why the rocket mass heater works so efficiently

This post is inspired by two things. I was working on some of these calculations anyway and I recently read the Comment Stream Paul is Impossible to work with.

Paul talks about "Supergirl" fighting with him on the physics of a rocket stove. After I read all of that meaningless drivel, I came to this forum (Rocket Stoves) to see if anyone has done an analysis (or explanation) of why the rocket stove works so efficiently. Seeing none right off the bat I though I would take a stab at it.

Science - In Physics, we all learn that energy is neither created nor destroyed. It merely changes from one form to another. Most everything about Physics is that. Wood burning is a molecular chemical reaction (oxidation) which is exothermic (the byproduct is heat). So, the first thing we need to know is how much total chemical energy is stored in a pound of wood (this becomes our Maximum Theoretical heat energy). To get efficiency, we look to see how much energy we can get from burning the wood then measure how much heat we got from the wood, in the room we are trying to heat. Divide one by the other and we get % efficiency. In Physics, and the other sciences we compare Theoretical maximum to what is achieved. You would think everyone would calculate efficiency that way because it makes intuitive sense. Sound fair?

The best reference I could find states the "theoretical" energy stored in a pound of wood is 10,800 BTU (British Thermal Unit) (7150 for burning of Carbon and 3720 to burn the Hydrogen). That is based on the chemical make up of wood and varies only slightly between species. See Figure 1 and Figure 2.

What does the efficiency label on a wood stove or pellet stove mean? Is it a % of that 10,800BTU? Turns out, no it is not.

Theoretical Design efficiency (gross). The design of a heating system has a theoretical limit beyond which humans can generally not tread. In other words, because of design, you limit yourself. This usually deals with letting gasses pass to a flue and escape the heating system unburned. That generally brings in the initial "fudge" factor to 8,600 BTU as the theoretical limit (80% of the actual theoretical limit). People also bring in laws, the "law of mass action", but there is too much argument for me to think it totally applies here. Could the industry be comparing their efficiency number to this number? Well ... To look good you might want to find a smaller number to compare to.

"Gross" and "Net" efficiency. This deals with heating water. Water created by the burning of the wood. The chemical reaction insists that the fire has to use another 570 BTUs of energy to vaporize the water created. That gives a new fudge factor. I have seen that fudge factor between 0.91 to 0.93 for wood. So, our new number to measure against is 8,000 BTU. So, that is it, right? They use 8,000 BTU to get efficiency. Well ... we will call that good.

"Combustion" and "Transfer" efficiency, moisture content of wood and excess air. All have our honorable mention as to what creates inefficiencies to a stove. Combustion is a measure of what does not burn. Transfer is a measure of how much heat came into the room vs. how much went up the chimeny. As mentioned in the Gross vs net efficiency, when a fire has to deal with water it becomes less efficient (roughly 1% less efficient per 1% water content in the wood). If there is more air than needed where the wood is burning, the combustion is more efficient but it steals heat.

EPA default efficiency. Yea, that was used when Paul was debating Supergirl, since then, EPA kinda dropped it. The default efficiency was something you could put in the sales material of the stove, without actually showing the real test results. It is kinda an average efficiency of what that type of design of wood stove could get and can be highly inaccurate.

WTF does all this mean? The efficiency rating on a wood burning device could be highly deceptive. And transfer efficiency depends on how the system is placed in the home (nothing to do with the device).

Something rated at 60% efficient may actually be 44%. If it has a poor transfer efficiency (typically 70%) that number goes down to 31% (about half of original spec.). If you replaced that with a device that was nearly 100%, you could burn 1/3 the wood.


What if there was something different? Something that, by virtue of its design, could re-write all the assumptions made about wood burning heaters.

Enter the rocket-mass stove.

It works by having an initial chemical reaction (burning) of some wood. The byproduct heat, gasses and materials from that reaction naturally move (through natural air flow) to a chamber where heat accumulates to a high steady state temperature. The temperature of the chamber achieved is much higher than achieved in a typical fire place, wood stove or even pellet stove (I need proof for pellet). Applying the "mass action law" if it truly applies, states at an equilibrium temperature the products of a reaction will be constant. Well, a very "rockety" rocket will be a very high temperature. That would change the completeness calculations how much can react. On top of that, the other compounds that would not normally burn, will burn. That would make the physical maximum change from 8,600 BTU per pound to a number that is higher. Maybe as high as 10,000 BTU. After the chamber, the exhaust serpentines through a mass until it vents outside. If the temperature of the exhaust going outside, is about the temperature of the air going into the system, then the transfer efficiency is nearly 100%. If all of that is true, a rocket mass heater could have 93% efficiency compared to the true theoretical maximum.

Size Matters. When a big piece of wood is shoved into a little hole it hurts. OK. get your minds out of the gutter. Anyone who has stacked wood can tell you that a cord of wood measures 4' X 4' X 8' (a volume). When you split it and stack it, the size you split it to will depend on the hole it is going into. If it is a big fire place the pieces can be bigger, going into most rocket stoves, the hole is small. When I have stacked a cord of fireplace wood up there is usually a lot of air voids. If you were to split the wood smaller the amount of void could be a lot less. I might even be able to stack 1.3 cords of regularly chopped wood on into 1 cord volume of finer chopped wood (a guess). So optically, it looks like less wood stacked, when in fact, it is less air stacked into the wood.

Size also matters when you bring the wood into the house. Smaller wood dries faster than big wood, it is kinda a surface area thing. We see from the article water decreases efficiency (1% loss in efficiency for every 1% of water content in wood).

How to prove all this. Yea, you could rely on anecdotal (empirical) data which is "science" as well. But maybe we could take a few measurements and make some calculations.

Transfer efficiency. I would simply measure room temperature air and the temperature of the air leaving the exhaust out the building. That one may be easy.

Combustion efficiency. Well, according to the article 1% of wood is ash, the rest is stuff. I would measure x pounds of wood burnt. Then remove the ash from the stove (as complete as possible) and weigh that. If there is a 99% reduction of mass, that means you have a 100% combustion efficiency.

Size of Cord. I would stack say 3 cord of normally cut wood. Then unstack it and split it as if it were going into a rocket stove. Measure how many cords.

Moisture. I already know from working at the Forest Service, how fuel moisture is determined in wood. Look at those tests.

