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019 – Rocket mass heaters


Paul interviews Ernie and Erica Wisner about Rocket Mass Heaters. Paul discusses the videos he made where he makes a RMH. Ernie and Erica review Paul's portable rocket mass heater design concept of putting a RMH on a trailer pulled by bicycles. Paul asks Ernie to critique his design in a constructively praise like manner. Ernie and Erica discuss the bucket heater. Paul explains that his set up took just about an hour and fifteen minutes to build. Paul explains why he is using rocks in place of sand. Ernie talks about the temperature differential required for long runs versus short runs of duct work. Erica explains outside versus inside temperature differential and the impact on the draft. Shorter runs of pipe will change the draft. Paul mentions the 6" pipe and five people foot rule for each 90 degree bend. Paul suggests a review of a standard RMH.

Erica goes through a brief history of stove efficiency and how master mason worked on these. The cost of these today could be as much as 30 to 100k. Erica explains how smoke indicates unburned fuel and how this is not good for people and the environment. Erica explains the basic parts of a RMH and how each part interacts with the heater exhaust and heat exchange. The different types of exhaust alignments is discussed. Ernie explains some of the criteria important to efficiency ratings. Erica explains that there are three different formulas used to determine efficiency. RMHs are incredibly efficient. wood stove are not allowed to put out exhaust below the dew point which gives RMHs another advantage. Typical temperatures leaving a Wood stove are 300 to 600 degrees versus 80 to 100 degrees for a RMH. Exhaust temperature also effects what gets left in the chimney like creosote and tar build up found with wood stoves.

Erica explains how clean and cool the exhaust is when leaving a RMH. Paul discusses the recent video put out showing a person smelling the exhaust. Erica recommends not breathing the exhaust. Paul discusses mixed results of the earth day burns. Ernie explains how warming the heat riser is an important first step. cob is explained by Erica and Paul. Sand and clay and a little straw makes up Cob. Paul likes to discuss his position on RMHs and how to do it without Cobb. Paul explains the advantages of making a RMH portable and some temperatures readings they measured. Ernie explains why he uses an attachable magnetic thermometer. Erica explained how insulating the barrel is a bad idea. Paul explains how all other RMH designs have the heat riser on top of the core. Paul's design is different. Paul thinks his design increases the pump. Erica explains how the brick burn tunnel works better than a smooth pipe. Paul's entire j tube was round duct just at the end. Ernie explains his rifled j tube and how cleaning it can be a problem. Breaking up the flame profile is discussed. RMHs have lots of options and Ernie explains how each one is a little different.

RMHs are great for people who want to tinker. Some of the questions coming from are best explained if the person builds a test stove in their backyard. Paul discusses the RMH book and how inexpensive it is. Another option is taking the book out at your local library. Paul brings up the pocket rocket. Erica explains what a pocket rocket is. Using a five gallon bucket and two pieces of 6" stove pipe. It shows how fire can burn upside down. Ernie explains how the j tube works and how a pocket rocket demonstrates this. They are smokey but are efficient. Paul thinks of apocket rocket as moderately comparable to a wood stove. Pocket rockets are cheap. Material costs run about $50. People have used a pocket rocket inside a wood stove and Erica explains why this can be useful. Paul reviews the reduction in wood when using the RMH. Ernie discusses a masonry heater versus RMH.

Erica notices that masonry heaters do smoke and the exhaust temperature is in the 200 to 300 degree range as compared to the RMH 100 degree temperature. The cost of masonry heaters installed is reviewed by Erica and how expensive they can be. There are some kits available for masonry heaters and they can be found around $6k. Masonry heaters tend to be more complex. Paul asks what the most expensive RMH would cost? Ernie does not have a good answer because there are very few contractors building RMHs. Erica says a best guess would be a couple thousand in materials and that same amount in labor, but that is a best guess because so few people are installing these. Paul says people can do it very very cheaply. Erica thinks that going through a combustible wall can increase price to around $500. Ernie gets his ducting for around $30 but the fire proof type items cost the most. Ernie suggest posting what parts you need and someone may help deliver materials to your doorstep. These need to be built to fireplace code and that can get a little expensive. Paul pauses for a brief commercial break in an attempt to get some free stuff from the local rebuilding center. Stove pipe can not be resold as stove pipe so use caution when getting used parts.

Paul discusses building code issues in Missoula County. Erica explains how they are helping develop sample building codes and how the EPA exemption letter to satisfy local code officials is difficult to secure. Erica explains a meeting with an EPA inspector and how permitting can be difficult unless you are aware of the exemptions EPA recognizes. Standards for testing wood stoves do not really cover RMHs. Erica and Ernie want to help write the code to keep things safe and meet the governments regulations. Paul feels the RMHs are the most sustainable type of device to heat a home and as such the government should be encouraging their use. Erica discusses energy conversion and how different systems lose energy for example going from nuclear to electric to heat.

Paul discusses other forms of heat and how expensive they are compared to a RMH. Erica give a review of the building codes and how they help and hurt us and some of the problems with manufacturers included by name in the code. Paul feels that there is place for government to encourage the use of RMHs because we could reduce our energy consumption. Paul changes the subject and talks about a Rocket stoves versus a rocket mass heater. Erica explains the difference. Most stoves were developed for use in third world countries. Paul tells his version of why the rocket stove was developed. Erica explains in more detail the many different types of stoves used by many native peoples and how no one person should really take credit.

Paul and Ernie discuss wood stoves and their efficiency ratings. Stoves generally cool down quickly while RMHs retain heat for days. Damping down a wood stove reduces its efficiency and as such makes RMHs the better solution. Paul confirms that Ernie and Erica are not contractors but they will come and give a class to teach people how to build these stoves. Paul reviews some of the costs associated with running training classes. Erica explains that how over the two days of class, the bulk of the rocket mass heater is done but how there is still a good deal of Cob work that still needs to be completed.

These stoves are typically tested to make sure there are no problems with smoke and how they have taken stoves apart when they do not work properly. Paul feels that as RMHs become more popular, more and more people will want these. Paul explains how there is a lot more topics to talk about in another podcast. Ernie adds that RMHs are a cheap stove but very sophisticated device. Erica explains that it can be cheap in parts, but assembling them properly is crucial to the success of the stove. Paul explains how reading the RMH book can increase your chances of success. Paul brings up heating water with RMH can be very very dangerous. Myth busters did a water heater episode and how dangerous they can be if not properly built. It is easy for a RMH to smoke if not built properly but it will not explode. Heating water is much more dangerous. Paul congratulates Ernie and Erica on creating the longest podcast ever. Paul gets kudos for creating an awesome portable RMH from Ernie and Erica.

Relevant Threads

Wood Heat
Wood Stove Efficiency
Solar Ovens
Butt Warmer
Underfloor Heating

Credit:Kevin Murphy

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