384 – Uncle Mud – part 2

Paul continues his conversation with Chris McClellan aka Uncle Mud about Rocket Mass heaters. He talks about the cottage style RMH. Paul describes a J-tube type system with a vertical feed system. Heat hits the barrel and then goes down the sides of the barrel. It acts like a standard 6″ J-tube system. There are a few exhaust tubes through the mass and allows for quicker heat response. These tubes lead to a manifold that eventually leads to the exhaust. It is possible for this to be a shippable core. The efficiency of this unit is close to 65% because of a shorter riser. With a short chimney, the cottage rocket is insulated with cob and it performed more poorly than expected near 70% efficiency. Once placed in the workshop it had a 20 foot chimney and it began to perform much better but time did not allow for testing. RMH’s built since then, building the heat riser with split insulated fire brick (instead of cob) which created a chimney effect and this increased efficiency above the one in the workshop. This removed the need to preheat the exhaust. This design benefits from a well insulated chimney and allows it to burn extra hot. Paul likes the idea that the cottage heater could have mass added to the top of the barrel rather than trying to heat a large cob mass alongside the barrel the mass would sit on top of the barrel. A clean burn and of course the concept of the mass giving off heat for a long time. Heating up a large mass takes time. The cottage heater would be used to generate heat quickly. It does not burn all that clean but it is much smaller so there are advantages to the smaller footprint. Paul likes the cottage style heater for tiny houses.

Next they do a comparison of the cyclone design and the cottage design. Paul likes the cottage style and sees it as a possible shippable core. UPS will ship the barrel as long as it does not weigh too much. This is an advantage compared to heavier designs.
Cyclone is a 4″ system with a matchbox. This would use kindling. The interior is only 4×4″. It is a tiny tiny wood feed. This will require burning it for a while before it heats up. This system was nicknamed mini mouse and took less than 20 minutes to heat up. It also does not have a large amount of mass. The need for mass is important in the cabin because of gaps in the floorboards. The cyclone has a casserole door. It sits at a 70 degree angle. The lid sits on the angled side and this allows for a larger wood feed. This allows you to load up the wood feed with about 4 times the typical wood feed. The temperatures for the cyclone run at about 1800 degrees. The size is about the size of a 55 gallon drum. It is narrower from side to side but deeper. It is also a little taller. It has a nice arched top and a cathedral roof of sorts. The riser goes up into the chamber to hold all the gases. The 4″ stove pipe takes the gases out the exhaust. There is some sand and clay acting as mortar. All the masonry gets heated up. The heat only gets to the room after the bricks are heated up and this can take a few hours. If the red cabin were better insulated it might only take 40 minutes. Once the bricks are at 160 the heat continues to pump heat out. RMH’s are designed to provide consistent heat through the night. The cyclone may heat things up even after the fire goes out. The cottage style takes about 8 hours to build nicely, but could be done quicker if aesthetics are not important. The cottage took about 2.5 days including the chimney install. The cyclone heater took more like 6 days and there was a lot of masonry done and it is a slower build. The cyclone has a lot more mass than the cottage. Each style has different missions the cottage RMH is good for heating a workshop versus the cyclone which is good for heating a cabin.

A discussion follows about using the clear casserole lids for doors and some problems of them breaking. Paul talks about different kinds of glass that could be used with better results. Some types of glass work better than others. The cyclone has a casserole lid as a door. Pyrex is a brand name and it used to be fairly high temperature resistant. The newer lids are no longer capable of handling the higher temperatures. When the cob is fresh and wet it tends to generate steam which cracks the lids. Moving the door away from the fire will help reduce the issue with cracking. The trick seems to be keeping the door farther away from the fire. There is a thread on permies about the casserole door. Pyrex has changed to soda glass and it does not hold up as well and it tends to explode when it gets too hot. Amber cookware seems to tolerate warmer temperatures. A conventional wood stove runs at about 1000 degrees. The amber glass can handle about 1300 degrees where soda glass maxes out at about 450 degrees. Paul reminds us that we are trying to get to 2600 degrees with a RMH so none of these glasses work well for that. The glass also works to cool the fire which is not what we want. Paul thinks that using the glass lets some of the heat escape before the secondary burn. Do not use too much glass and it is something to use with caution when designing a RMH. Paul loves the idea of using the casserole lid instead of the metal frame door. The door fabrication alone takes several days. The casserole idea is a magnificently simple solution. One thing to note, the cyclone heater has no barrel. If you wanted a RMH that does not look like it has a barrel, you have a trade off because it has not barrel. You have to be careful if you add to much cob then the RMH will not work properly. Kirk (aka Donkey) has come up with a good design that strikes a balance using a stratification chamber. This design is slow to heat up but once started it stays hot, maybe even too hot but it does keep the space warm through the night. The cyclone does provide a stable temperature over a long period of time because of the large mass. Knowing the benefits of thermal mass is important when using the cyclone. If you are only looking to use it for one day, then it may not be the right solution. There are some Rocket Mass Heaters coming out of the Ukraine that are square instead of round. These are batch rocket based masonry heaters. These may be acceptable to insurance companies because masonry heaters are pretty well defined in the codes.

A discussion of the season extender is next. This was built with seven barrels laid end to end to create a giant Pringle type can under a hugelkultur mound. Paul hopes he can use this to plant tomatoes earlier in the spring. This giant can is perfectly level and is very wet tolerant. Inside the barrel they started with a basic 8″ RMH J-tube. The core is made with firebrick and insulated firebrick around the heat riser. The riser is two drums welded together so it sits about six feet high. The riser is insulated with perlite and ceramic wool. There is an exhaust tube that goes down into the Pringles can that sits under the hugel bed. There is some discussion that maybe five barrels would probably have worked better than the seven unit design. The barrels are the radiant surface with the ends are exposed for cleaning and inspection purposes. This design also has a small seat where you can sit and feed the fire. The exhaust goes back up and out through the manifold where the exhaust leaves the heat riser. The chimney needs a little heat to help it function better when it is very cold. The heat is being pulled from the bottom of the stratification chamber. The design of the chamber size is also an important factor. A discussion of the Facebook group versus the permies forum on RMH’s comes up. Paul is NOT a fan of Facebook and thinks it is better to have the information at permies because Google search will not include Facebook links.

Credit: Kevin Murphy

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Relevant Threads

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wade L
Suleiman ALAQEL
Ash Jackson is The Scrollbard
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