Paul continues his conversation with Chris McClellan aka Uncle Mud about Rocket Mass heaters. They start this podcast talking about the Minnie Mouse, who now lives in the Love Shack. She’s a rocket mass heater contained inside a barrel, inside a wooden structure. Peter cleverly lined the back of the barrel very tightly with bricks, so that the stove doesn’t radiate a bunch of heat backwards to the wall. Thus, the heat shield is built in – no metal or brick had to be put over the wooden wall. Mud points out that this heat shield is also functioning as thermal mass, which is awesome double function action.
Minnie Mouse throws off a lot of heat quickly, but she also holds onto some heat, making her sort of a hybrid stove. She has a metal core – a metal wood feed and a metal burn tunnel. The heat riser is not metal. The reason this works is that the wood feed and burn tunnel are both air cooled. Steel will spall at 1600 degrees F and melts at 2600. You need to keep steel around 1400 degrees. This is done by having a smaller system – a 4 inch J tube system (4 inch batchbox would probably get too hot). The heat riser is vermiculite and can take a bit more heat. Paul thinks that if it was a 6 inch system, you couldn’t have a metal core.
Mud notes that “mild steel” will become challenged at 906 degrees F. You can use metal for a rocket cooker, but that’s because it never runs for very long. Peter uses metal to test various designs, because he’s just seeing how things work, he’s not going to put it into someone’s house. Metal cores tend to fail.
Mud says that although you can “hook” people into RMH with the economy of heating with less wood, and cheaper wood, the real value is in the increased safety. Paul agrees “Hey, you wanna save some money?” is the calling card of the RMH. Heat your home with 1/10th the wood *and* you don’t burn down your house with a chimney fire.
The Liberator is UL listed, so that should make your insurance company happy. It is a fully steel system that uses fins to dissipate heat. It’s a 4” system, another reason it works. (It’s not as efficient as ceramic cored rocket mass heaters for this reason.) Paul is still hoping someone will develop a non-metal shippable core. When he’s seen rocket mass heaters made with metal cores, they don’t seem to last. If something was made of ceramic, then you can have a 6” or 8” system. You can’t do a larger system in metal, it will get too hot and then it will fail. Paul has seen many people build 6 or 8 inch systems in metal, and they work grea… until they fail.
Rocket mass heaters with basic firebrick cores do great – they heat a home for years, no problem. We need more videos of the good builds, to compete with all the flaming freak shows of death that you can find on YouTube.
Moving on to the tipi. The rocket mass heater there was no longer working well. Ernie went up there to check it out. He thinks the core had some silt (instead of clay) and over time, this led to breakdown and holes in the core. Where the burn tunnel met the riser, there were holes leaking into the manifold, thus bypassing the heat riser. Ernie opened it up and rebuilt it with insulated firebrick and now it works great again.
The double shoe box is a batch box system that doesn’t have a heat riser. Instead, it has another box above the batch box, with a slot in the top of the bottom box. The fire goes up through the slot and does the cool ramshorn thing in the upper chamber to mix the gases and get the secondary combustion. They’ve built one up at Allerton Abbey. It’s got a glass cooktop, so you can see the curlicue flame path (and cook on it as well).
Having two chambers solves a problem that batch box heaters often had. People would overfill the batch box and it would interfere with fire leaving via the back wall. Then the fire wouldn’t burn cleanly. Now, the flame leaves via a slot in the top, so it’s much harder to block. The stove in Allerton Abbey has a heating bench attached to it, an open stratification type bench. The batch box has a casserole lid door, and once they built a bit more cob to get the glass further from the fire that’s working well. The thing burns really cleanly. The exhaust has no smell “not even like socks.”
Mud says there are lots of new designs showing that the secondary combustion doesn’t require a barrel with an insulated heat riser. People are making really interesting designs, especially with cooktops. As long as the system moves air well, multiple forms can lead to secondary combustion of the wood gases and thus impressive efficiency. Paul feels that the key factors that make a heater efficient are that the gases move through an area that’s over 1400 degrees (up to 3000 degrees) and that the exhaust is not nearly as hot as that of a conventional wood stove. He’d like to see how efficient these stoves can become – there is still room for optimization.
Mud says efficient is cool, but we’ve got to get people to use the things. Lots of people hate the idea of feeding a J-tube system every 20 minutes, so having the batch box system can help. The usefulness of the cookers may help these appeal to different people as well as the different aesthetic (no barrel). When he proposed building a rocket mass heater in his home, his wife said that saving $150 in fuel was not worth tearing up the living room. More designs, more choices in shape and function, this is how we get lots of different people using this better technology. Burn the fuel in your stove, don’t let it build up in your chimney to later burn down your house.
Paul says we’re getting to the point (in design) where a person might heat their house with sticks from the yard, plus garbage. If you pay for garbage hauling, now you’re saving money a couple of different ways. One idea for saving energy was to plug the chimney at the roof line, but Paul is here to say – don’t do this. Humans being fallible, somebody’s going to build a fire when the chimney is blocked, and it takes too long to figure out there’s a problem.
The last item on Paul’s list is the bun warmer. They already talked about it, but Paul wanted to note a couple more things. The original ring of fire used half barrels not as stratification chambers but sort of as ducting. Over time the ring of fire pulled sand into the barrels. Mud took over building the bun warmer from Ernie (whose leg started to trouble him and he had to take a break). He saw this as an opportunity for students to build a basic thing with their own hands. They made it a little harder than it needed to be, trying to bend barrels. In retrospect, it should have been built as a long bench, but having it bent in a U shape allows folks to sit and talk amongst themselves. Having it set up as a stratification chamber allows the heat to equalize all over. Paul says yeah, the surface was 85 degrees anywhere you might choose to sit – very nice. Mud says the fundamentals of the bun warmer are very cool, and he’s thinking of building an herb dryer using a similar design. (He also mentioned constructing a clothes dryer using a rocket core, and that sounds cool -JW)
Paul wishes that a thousand times more people knew about rocket mass heaters. Why would anybody buy a conventional wood stove when you could have a rocket mass heater?. OK, some of it is backlash from the lame so-called rocket mass heaters that people see. Still, there’s still a mystery here – come on, they are cleaner than natural gas heaters!. How do we get this information into more brains. We need to have more rocket heating jamborees – all over.
Credit: Julia Winter
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2018 Appropriate Technology Course in Montana
2018 Permaculture Design Course (PDC) for Homesteaders
384 – Uncle Mud – part 2
all of the rocket mass heaters and rocket stoves at wheaton labs (with pics!)
Rocket mass heater forum
pebble style rmh – in the fisher price house
Better Wood Heat: DIY Rocket Mass Heater Videos
Podcast 315 – More on Rocket Mass Heaters with Ernie and Erica Wisner Part 1
all of the rocket mass heaters and rocket stoves at wheaton labs (with pics!)
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