Newbie - Rocket Mass Heater Build

[pa_friendly_guy]

I have thought about some of the matterials used in Rocket Stoves as well Paulbee. I agree that if I were building one the cost difference to get stainless steel pipe would be well worth it to me. The other weak point that I have wondered about is the top of the 55 gal drum that takes all of the blast from the flames as well as the heat. I have seen some designs where they weld a 1/4 steel plate to the inside top of the drum to take the abuse. The bottom line however is that the RMH is cheap and easy to built. They can be de-constructed easily and many of the matterials re-used. So if in 10 years your 55 gal drum burns through, Oh Well, just get a New one and put it in. That is one of the beautys of the design.

[matt walker]

The inner flue pipe in the riser should be thought of as a component of a temporary mold, not as a permanent part of the system. In my house stove, the inner pipe came out in pieces in the first few weeks. It just disintegrated and I pulled small pieces out of the fire box during routine clean out. If you look at my half barrel thread, you can see that the mix of clay and perlite is sturdy enough to remove the outer mold after one burn, and the inside pipe can be allowed to burn away.

As for harmful stuff, well, I don't disagree, but I'm the kinda guy who has spent most of my life sitting around an open fire, so it's gotta be better than that. I do actually notice an improvement in that I don't feel "smoked" when I wake up after a night of sitting around the fire. Paul, on Donkey's Rocket Stoves board there is a fellow with a "sniffer" who is producing really good graphs of the CO and O2 levels in his stove. It's pretty amazing how cleanly they can actually burn, and i think they can approach 2000 degrees in the tunnel. I built a small oven to put on top of my outside stove last night and while burning out the mold the whole thing went incandescent, and that was at the top of the barrel.

As for the barrel and longevity, well, the prevailing logic is that for the steel to "burn out" it needs both high heat and oxygen, and those two things don't exist at that location at the same time, ever. Whether or not that's exactly accurate, the reality is that replacing a barrel is a five minute job, and there is never enough fuel in the small feed to be dangerous if it did fail dramatically, so I feel like it is a non issue. One thing about these stoves I've learned is that I was thinking about all the wrong things before I built one. Things that seems like problems that need solving just aren't, period. The classic one is the way folks are always speculating about a way to get a long burn or a fuel hopper or such. That is an idea that we have from traditional heating sources, but in a rocket mass heater you quickly learn that it just doesn't matter. You burn it when you are around, let it go out, and light it again when you return. The heat stays in the mass and the temperature in the space varies very little, regardless of the fact that the stove is out.

[matt walker]

Oh, and Hardiboard. I think it would be a good choice Paul. I actually just used some as a base for a small oven for the top of one of the outside stoves. It broke from the weight of the mud, so wasn't a good application for it there, but it held up well to the heat. I think it would make a fine mold material for the inner tunnel mold, and would probably last a long while. Good idea!

[paulbee]

Thanks folks!

Hardiboard is available in a bunch of different thicknesses and intended applications. Been quite a while since I looked at their product portfolio, but I'd try looking around their corporate website to see if they have complete product catalog with MSDS or similar engineering worksheets. Odds are you are making do with thinner and cheaper Hardiboard products that aren't optimal for this application.

Other companies have straight cement board available that should do the job. Obviously the stuff isn't cheap or light. Hardi is just a well known and distributed brand

Not an engineer or chemist, but perhaps a hardboard with metal grid/bracing built in is the correct solution. Very common to pour cement with a metal rebar or similar mesh sandwich for integrity.

Was fussing with steel strips in my brick pile to support spans without brick obstructions in the burn chamber (that is why that all came to mind).

As for the 55 gallon drum, burnout and or cracking is inevitable. Unsure if the inner black pipe or the barrel will fail first. I'll post elsewhere to see how others are fairing with their older rocket stoves and metal failures. Expect if the chemical reaction, condensation / oxidization doesn't create the problem the temperatures will warp the barrel in no time.

The solution (if there is one) in general with these parts is to stick with stainless steel 55 gallon drums (hard to find here and expensive new) and ditto for the inner duct black pipe, stainless instead. Ceramic would be even better, but I never see it around here and consider it exotic (can't salvage it from teardowns, scrap, etc.)

As for the 2000 degree temperatures, it is possible but only in tight spots in very optimal fire with a rather big pile of hardwood coals. Get 6 inches away and the temps drop massively. What we are doing with these stoves typically is two notable things:

1. Reducing the burn chamber down to a typically very small area.
2. Reflecting and concentrating heat in the burn chamber

I have yet to see a rocket loaded with massive logs or what most people use to bank a fire in a box stove overnight. Since the burn chamber accepts and burns less we are heating the same very common area. As where in a traditional stove, you have fire wherever it has fuel and is ignited. Meaning, the cold spots in the traditional stove are many and always different.

In many ways this concentrated burn spot is similar to the designs we see for pelletized biomass. That is why those stoves are also far more efficient per se than boxwood stoves.

The reflection and concentration are common in these stoves and good stoves of all types. Problem with other stoves is we are reflecting fire temps in a much larger burn area. It is like burning a match in a 10 sq ft room compared to burning a match in a can. You would notice the heat in a can, but in the room, only the smell/smoke.

Wondering how suitable most rockets would be for prepping batches of biochar/charcoal. Seem to be very close to environment one would want to create such.

[Oddmar]

Just complete my move to southwest Arkansas, near Arkadelphia.

I'll be building several new rocket mass heaters at the new property. We were going to build a 30x50 shop with strawbale in-fill, but straw bales are $6.05 locally...Ouch!! So we are going to use blown-in cellulose. If we make it 14 inches thick we'll still have an R value of 50.

[Lollykoko]

That price for straw is really out of sight! Is the weather in that part of the world extreme enough to require an R-50 wall?

[matt walker]

Right on Oddmar, glad to hear you are done with the move. I'll look forward to your next project. Any updates on how the last one is working for your friend?

[Thundersnow]

It's been inspiring to follow this thread... makes me want to start a build !

[Oddmar]

[Oddmar]