Now, for determining the Gross efficiency limit ... Well, I first would suggest you measure the internal temperature of the chamber of your most "rocketty" stove. Next, find out the temperature of normal wood burners. Then find out the maximum temperature where you would get 100% of the wood products to burn. Using the "law of mass action", if it applies, regress (or interpolate) the new efficiency. I have not found the temperature variable just yet but I suspect it is not linear in this case. There is evidence it is exponential, but it could be parabolic, I am really not sure.

I do not have a rocket mass heater to test on. That sounds more like a job for a Lab that actually has these kinds of stoves. But where could that be?

Conclusion? Based on what is stated, rocket mass heaters can appear to use 3 to 4 times less wood of the system they replace maybe more.

http://mha-net.org/docs/v8n2/docs/WDBASICS.pdf
http://forgreenheat.blogspot.com/2013/05/a-review-of-wood-and-pellet-stove.html
http://demonstrations.wolfram.com/TheLawOfMassAction/

Thank You Donald: That was the clearest technical explanation I have read to date! Most people I that I give the rocket mass heater description too are willing to accept my numbers as fact ...however there are those that insist on arguing that their wood burner or pellet stove is better... (You Got This) and with my limited scientific knowledge I haven't had the scientific (ammo) to respond. Now thanks to your excellent post I will have a much taller pedestal to stand on while I preach the virtues of replacing every other wood burner with a RMH !

thomas rubino;

First of all. Isn't that picture from the road that sluffed off here in Washington, on highway 12 or 2? Stevens pass or somewhere left of Yakima?

Thank you for the kind words. I guess I needed to write it down somewhere to try to explain it to myself. I picked here.

Yes, it sure is the pass near Yakima. I don't know who photo shopped the pic, but to me it represents the attitude of most people today. If they see it in online or hear it on fox news it must be true...Don't look just leap... LOL ...I won't even get into our "choices" with this years upcoming election. Great post Donald, thanks for sharing

Chapter 2

So, in the first post of this string I gave data and some "proof" of what I was talking about. However, there were some holes. Holes that I think Wheaton labs can fill in with some experimentation. Likewise, I will be doing my stuff, and sharing the experience with you. Sound fair?

I went to google and asked it if there were a more complete reference. It showed me this textbook "Combustion and Incineration Process: Applications ... ", By Walter R. Niessen. This book is huge and chalk full of sciencey/mathy goodness. Warning: If someone puts a bunch of stuff in a book that does not mean it is gospel. However, I will use it to try to better understand what scientists define as fire (heat transfer) behavior better.

Hole 1: Mass Action: The temperature Remember above I said that there was some law (Law of Mass Action). And things are calculated to burn 80% because wood stoves (and stuff) do not burn as hot as a rocket stove. All the previous references said, at a "given temperature" the mass action will be constant. I assumed that constant changes
with higher temperature. I found the formula they use to calculate this constant "K" and is shown in Figure 1 from page 39 of book. The relationship of temperature to the constant is inverse logarithmic, but a little more complicated than that.

Translation: It approaches 100% burn slowly as temperature is raised. If you double the (assumed) temperature, you can get a few more % points up from the 80%. My guess ... about 10% points at most (I think that is generous) (that is an increase in theoretical limit of 12.5%). This constant "K" will be different for different substances. Figure 2 (page 40 of book) shows a graph of "K" for a lot of different materials. I do not know what those materials are but I use that to illustrate it has been calculated or measured. And surprisingly, some values of K go up with temperature and some values of K go down.


Hole 2: Mass Action: The Time This book suggests that there is a "half life", if you will, for things to burn. That means, in general, the longer burnable stuff is in a hot area the more of the burnable stuff will burn (giving off heat). If I understand the rocket stove build, after the initial burn the products (gasses, particles) are accelerated into a chamber where it creates an eddy (a donut of disturbance). This donut is at the top of the system, trapping (light) burnable gasses into a kinda vortex than makes them stay in a heated area for more "half lives" than they would in a wood stove. In a wood stove, as soon as something is burnt, the products head immediately to the chimney. Let us do some simple math for a couple of seconds. If the burnable material is in the vortex for 1 half life (HL) 50% of that material will be burned. In for twice as long (2 X HL) 75% burnt, in for two times longer (4 x HL) 93.75% and so on. Now, I am guessing that the half life will be different for different material. I do not know what this half life time is and the book does not come out and say it yet. After the gasses have given up energy in the process (burned) they "float down" to the exhaust of the chamber and out into the serpentineing tube.

Hole 3: Mass Action: Turbulent Mixing Yea, this "donut of disturbance" violently mixes the gasses and the heat together. This is kinda like a feedback loop of mixing heat and gas. The draw of the stove starts the flow going, then, as the air is moving to the chamber, it turns a corner and goes up, accelerating the gas and stuff. When it gets to the top, it is a gas with heat energy and kinetic energy (energy of motion) wanting to continue up. It finds a barrier and gets deflected. This adds heat and pressure to the donut (a feed back). As the gasses (and other stuff) mix with the high heat, it burns more completely and loose energy. When the gas has lost enough energy, it gets off the donut and travels down the chamber to the exhaust. A wood stove, by comparison, will not have a "heat feed back" and as soon as the wood burns there is no real mixing, it just goes to the chimney.

What do Hole 2 and Hole 3 do to add to efficiency? Not quite sure. What we need is a measure of the "half life" of burning fraction. Then see what turbulent mixing does to that half life. I think it may be pretty complicated to find a new "half life" because the feed back may make create a less predictable formula than the normal half life formula. I say that because I suspect the half life was calculated with laminar (steady) flow.

Hole 4: Burn Products Photosynthesis produces one product in general. Sugar. Technically the formula is CO2 + H20 + sun light energy makes C6H12O6. That is the sugar molecule. That is the only thing in cellulose. So the calculation in the first post is to burn only sugar (C6H12O6). The article that calculation comes from also mentions there are other things like ammonia, methane, and a bunch of other things. These compounds contain a different amount of chemical energy than sugar. I will have to do more research on how much more heat they can contribute and how much they would in a rocket stove. In a wood stove, chances are, the temperature would not necessarily get high enough for the "other stuff" to burn.

It looks like I may have a little more reading and calculating to do. my best guess of what the differences are. I believe the physical max (80% of theoretical) can be changed for the rocket stove to maybe 90% of the theoretical. That is a change of 8,600 BTU to 9,700 BTU where the theoretical is 10,800 BTU. That 10,800 BTU is derived from burning a pound of sugar and should be changed to a theoretical value that represents all the stuff in the wood. Even if it makes the new theoretical value smaller... yea, I said that.

