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Rocket Mass Heater concerns. Advanced combustion wood stove versus RMH. Whats the difference?

Brian Knight


Joined: Nov 02, 2011
Posts: 389
Location: Asheville NC
    
    7
Ive been intrigued by RMH ever since finding this site but there are two big things about them that are problems to me.

1. Indoor Air Quality. Because of their design, they seem to allow smoke to back draft into the home especially when starting the fire. The most recent YouTube video is rather alarming to me when Ernie opens the drum top, a big poof of ash is released into the house. Studies have found that households that burn with wood are more likely to have occupants develop a certain respiratory condition. These stoves seem to be a bit dirtier than the woodstoves Ive lived with and they were fairly dirty.

2. Code Acceptance. Hopefully this will be resolved soon but it an obvious problem for those that must follow building laws. Lets assume for the sake of good debate that building codes dont matter in the following comparison.


Surely the difference between Advanced Combustion Woodstoves and RMH have been covered on this forum. I have searched pretty hard to no avail. Could those better educated about this stuff enlighten me?
Advanced Combustion Woodstoves have an advertised efficiency from 60-72% Not bad. What would an average RMH have?

Catalytic Wood stoves have 70-80% advertised efficiency. I dont like the idea of these due to the maintenance involved with changing out the catalytic components but perhaps someone who has done it could tell me its not a big deal. To me, a new 100$ part is a big deal if it needs to be done every year.

The biggest advantage over RMH is that new wood stoves seem to have better indoor air quality characteristics. The next would be they are more aesthetically pleasing, especially since you can see the fire and the newer designs claim to keep the glass cleaner.

The biggest advantage I assumed of RMH is their DIY nature but Iam not so sure about that anymore. There is quite a bit of physics and site variability involved and the plug and play ability of a mass produced high efficiency woodstove is pretty appealing. I hope I didnt raise too many questions for one thread but would love to hear everyone's thoughts on this stuff. Thanks!


"If you want to save the environment, build a city worth living in." - Wendell Berry
Daniel Truax


Joined: Jan 07, 2012
Posts: 61
Uhmmm...

Where to start.

Efficiency can be measured in two ways: how well the wood gas is completely oxidized and how much heat is harvested and kept in the house. Doesn't make much sense if you have 99% combustion efficiency with 80% of the heat going up the chimney.

-Dan

Kirk Mobert


Joined: Jan 07, 2011
Posts: 128
Location: Point Arena, Ca
    
    2
Brian Knight wrote:Ive been intrigued by RMH ever since finding this site but there are two big things about them that are problems to me.

1. Indoor Air Quality. Because of their design, they seem to allow smoke to back draft into the home especially when starting the fire.


This isn't an issue in a well built and maintained rocket stove. Also, I've rarely met a standard issue box-type stove that didn't smoke when you open the door to feed it or start the fire. Even the "super efficient" box stoves will do this. With rocket stoves, there's little to no smoke outside either, improving EVERYONE'S air quality.

The most recent YouTube video is rather alarming to me when Ernie opens the drum top, a big poof of ash is released into the house. Studies have found that households that burn with wood are more likely to have occupants develop a certain respiratory condition. These stoves seem to be a bit dirtier than the woodstoves Ive lived with and they were fairly dirty.


I saw the video last night.. Ernie opened the top of the barrel, that's something you do around once a year when you clean the thing. Rocket stoves produce LESS ash than yer box stove, and almost NO creosote. The annual cleaning of the things is a snap and poses a significantly smaller risk to your health. One thing about rocket stoves, the ash is lighter than most other stoves and can get into more places. Generous and proper placement of clean out ports is a MUST.

2. Code Acceptance. Hopefully this will be resolved soon but it an obvious problem for those that must follow building laws. Lets assume for the sake of good debate that building codes dont matter in the following comparison.


Surely the difference between Advanced Combustion Woodstoves and RMH have been covered on this forum. I have searched pretty hard to no avail. Could those better educated about this stuff enlighten me?
Advanced Combustion Woodstoves have an advertised efficiency from 60-72% Not bad. What would an average RMH have?


There's that word again... "Efficiency". Tossed out with no quantifiers, qualifiers, classifications or context, the word means little to nothing. Efficient at what? Turning wood into flame, burning off pollutants, producing heat, storing heat, or giving it back to heat the people? In the right conditions, one can get an old fashioned potbellied stove to turn wood into fire at 90% efficiency. It's making the heat produced USEFUL that's missing in that equation. Just about any stove can be made to run at high efficiency (on SOME measurement or another) in THE RIGHT CONDITIONS. In our society, we assume a one size fits all solution that will work under all conditions but the truth is that ALL workable solutions MUST be context sensitive. Context is EVERYTHING.
Here's a graph from a Testo analyzer of an experimental rocket stove, it's a 100 minuet test run:

Notice the pink CO line. For the first 30-40 minuets the stove is coming up to heat and it isn't burning at max efficiency, there is a spike around the 50 minuet mark where the filter on the Testo is changed, and at the end as the fire is allowed to die,once again CO levels rise in the exhaust. For much of the burn cycle, CO levels are just at or below the analyzers sensitivity level. The red line is the "efficiency" line, we find in this test an average efficiency (of burn, turning wood into flame) of 90.3%. Remember, this test is under tightly controlled conditions, this stove moved into your home would behave differently. Also, this stove is a burn chamber test ONLY and has no thermal battery. It's efficiency at making the produced heat useful is (in it's current state) VERY low.

