given that we are generating energy to store in a “battery” — the Thermal Storage Mass — how fast we release it is semi-irrelevant. As long as the temperatures inside the Burn Tunnel and Heat Riser remain high enough to produce the secondary ignition, then we are fine.
Rocket Mass Heaters Are
At this point, I want to lapse into a bit of personal philosophy about Rocket Mass Heater systems. I have been researching into the subject now for well over 9 months and while I hardly claim to be an expert, there are a couple of things I commonly see people brand new saying or doing that I think bear commenting on.
- Simple: Build an insulated tube following the “1x 2x 3x” size plan, light a fire and go. There is no magic, but there is some physics you have to be aware of to get “better than good” performance.
- Grid-Free: Rocket Stoves & Rocket Mass Heaters do not require electricity to run. If you can get the fire started somehow, you’ll never freeze to death because a blower or feed pump doesn’t have power.
- Sustainable: You do not need to use firebrick like I did — you can build them with clay, straw, rocks and reclaimed old masonry. As a result of the efficiency which they heat at, keeping upwards of 85% of the energy available in the wood inside the building to be heated, they use up to ¾ less wood than conventional wood-block stoves. I have read accounts of folks keeping their 1200sq/ft mountain home heated with nothing but deadfall branches from the surrounding forest.
… But Are Not
- Free: There is a gulf of difference between “Free”, “Inexpensive”, and “Valueless”. Rocket Stoves & Rocket Mass Heaters are not free. Even if you do not build a firebrick Cadillac, there is perlite, stovepipe, bricks, blocks, time and tears. However, the simple fuel and heat efficiencies of these systems make them well worth what you pay.
- TLUD: A barrel-based “Top Lit, Up Draft” burning system is not a Rocket Mass Heater. Yes, it uses radiant heat to set up a wood-gas re-burn. Yes, they are remarkably efficient for many of the same reasons that “Rocket” systems are. Rocket systems burn side ways, not top down. TLUDs are cool, but they are TLUDs, not “Rockets”.
- Vortex Burners: Similar to a “Rocket” system in how they work, Vortex Burners engineer the inside of the burn chamber to induce a vortex in the rising gas column to trigger the “afterburner” effect we see in the Heat Riser of a “Rocket” system. This produces remarkable results in terms of heat efficiency and burn temperatures. But … it’s not a “Rocket”. To build a Vortex Burner, you need a machine shop and casting tools. To build a “Rocket”, you need a pile of bricks. Vortex Burners are cool, but they are Vortex Burners, not “Rockets”.
- Magic: You do not get any more energy out of a piece of wood or whatever feedstock you are burning than is actually there. There is no magic where you get something that isn’t there. Instead, you get to use and keep all of what energy is there, which less efficient systems throw out for a variety of reasons. Magic is cool, but it’s Magic, not “Rockets”.
- Hard: As I noted in a previous post, I’m a telecom geek and a Steampunk / Fantasy-fiction writer. I am hardly a model DIY Handyman, or highly motivated and well-funded Prepper. These things are not hard to build. Once you understand how they work, and why they work, you can build one out of two tea-lights, three soup cans and four bricks. It won’t heat to large a space, but it will “Rocket”. Give it a try. This is “Rocket” science, not Rocket Science. You can do it.
Ok, so this is another post that is waaaaaay longer than I had expected it to be. As before, hopefully you are still with me, and better yet, some of this was valuable to you.
For now, I think I’ve exhausted my “heap of thoughts” on Rocket Mass Heater systems. If you like watching YouTube videos, here is a set you may enjoy to help you on your way:
*Rocket Stove Classroom Instruction with Ernie & Erica Wisner … these two are amazingly good, and are fun to listen to.
*“Rocket Mass Heater Workshop” by Verge Permaculture … another good one, reviewing a lot of the benefits to these systems.
*“How to Make a 16 Brick Rocket Stove” by Dr. Larry Winiarski … They make everything, including the bricks, from scratch. Proof that almost anyone can build one of these.
*“Rocket Mass Heater Playlist” by Bigelow Brook Farm … Remarkable work using a “big bore” RMH to heat an production aquaponics greenhouse.
*“Greenhouse RMH Build Progress Review” … ½ way done in my own Rocket Mass Heater build project. I talk about a couple of the “quirks” in my own design, and what I’m trying to do.
*“Rocket Stove Mass Heater with Homemade Mortar & Cob” by MyLittleHomestead … a good talk about the design, the build and the very pretty finished system.
If you have any constructive comments, questions or suggestions, please leave them in the comments section below this post. I would absolutely love to hear from you! I hope this series of posts helps someone else get into this fascinating technology.