I will keep going on it. I hope you still want to join me for the ride?

It's good to be investigating the theoretical basis for improved combustion. There are a couple of points you mention here that want clarification, though. Holes 2 and 3 are primarily covered in RMH practice by the height of the riser and the turbulence generated in the bend from burn tunnel to riser (and by the P-channel at the beginning of the burn tunnel, if provided). The time required for combustion is generally achieved while the gases are moving up the riser (mixing all the while), and combustion is essentially complete on reaching the top. The torus at the top of the barrel may induce a bit more combustion in some cases, but is secondary.

A test J-tube in open air may have flames shooting out the top, but when the barrel goes on, the velocity will be reduced so the combustion generally takes place before the gases reach the top of the riser. I have seen some videos where pyroceramic glass panels allowed visibility inside a finished system in operation.

Chapter 3

Hi, everyone. Yea, we can talk about the riser/donut and completeness of burn between the two some time later, let's get on with the subject

Let's try to get some of more holes filled in.

I did some research and when Paul was arguing with supergirl the industry was performing a standard test on wood stoves called CSA B415.1-00. It has changed a little now (2015 standards are different) but this method of testing is worth mentioning. By the way, the test method is about $200, talk about a barrier to investigation. I found a test performed in 2009 on a batch of 16 wood stoves and that can be found in the link below. There was argument in the late 80's about the accuracy of the test, but it was better than anything they had so they went with it.

Test conditions: The goal is to burn a fire for a time and measure different parameters. Fuel-Kilm dried wood, as low on water content as they could possibly get. Exhaust measurement - particulate and flue gas levels were measured. Wood was burned steady at a low temperature (LT) and a high temperature (hT). They measured the temperature at two points in the exhaust, and did some other things.

Looking at the test ...

I agree. The tests sucks. It is highly inaccurate. I can judge that by the high variation in the measurements and most all measurements had high variance and variability. If I did this in a lab, my bosses would tell me "you suck!" and to construct a better test, immediately. On top of that, even if my bosses did not do that, I would quit before allowing my name to be on the report. But it is the "best" we could come up with in 1989 (did we really suck that much in the 80's?).

The test were usually about 7 hours. The low temp tests averaged at 725 F and the high temp tests averaged 830 F at 1 foot high inside the chimeny.

Other stuff (residues): Remember I stated there are things, other than sugar, burning. Some of those things can be found in Figure 1 labeled exhaust constituents. That is a partial table (there are many other substances tested for) of the things they found in the exhaust. As you can see the variance is quite high in those numbers. However, the table suggests two things. 1) the higher the temperature, the more completeness of the burn of these other things. 2) It is hard to measure consistently. I submit Figure 2 as a table of ignition temperatures of different compound shown in the table in figure 1. It is my assumption that a lot of the residues from a burn in a normal wood stove do not see the temperatures required to ignite them. However, they might in an environment of a little higher temperature.

Fuel moisture: I said Kiln dry wood was used in the test, but that is a standard, this was cord wood, however, measurements reveled the average is about 17% moisture for the wood burned. A few websites recommenced less than 20% but most people would burn and get a reasonable fire at 23 to 25%. These web sites suggest stacking your wood for 1 entire year to achieve less than 20% (but no guarantees). I stated above, you generally get about 1% point change in efficiency for every 1% of water content in the wood. So for most folks (using a wood stove), the wood would automatically be 5% to 10% points different in efficiency over the fuel tested. A rocket stoves wood, in general, would have an advantage because of the higher surface area exposed to air. You may even get kiln dry levels.

Temperature I stated at the high temperature test (830 F at 1 foot) and a low temp test (724 F at 1 foot). I have seen some things about rocket stoves to get as hot a 1,800 F but in general maintain at 1,300 to 1,500 F (in the rocket). (Please, correct this).

That suggests to me that much of the "residues" did not see temperatures high enough to ignite (or they would not be detected in the exhaust). Looking at Figure 2, I would say that there is good probability for more burning in a rocket stove (if not all) than a wood stove. What does this add? Like I said, I would have to find out how much of this stuff the wood would produce. However, in the tables it looks like these residues add about 20% to the calculation of Theoretical energy.

This temperature difference also helps with our "law of mass action" adjustment to the completeness of the burn and theoretical energy stored value. Now, I did say that it is complicated but we can take a quick look. This wood stove test had two temperatures (725fF and 830F) and the general difference between the two theoretical max energy calculation for the two was about 175 btu/lb. If the difference were linear ( a straight line) the new theoretical max becomes. 850 btu higher per pound. That is assuming linear. However, we may be able to just say about 500 BTU for a theoretical max energy of 11,300 BTU/lb. That is a difference of about 5%. Again we would need to add in the effect of the residues igniting.

Chapter 4

Transport efficiency This is the efficiency of getting the heat generated into the room being heated before it leaves the house in the form of exhaust. Now, I commented before on this. I do not want to get into it too much. There are 3 mechanisms for heat transfer, conduction, convection, and radiation.

For a wood stove there is little surface area devoted to these 3 forms of heat transfer. The "conduction" is the fire warming up the stove and part of the pipe. Since the stove is metal it is a good conductor of heat. Infrared radiation will "radiate" from all (very limited) surfaces of the stove and pipe. Convection will only generally occur above the stove, not the chimney.

For a rocket-mass stove: The "rocket" and the"mass" has a lot of surface area and a lot of volume (compared a wood stove). Therefore, there is a very high potential for storage and transport of the heat in to the room. On the "rocket" part, it is insulated until you get to the riser which is metal (a conductor of heat), that focuses more of the heat from the burn into the riser. In the "mass" part, the exaust pipe is in contact (conductive connection) with rocks and stuff (can be anything). Then, the "mass" has a "semi-conducting" shell (the outer part of the bench or bell) for slow heat release. This "mass" has a huge volume for storage of heat and the outer shell has a huge surface area to radiate and convect. And if you sit on it, conduct.

What I noticed in the report was, for the stove testing at 825F (in chimney) at 1 foot, it was 650F at 8 feet (in chimney). What that implies to me is the transport efficiency might be much lower than the 70% I stated in the first post. Subtracting room temperature (70 F) from both and dividing suggests only about 25% to 30% efficiency. Ok, there is something I am surely missing.