A note about the analyzer... The gizmo was made under certain assumptions. It measures 3 things (in the exhaust), temperature (in C), O2 levels (percent) and CO levels (parts per million). It ASSUMES that temperature, CO and O2 contents fluctuating within pre-set ranges, equals high efficiency and it looks to me that O2 content is heavily weighted as a factor. I find it interesting that at around 76 minuets the graph shows a major spike in O2 level and though none of the other (measured) data points respond, the (assumed) efficiency level dips dramatically. For me, the assumptions that went into designing the tester are as important (probably even more so) than the test results that it returns.

Catalytic Wood stoves have 70-80% advertised efficiency. I dont like the idea of these due to the maintenance involved with changing out the catalytic components but perhaps someone who has done it could tell me its not a big deal. To me, a new 100$ part is a big deal if it needs to be done every year.


Catalytic converters were invented to solve a problem, namely that airtight stoves are smoky and dangerous. They burn out (that's a design feature) in a handful of years and must be replaced. The problem here is that most people don't know this and forget to do it. When the converter dies, a whole lot of creosote can be generated VERY quickly which is dangerous. My folks had an old airtight, I remember my pop on top of the (wood shingled) roof with the garden hose, screaming orders into gale force winds, scrambling to put out the chimney fire.

The biggest advantage over RMH is that new wood stoves seem to have better indoor air quality characteristics. The next would be they are more aesthetically pleasing, especially since you can see the fire and the newer designs claim to keep the glass cleaner.


The air quality bit I've already addressed and it's nonsense.
As to aesthetics, I agree that seeing the fire is nice, though there are some oddball rocket stove designs out there that solve that problem, glass and all. Rocket stoves can be made to be VERY beautiful, the thermal mass can be any shape, plastered in unending combinations or gussied up as custom as anyone would wish. Heated benches are GREAT to lounge on, though one needn't be stuck on that idea. These stoves can be built to look (and act) just about identically to a masonry heater or perhaps you would prefer a thermal mass dividing wall between two rooms. The ugliest feature in my mind (the barrel) doesn't NEED to be an actual barrel but can be made of any suitable container, found or manufactured.

The biggest advantage I assumed of RMH is their DIY nature but Iam not so sure about that anymore. There is quite a bit of physics and site variability involved and the plug and play ability of a mass produced high efficiency woodstove is pretty appealing. I hope I didnt raise too many questions for one thread but would love to hear everyone's thoughts on this stuff. Thanks!


The biggest advantage is that these stoves provide a whole lot of comfort, for very little financial outlay, on less wood than can be believed at first look.


Build it yourself, make it small, occupy it.
dan murf


Joined: Feb 17, 2012
Posts: 50
Location: Michigan West Side
What is being done to get this heater being expectable UL ratting. I just spoke with a ex-spert today. At a fire brick & call fire clay distrubuture here in west Michigan. He said you would have pay a grand & take your stove to Ohio(I think, will check notes) to get rated. Then you would have build ita exactly to those specifications... is anyone working on this. Can I help them on this in anyway?
Murf.


"to Tinker or not to tinker, that is the question!"
If you build it better than the one profiting from it, don’t tell them, they'll get pissed! "I challenge anyone to challenge me" ... Murf! "I am responsible for the comment in this comment section"
Kirk Mobert


Joined: Jan 07, 2011
Posts: 128
Location: Point Arena, Ca
    
    2
dan murf wrote:What is being done to get this heater being expectable UL ratting. I just spoke with a ex-spert today. At a fire brick & call fire clay distrubuture here in west Michigan. He said you would have pay a grand & take your stove to Ohio(I think, will check notes) to get rated. Then you would have build ita exactly to those specifications... is anyone working on this. Can I help them on this in anyway?
Murf.


The thousand dollars is typically on a "per pop" basis.. This means that if something needs changing, you'll need to put up another grand for a second "pop", likely that several visits to the lab will be needed to move the project forward.
Ernie and Erica have already put a WHOLE lot of time and effort into this endeavor. I recommend that if you are interested in helping, you go to their website and see how you can get involved.
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
OK couple things to reply to.

1. I took the lid off my stove to let folks see what was going on inside it. the fine ash is very fine and will rise at the smallest breeze. Erica and I know this.

2. I can take the clean-outs off the stove while it's running and get no smoke in the house.
3. price those fun catalytic converters and see why folks forget them.

4. the cost for the testing we need to do is around 5000 a pop. and i will have to use all new code approved materials for about another 1000 or so.
figure that when all is said and done the whole thing will cost around 7000 to do the tests. then we need to get accepted by the UL folks and the govment.

look at Kirks graphs, do you actually think the govment will let this simple stove pass? We believe the best way to do this is to get it into code locally then make the big move. I so dislike getting arrested for crossing some huge manufacturer.


Need more info?
Ernie and Erica
Wood burning stoves, Rocket Mass Heaters, DIY,
Stove plans, Boat plans, General permiculture information, Arts and crafts, Fire science, Find it at www.ernieanderica.info


Brad Davies
volunteer

Joined: Sep 22, 2011
Posts: 212
Location: Clarkston, MI
    
    8
Brian Knight wrote:1. Indoor Air Quality. Because of their design, they seem to allow smoke to back draft into the home especially when starting the fire.


I sometimes have the same problem with my fireplace insert if I don't preheat the flue by burning some paper. I wouldn't consider it a design flaw for either application, but I would consider it an incentive to start a fire with the proper procedures.


SE, MI, Zone 5b "Opportunity is missed by most people because it is dressed in overalls and looks like work."
~Thomas Edison
 
 
subject: Rocket Mass Heater concerns. Advanced combustion wood stove versus RMH. Whats the difference?
 
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