Multiplicity: When you look at the individual efficiencies (i.e. design and transport efficiency) in any "system", usually the efficiencies multiply (not add). So let's pretend you have a "design efficiency" of 60% and a "transport efficiency" of 50%. The "system efficiency" would be 30% (0.6 X 0.5 = 0.3).

At this point it is looking like heat transport is a bigger contributor to inefficiency than the inefficiency of the design. let us say we calculate the wood burning stove to be 80% efficiently. If the transfer efficiency is 25% , that would mean the "heating system" is only 20% efficient at actually heating the room. Wow, no way!! is it really as simple as that?

Summary in Chapter 6

http://heatkit.com/docs/ec_report-verification_of_emission_factors.pdf

Donald Kenning wrote:

Transport efficiency This is the efficiency of getting the heat generated into the room being heated before it leaves the house in the form of exhaust. Now, I commented before on this. I do not want to get into it too much. There are 3 mechanisms for heat transfer, conduction, convection, and radiation.

For a wood stove there is little surface area devoted to these 3 forms of heat transfer. The "conduction" is the fire warming up the stove and part of the pipe. Infrared radiation will "radiate" from all (very limited) surfaces of the stove and pipe. Convection will only generally occur above the stove, not the chimney.

For a rocket-mass stove: The "rocket" and the"mass" has a lot of surface area and a lot of volume (compared a wood stove). Therefore, there is a very high potential for storage and transport of the heat in to the room.

What I noticed in the report was, for the stove testing at 825F at 1 foot, it was 650F at 8 feet. What that implies to me is the transport efficiency might be much lower than the 70% I stated in the first post. Subtracting room temperature (70 F) from both and dividing suggests only about 25% to 30% efficiency. Ok, there is something I am surely missing.

Multiplicity: When you look at the individual efficiencies (i.e. design and transport efficiency) in any "system", usually the efficiencies multiply (not add). So let's pretend you have a "design efficiency" of 60% and a "transport efficiency" of 50%. The "system efficiency" would be 30% (0.6 X 0.5 = 0.3).

At this point it is looking like heat transport is a bigger contributor to inefficiency than the inefficiency of the design. let us say we calculate the wood burning stove to be 80% efficiently. If the transfer efficiency is 25% , that would mean the "heating system" is only 20% efficient at actually heating the room. Wow, no way!! is it really as simple as that?

Summary in Chapter 5

http://heatkit.com/docs/ec_report-verification_of_emission_factors.pdf

I only glanced at the report really, so I could have very easily missed the detail. Those figures of 825 at one foot, I vaguely recall that it was specified as 'within the chimney'?? Heck, I should check again, might save us both time! Luckily the download was still in the toolbar, so can keep this reply open.

Table 2 specifically mentions 'low chimney' and 'high chimney', and the preceding blurb says 'In general the higher emissions heater had lower flue temperatures than the lower emissions heater.', again specifically mentioning flue temp.

Table 8 has explanatory 'One foot (30 cm) above heater in chimney, average', again in chimney.

I scrolled quickly thru again so could certainly have missed it, but using those figures from 'in chimney' to have a stab at the heating transfer efficiency could lead to skewed results. Which flatter the stove I might add!

You probably read it more deeply than I did (yawn haha) so apologies if I missed the part and have the wrong end of the stick.

Chapter 5

The Fuel: OK, this may not be considered part of the scope of the discussion but I feel compelled to mention it. The fuel does not magically appear next to the stove. It takes energy, time and in some cases garbage to put that fuel next to the stove. In this discussion of efficiency, should this argument be placed?

Suppose I break a twig off a tree outside my door and put it into the "wood heater" (here I mean, rocket stove, wood stove or pellet stove), that would be a different expenditure in energy, time and garbage than shipping the fuel a few hundred miles in containers that will be garbage.

Cord wood or mill ends: Free is relative. You still need a chainsaw to cut it (fuel and time), an axe (or wood splitter) to split it (time and/or energy) . Stacking and bring into the house. Cutting a cord of wood and putting into a pick-up may take an hour and cost about 1 quart of fuel in the saw. I will assume in a pick-up truck you will expend 50 ton miles per gal. ( pick-up to carry 1 ton of stuff 50 miles on a gallon of fuel). So say the wood is 10 miles away and an average cord of season wood is 2 tons, that is 0.4 gallons of fuel to transport 1 cord (and the same to get there). Now, you need to split and stack the wood. I would estimate 1.5 hours for normal wood stove splitting (per cord) but 2.5 hours for splitting a cord of rocket stove wood. That may seem like an argument against rocket stoves. However, most people who use rocket stoves see the fuel resources all around them (it does not have to be cord wood, it could be junk mail). So for each cord you may have to invest 3 to 4 hours and a gallon or more of fuel just to get the wood pile next to the wood heater. Good seasoned cord wood has a fuel moisture near 20%.

Pellets: Yea, it costs money to buy bags of these things. It saves the user a lot of time. just drive to the store, buy the bags, stack the bags at home, put the pellets in the hopper. It is like $5.00 a 40 lb bag here in the tri-cities. So, it is like $250 per ton of pellets. Driving to the store and time may be 1 hour and 2 gallons of fuel per ton. However, that is not the true cost. The factory is more than 200 miles from here. This factory uses wood shavings (from a mill) and spends energy making the pellets (I do not know how much). These pellets go into plastic 40 lb page (about 0.1 lbs each). They are stacked on a pallet in 1 ton increment and wrapped in shrink wrap for the trip. They next get a plastic cover over the top and put on a semi. So for me to buy 1 ton of pellets it consumes (at a minimum) 4 gallons of fuel, 6 "man hours" and $250 to me. It would also create about 10 lbs of plastic garbage (not very recyclable). Pellets have a moisture content between 5% and 10%.

Junk mail and twigs: yea that is the stuff, it comes to your house or in your yard for free. You can even take the junk mail and twigs from your neighbors. A rocket stove will achieve a high temperature and I speculate it could break down any added "nastiness" of the junk mail.

The Junk: The report in the previous post measured the "byproducts of burning" that go up the exhaust. But that is not part of the efficiency argument and is a great argument for another time.

An argument for (or against) pellet stoves: A pellet stove works by feeding in small amounts of pellets into a burning area augured from a bin of pellets. A fan (roughly 75 watt on low) pushes air though the burn area and up the chimney. There is a separate channel that air enters the stove and is pushed by the burn area (heating the air but not picking up smoke and particulates or fumes) which may be another 75 watt. There is a controller that manages all of these processes (including a start up protocol). The pellet stove can not work without the input of electricity and the fuel can only be in the form of a pellet.

While the stove makes up (somewhat) for not having a mass by forcing air around the chamber, the exhaust pipe is still hot (maybe 300F). This is called avection (force air convention). The burning temperate is a little higher due to the other forced air (through the burn chamber). So, the design efficiency may be higher than a wood stove it still does not reach the internal temperature of a rocket stove (so probably a little less). My estimate, 3% more than a wood stove for the burn. The fuel moisture being about half (10%) of regular cord wood can give an extra bit of efficiency. The transport efficiency may be much higher than a wood stove but the exhaust is still hotter than the exaust of a "mass". However, we must add in a per pound measure of BTU's per lb of pellets to operate the fans and auger. Assuming you will use 170 watt per hour to operate the fans and auger that is 580 BTU's per hour. That is using the stove on low flow, so if the rate is 2.5 lbs per hour, that means it is roughly 230 BTU/lb of electricity used to operate a pellet stove.

So what does all that mean. You get better design efficiency and transport efficiency in a pellet stove then a wood stove, however, from the temp of the exhaust and the temp of the burn area, it will not be as efficient as a "rocket" - "mass" stove. The efficiencies gained (over a wood stove) by the overall design are also augmented by the use of electricity to make it work.

Stay tuned for Chapter 6. Final summary.

http://cta.ornl.gov/vtmarketreport/pdf/chapter3_heavy_trucks.pdf
http://www.rapidtables.com/convert/power/Watt_to_BTU.htm

Thomas, Great picture, Giant gap where the road has fallen away. Thanks for posting

Richard

Donald Kenning wrote:

Multiplicity: When you look at the individual efficiencies (i.e. design and transport efficiency) in any "system", usually the efficiencies multiply (not add). So let's pretend you have a "design efficiency" of 60% and a "transport efficiency" of 50%. The "system efficiency" would be 30% (0.6 X 0.5 = 0.3).

At this point it is looking like heat transport is a bigger contributor to inefficiency than the inefficiency of the design. let us say we calculate the wood burning stove to be 80% efficiently. If the transfer efficiency is 25% , that would mean the "heating system" is only 20% efficient at actually heating the room. Wow, no way!! is it really as simple as that?

Summary in Chapter 6

http://heatkit.com/docs/ec_report-verification_of_emission_factors.pdf

Hi donald

not sure quite where you are going with this. Originally I thought you were doing rough back of the envelope calcs to get an idea of the efficiency of the transfer of heat to the room, but using (as it turned out) the wrong figures. The idea was sound enough. But this section did not change so am left a bit unsure now. I presumed the rough figure of 20% was that 625 is roughly 80% of 850...excepting now we can't use those figures...in order to multiply 20% by 60%.

As they are in chimney figures, well I think about the best you could use those figures to tell us anything would be that that is the heat, or efficiencies if you want to look at it from that angle, of the flue itself. Apart from making it easier, and cheaper to install and keep clean, I would imagine a lot of the reason for exposed flues in the space is to gain extra heating?? I am not sure if it was stated in the paper, but where was that temp gathered?? Ie, was it the surface temp oif the flue at those places? I ask, because (tho I could be completely wrong) I find it a little hard to believe/grasp that if it was measured in the centre of the gas stream it could lose that much heat in eight feet. One would presume there is quite a bit of flow, how long would the gas take to move eight feet I wonder. Anyway, just sounds implausible but then again I am certainly not an expert.

Maybe one way you could use those figures (flue temps)...it seems you like a bit of thinking and number crunching...we are told how much fuel is burned and in what time. That will give us a mass of air flowing thru the system in unit time. We already have the temps, you could then calculate the mass of air at those temps leaving the system and come up with a loss figure of the heat via the exhaust.

Chapter 6: Final Summary

We are almost there. Bear in mind, this is a simple literature search. People with fancy letters next to their name study fire/combustion/heat dynamics for years, I started last week. I could be wrong on some or all of this stuff.

I wish I had a lab filled with highly accurate instruments, rocket stoves, pellet stoves and a suite of high power thermodynamic and mathematical software.

So, here goes:

Executive Summary:
The main topic of this discussion is "efficiency" and the perception of it for heating systems. I believe, with my limited research skills, enough anecdotal data and scientific research exists to "prove" or at least empirically define what efficiency actually means in this case and what people have mistakenly think it means.

"Design efficiency" is defined as the efficiency of the wood burning device to turn the chemical energy in the wood fuel into heat energy. "Transport Efficiency" is defined as the ability of the wood burning device to transport the heat generated in the device, to the room which it is heating instead of out the exhaust. "System Efficiency" is the total of or combination (product) of the two other efficiencies (SE = DE X TE). In my estimation, wood burning manufacturers specify the "design efficiency" and people perceive that at the "system efficiency". This causes large confusion and "heated" debates (pun intended) based on misinformation (or incomplete information).

While that may be the case, the estimates of efficiency may also be flawed by the testing instrumentation or procedures causing major inaccuracies (uncertainties). Also, "design" efficiency is usually compared to what theoretical limit of the design, not the theoretical limit of the energy stored in the wood fuel. Likewise, most people embroiled in this type of argument fail to analyse the wood fuel and waste of the wood burning device and ignore this portion of the efficiency calculation. This wood fuel does not magically appear next to the wood burning device and the ash/waste does not magically disappear from the device.

My limited knowledge of thermodynamics and the barriers the wood device industry places on transparency have hindered some of these effort but I feel I have made a reasonable effort for the time constraint (do not want to do more) and limited resources (I have never even touched a rocket stove). I am reasonably assured, without further investigation, that a "rocket" - "mass" stove (compare to wood or pellet) IS the most efficient heating alternative of the three. That is, from the standpoint of conversion of chemical energy into heat energy filling a room. Likewise, it is my opinion that it also is the most efficient from a fuel gathering and waste minimization perspective in the parameters of labor, transport fuel, garbage created and money spent. While I did not do a complete analysis, I believe the initial creation and end of life environmental considerations also point to the "rocket" - "mass" stove being the most advantageous.

The Numbers:
I compared the three systems and gave a rank for many factors for the "wood stove", the "pellet stove" and the "rocket-mass stove". I submit Figure 1 as a table of that ranking. Likewise, I tried to come up with estimate of actual numbers corresponding to the issues raised in the Executive Summary. I submit Figure 2 as a table of those number. I would like to point out, these are best efforts numbers based on the investigations of the previous chapters. In the end, these are just (educated) guesses. I assume in the first line of "design" efficiency the numbers are compared to 8,600 BTU/lb for the wood fuel.

Notice, I do give a "system efficiency" estimate for the three. I would like someone else to provide some rigger into verifying some of these estimates. Since these are guesses, I do not want people to state the number to others as if they were gospel. If there is at least a little consensus with the people that work with the stuff, that would be good.

What I did not do?
Well, I did not feel like including estimate of energy release for extra constituents (flue gasses) that are burnt in the "rocket" stove that are not burned in the wood or pellet stove. I did not provide an estimate of uncertainty (precision, accuracy) in the values in the table, which I would normally do but I am lazy. Look both ways before crossing; in other words, I did not try to approach every consideration from as many angles as possible (like include a estimate of "excess air").

I also did not discuss certification or ascetics. Many people may not be scientists but they look for scientific approval and safety in the products they buy. Getting UL or some other type of thing is a process that mass produced wood burning systems went through. Even if it was very inaccurate, they were pedigreed in some way. "Mason" type stoves usually are custom made and in general would not get a certification (that seal of approval from "science"). Ascetics is way out of the scope of this conversation, however, I believe that sometimes it is the basis for people to do the science argument. They may think the stove is ugly, but instead of saying that, they argue about certification or efficiency or both.

What I would like to see:
My eyes, nose, mouth and hands are instruments to detect things like temperature, particulate levels, and other stuff. I would like to see more accurate measurements of "rocket - mass" stoves with some actual scientific instruments. Temperature at bends, temperature in riser, temperature of air entering, temperature of air leaving and temperature everywhere else. Air flow everywhere. Measurement of the particulate levels and gasses. The instruments exist to do this. I would like to resolve the questions people brought up (products burn in the riser or in the donut of disturbance) during this write up.

I would also like to see the RMS come out of the DYI mode and actually become a product people would have the patients to install (2 hours instead of 2 days).

Science: Should never be intimidating to anyone. In many ways, it is like permaculture in that you "observe" and then you do some other stuff (OK a bit thin). However, NO ONE SHOULD EVER TRY TO INTIMIDATE YOU WITH SCIENCE, the principles of science are there to help explain (the most trivial things about) the world around you and are free to all (third ethic?). Physics helps you understand the energy around you and allow you to harvest and store it (2nd principle?).

Do you need hard science?
In this debate? Not really. Paul says, even anecdotal evidence is science. I agree. Even though I went through this week of thermodynamic hell, most people do not need that. You can show them that people are burning less, working less, spending less. Most people who say they want a scientific explanation would not know how to scientifically argue themselves. However, if they insist, you could point them to this post and have them spend a few hours studying.

A story:
I love my flat screen TV. When I put the flatscreen TV next to my old school TV (CRT) I could see the flatscreen has amazing color fidelity compared to my current TV. I also realized, the resolution made things look so much more like I was actually there. There were all kinds of plug-ins in the back (HDMI, VGA, and much more) that I did not have on my old TV. I could imagine that some of my shows, movies and gaming would be a completely new experience even in reruns. I saw the "pretty" and I wanted it now!!! But wait, I do not know the science behind it. I asked the sales person to provide a more scientific explanation to me before I buy. The resolution, HDMI, color balance explanations from the sales person were not very technical. That could have prevented me from buying the TV, and then I realized, I love the "pretty" TV. I want it now!! I guess I can figure some stuff out later. I like the pretty!

OK that sounds shallow, but from my standpoint, I do not need to know how everything works before I buy it. I realize and appreciate the science that went into it, but that does not always mean I have to master it. (So, don't hate, K?)

If you are reading this sentence, you have come to the end of the ride. I thank you for taking this journey with me. I would not wish this on my worst enemy but I couldn't recommend it enough to my best friend. If you are reading this, I hope to count you as a friend as well. Thank you for your indulgence.

http://energy.gov/energysaver/wood-and-pellet-heating
https://www.epa.gov/sites/production/files/2016-03/documents/discussion_paper_-_process_for_dev_imp_cwtm_030916.pdf

Interesting topic. It has taken me back to a few places, and made me consider some new ones.

Perhaps you have some confirmation bias creeping in Donald But I will say I like your approach, before I provide some critique.

Some of the answers you are looking for are. I believe, in places you haven't yet looked. The Woodstove Decathlon had a rocket heater in the mix, the cool Walker Stove, and this was tested alongside a range of other stoves.

The New Zealand method for testing stoves is done in a calorific room, all the energy entering or leaving the room is measured. So changing the useable calorific value of the wood would not change the efficiency of the stove in this test. This alone might cause you to re-evaluate some calculations and inferences you are making. There is one model stove sold in both the USA and New Zealand that tests as 78.5% using, I assume, the British Standard, and 72% with AS/NZS 4012. Am I right in assuming the British Standard uses the stack loss method?

In regard to the reduction in efficiency of water, calcs I did at one point showed the difference between green wood at 50%, and air dry at 20%, is only 6%. This was working out the BTU's to boil water. Also, if you assign a greater calorific value to wood, then the percentage used to boil off the water reduces.

Can I assume that a typical Rocket stove is a lower overall output, and therefore a lower mass of wood used? If so then there is a negative side to the Law of Mass Action to join your positive ones

The second law of thermodynamic may work against some mass heaters. A very hot box made from steel or cast iron gives off heat a lot faster than a warm mass. There are other factors at play arising from this law here that I won't go into here.

FYI the IntensiFire saw 1635°F 2' up the flue, and CO levels started rising exponentially once that temperature dropped below 1380°F. That looks to support the ignition temperature figures published. Note that despite these high flue temperatures the IntensiFire outranked two stoves for efficiency in the Decathlon that have been measured at 80 & 82% (also I assume using the British Standard).

I think it's hard to say anything about rocket mass stoves in general. There are a lot of designs out there.

In general, it's possible to get 90+ thermal efficiency out of any heat source with the right heat exchanger. There's Mother Earth News article from a while back where they built a masonry stove and got 93% on their first try. Just takes a convoluted flue that's matched in length to the heat-transfer rate and capacity of the flue materials. The heat source could be in engine, a fire, thermonuclear system, etc. So efficiency is largely controlled by the flue, regardless of the heat source.

A clean burn is another part of the efficiency. High-efficiency stoves usually have some sort of way to circulate the gas inside a stove. I don't see anything about the rocket mass stove that makes this happen. I suspect that we're all getting excited about a carefully managed fire. That's great but let's not over-sell it.

I've still to see any data that's credible and reasonably accessible.

Also, a 200 $ test is no big deal. Most standards labs cost 1000's for any battery of tests!

I see in the first post you talk about the theoretical energy stored in wood and say that it doesn't vary that much.

Here are some actual numbers to reference (note the values & units are millions of BTUs/cord of wood - so energy per unit volume):

Osage Orange (Maclura pomifera), 32.9 BTUs
Oak, White (Quercas alba), 29.1 BTUs
Locust, Black (Robinia pseudoacacia), 27.9 BTUs
Ironwood (Ostrya virginiana), 27.9 BTUs
Hickory, Shagbark (Carya ovata), 27.5 BTUs
Apple (Malus domestica), 27.0 BTUs
Honeylocust (Gleditsia triacanthos), 26.7 BTUs
Hickory, Bitternut (Carya cordiformis), 26.7 BTUs
Oak, Bur (Quercus macrocarpa), 26.2 BTUs
Mulberry (Trees from the Moraceae family), 25.8 BTUs

These are taken from this blog post https://www.permaculturereflections.com/top-10-fuel-trees-for-zone-5-and-above/
which reference a "a study done by the University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources. I also consulted a study published by the University of Missouri-Columbia along with various other online resources."

I think the University of Nebraska paper might be:
http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1858&context=extensionhist
titled: G88-881 Heating With Wood I. Species Characteristics and Volumes by Mike Kuhns and Tom Schmidt

This paper provides the amount energy stored in the wood (as seen above) but lists many more types of wood than the 10 listed above. It also lists some qualitative properties for each type of wood - such as ease of splitting and how many sparks are produced when burning the wood.

I think the University of Missouri paper might be:
http://extension.missouri.edu/p/g5450
titled: Wood Fuel for Heating by H.E. “Hank” Stelzer

Something that the Missouri paper points out is that "One pound of oven-dry wood of any hardwood species has an available heat value of about 8,600 Btu. Resinous softwood species, such as shortleaf pine, tend to average slightly higher at 9,050 Btu per oven-dry pound." Another interesting tidbit from that paper is "On a volume basis, the heavier air-dried wood is, the more heat it will produce. Therefore, a given volume of the heavyweight woods, such as oak or hickory, will produce more heat than the lightweight ones, such as cottonwood and willow." In other words, if you are looking for a greater amount of heat per volume of wood, take a hardwood, but if you are looking for a greater amount of heat per unit weight of wood, take a softwood.

Anyway, I hope these data points might help some in the discussion.

--matt

All of the calculations here have been on the basis of mass, not volume, so the different amounts of heat available from a cord of different kinds of wood are accounted for. Resinous vs. hardwoods are an interesting consideration.

I live in central PA. Does anyone reading this who operates a RMH live in central PA? I would love to come visit you to see how this thing really works and if it might be something we could incorporate into our next home (we are in the early planning stages). Thank you for considering it!

I am just east of Binghamton, NY, not exactly central PA but close. My home RMH is nearing completion but not quite operational yet, and I would be happy to show it off later in the summer or fall. I also have a rocket-fired bake oven which works nicely.

Glenn, Would you happen to have a link to photos of your rocket-fired bake oven that you would be willing to share? Thanks.

Glenn, thanks for the invitation! How do I give you my email so you can let me know when your RMH is up and running & open for a visit?

Thank you all for the discussion.

What I particularly liked is Emilie McVey participation in this topic. If 1 person can look or re-look at rocket-mass stoves, I feel all the analysis is worth it.

So, let's address the other issues.

$200, A barrier to investigation:
In chapter 3 of this discussion, I talked about The test method costing $200 not to have a test performed. I have spent and charged a lot of money over the years for tests, however, the documentation that tells you how to perform that test has always been free. Labs and stove manufacturers might buy this test. I was not going to spend $200 for my curiosity and this post. Besides, I had enough evidence for free.

Accuracy of the analysis:
There are about as many variables here as there are homes to heat, I recognized that from the start and I am used to making more accurate analysis for more controlled conditions than what is presented in the subject of "Efficiency of wood heating systems". However, I approached it in the same way I mostly do. In the end, all I had, was generalities and inaccuracy in measurement. However, certain parts of the analysis stood out to show that there is a difference between the systems. Focusing on analyzing the differences, I believe, was the most fruitful way to determine the differences in efficiency. I stand by my methods of trying to obtain the closest approximation to a comparison I could get. Could I have researched it more? Sure, you betcha (in the summary of chap. 6, I state people study this stuff for years). When I wrote chapter 6, I felt like most other "facts" unveiled would probably add in only a small way to the overall analysis.

When creating this string, I felt compelled to analyse the efficiency of the three systems (system efficiency). Then my obsessive nature took over and I decided to analyse the "entire" efficiency. That is to say, I took into consideration the costs in time, money, transport fuel and garbage of these systems from cradle to grave (or even cradle again).

My measuring sticks:
I noticed early on that BTU/lb was a weight measurement when eventually I would have to say something about a measurement of a volume (cord of wood). Thank you Matt Goto for providing those measurements in terms of million BTU per cord. However, I decided to use the measure the industry uses so that we talk about their standard. In the end, some of those considerations fell out. I also used 11,300 BTU/lb which may seem a bit "cheesed" up. The point I was making here is two fold, the 10,800 BTU/lb may be inaccurate at higher burn temperatures. In the "mass action law" they say the burn is dependent on temperature and I also realized some of the minor gasses would also burn at those higher temperatures (adding btu's). The number could be higher or lower, however, I could not find a reference to help me with this, so I took an educated guess. The temp of 1,300 to 1,500 came from another string on this forum, I can't find it now, but it is around here somewhere.

In the first post, I say something about excess air and it's effect on efficiency. I was not quite sure how to treat it for the chapter 6 summary so I left it out. But yes, excess air is a factor in efficiency. In a post on another string, someone talks about mixing gasses at high temperatures. I believe that is addressed here. My main point with that is that all gasses and solids will "see" that higher temperature for long enough (several half lives) to have a reasonably complete burn.

Masonry Stoves:
Should I have brought Russian, Swedish or some other masonry type stove into the discussion? Well, since these are custom like rocket mass heaters, I originally thought the "entire efficiency" calculation may take me some time, that I did not want to spend. The points raised are valid about them having similar "system efficiency". And any arguments about maintenance and "efficiency of gathering fuel" for the fire are similar to a rocket mass stove.

If we cull those points out of the discussion we are mostly left with the "birth" and "death" considerations of a masonry. Birth: Well, we are looking at a similar expense in refractory bricks and stuff, however, the $10,000 expense for the brick layer time and the environmental impact of the bricks and mortar used in the "mass" part are very different than a rocket stove's earthen approach. Likewise, people doing the bench tend to use linseed oil, where the brick and mortar may be painted with stuff. I do not know about the "bell" or "batch" systems. Death: I am not sure about longevity of the rocket mass stove verses the masonry stove. I am guessing the masonry can last a lot longer. But at death, where can all the stuff go? For a rocket mass stove, most of it is "earth" already, and can probably go back into the earth. Can the bricks and mortar? And what about the "bell" build and "batch" build of the rocket stove? Can someone who actually owns one, chime into this string and tell me? I am also not sure about the riser and barrel, how recyclable it is at the end of life.

In the end:
I realized early on, that going down this path, may have been a flight of fancy. However, it was a journey I was taking at the time. I did not find what I was looking for right away, in this forum, so I ... decided to create a post for me to think this through. I want to thank permise.com for their indulgence in allowing me to take this walk freely, on their medium. I also want to thank the readers of this post, you keep me honest and on-task. You guys are great!!!

This may not be the best place to ask this question but if someone knows where it belongs, can they please move it. The higher temperatures of combustion appear to be a key element in this thread. As I see most RMHs built with a standard steel barrel I can't imagine that barrel lasting very long at those 1300 - 1500 degree temperatures. I have seen some cases where people have taken there RMH apart to replace the barrel. Can anyone provide any information on the life of the barrel? Is there a better option than the standard high carbon steel barrel?

Glenn Herbert wrote:

A test J-tube in open air may have flames shooting out the top, but when the barrel goes on, the velocity will be reduced so the combustion generally takes place before the gases reach the top of the riser. I have seen some videos where pyroceramic glass panels allowed visibility inside a finished system in operation.

i think you can see the riser flames in this system with glass

There is a major difference in how the wood is loaded in a RMH vs other wood stoves.  With a RMH the wood is external to the combustion chamber and exposed to the heat of the stove.  This allows the wood to dry prior to being burned.  With other wood stoves the 'charge' of wood is internal to the stove and all of the moisture must be cooked off in order to burn.  The phase change from water to steam requires gobs of energy.  

I don't think that is actually such a big difference. Yes, the surface water in RMH wood is driven off before the wood gets into the burn tunnel and burns, but if there were less water, the radiated heat would preheat the wood itself more. I think it's pretty much a wash.

Hey Donald, thanks for the good write up. It is a lot to think about. We are getting ready to build our "forever home" with a thermal mass heater as a major design feature. One thing that I wanted to add, is I think the "size matters" claim is actually wrong. Size does matter... but smaller is actually bigger in this case. You wrote--

Donald Kenning wrote: If you were to split the wood smaller the amount of void could be a lot less. I might even be able to stack  1.3 cords of regularly chopped wood on into 1 cord volume of finer chopped wood (a guess). So optically, it looks like less wood stacked, when in fact, it is less air stacked into the wood.

But actually, cutting up the cord smaller makes for more air pockets and a larger stack of wood. You can reference this thread for some amusing back and forth-

http://www.arboristsite.com/community/threads/true-or-false-a-cord-of-rounds.141219/

Particularly good is the practical mini experiment with carrot sticks. Try it! Isn't it great when science completely bucks our intuition? If only it would buck or logs too.

Also your other point in the "size matters" section about smaller logs having a faster drying time is definitely true.

Thanks again.

"I wish I had a lab filled with highly accurate instruments, rocket stoves, pellet stoves and a suite of high power thermodynamic and mathematical software."

So Donald, set up a "GO FUND ME" Account and get with it! I'll help with a donation, and I would think Paul would be interested in that as well, If he is interested in taking this to the next couple of levels. After all this can't be a Mickey Mouse operation.
" UL Listed" they do all kinds of testing for safety, and how efficient the product is. If going on a grand scale .....building a ship able core say, would it not have some kind of testing?

The problem with UL listing is that they only cover complete appliances, not components like a core. It would be pretty much impossible to get a UL listed RMH, as far as I can tell.

Some products that are listed are more stoves than mass heaters.

Glenn Herbert wrote:The problem with UL listing is that they only cover complete appliances, not components like a core. It would be pretty much impossible to get a UL listed RMH, as far as I can tell.

Some products that are listed are more stoves than mass heaters.

Well, in the case of a J tube. It's hard to get it "listed" But a batch, being soo close to a masonry heater, it should  be far easier, if fitted by someone doing masonry heaters on a regular basis. JMHO tho.

My attempt at a quick explanation:

Here's my take on the bottom line: When burning a given mass of a fuel, the device that puts the lowest temperature exhaust out of the building while still burning the majority of the combustibles wins.

we address some of those exact points in this new video series.   This is me, erica, mud, donkey and peter taking a stab at the most important myth:

It has now been three years and Peter makes a summary about this batch box project - including the efficiency testing.

To see the full build, check out the DVDs.

This was recorded at the 2017 Rocket Mass Heater Workshop Jamboree.   Me, Erica, Donky, Mud and Peter take a stab at carbon monoxide stuff while Ernie just looks pretty.

Addressing the suggestion that rocket mass heaters violate the laws of physics and thermodynamics

Hi...i am a new user here. In regard to the reduction in efficiency of water, calcs I did at one point showed the difference between green wood at 50%, and air dry at 20%, is only 6%. This was working out the BTU's to boil water. Also, if you assign a greater calorific value to wood, then the percentage used to boil off the water reduces.

https://youtu.be/jyQwgBAaBag

Another seeming violation of the law of conservation of energy, this one still has me scratching my head.   I think it's like a ram pump principle applied to a land sail vehicle?