how a RMH uses 1/8 of the wood of a 75% efficient wood stove

paul wheaton wrote:


Wouldn't the RMH effectively have the same short term output (per unit of propane) due to the steel barrel?  So all of the gains that you get with the mass would be from the stuff that was gonna shoot out the chimney anyway?

Classic TMI, lost in the woods, these are the things that can be corrected in the classroom vs here where time has been spent with the wrong direction of focus! It is why I am not sure effective online teaching can exist.

About the fifth line of the hyp: "for this exercise we are disregarding the steel drum portion of the RMH, not because it does not play a role but because it would be similar/the same as the steel wall of the CWS."

From a practical view we are throwin 1/4 pound of wood in each, that would be equal to just about enough energy to start heating the brick and steel. Do not forget this is not an accurate number portrayal of what will happen, it does however allow one to explore percentage differences in performance using identical fuel and burners and how the RMH has some comfort heating advantages over a CWS regardless of an efficiency comparison.

Rich, do you have a lab where you can put a person and a CWS and, say 20 pounds of wood, measure use, comfort, time, etc.  And then be able to repeat this with a RMH?  The reason I ask is that it seems to really come down to "the claim".  If "the claim" can be proven in a lab, it seems that would be mighty powerful. 

paul wheaton wrote:
Rich, do you have a lab where you can put a person and a CWS and, say 20 pounds of wood, measure use, comfort, time, etc.  And then be able to repeat this with a RMH?  The reason I ask is that it seems to really come down to "the claim".  If "the claim" can be proven in a lab, it seems that would be mighty powerful. 

Paul,

I have no such a room at this time, all of my power venting over the lab is within the main access. I do have a room in which I could duct over to the venting, however it is very small, smaller than pretty much the RMH heaters I have seen. It would be ok, however it would require extensive modification, (sealed room, no combustion air at all) and it would be a ways off before I would explore it, just being truthful.

I would also require the assistance of a skilled cob mason even if it was scaled down. While I am sure I am capable of learning to do it, if we were to be hedging bets on such a test I would want any prototype to be constructed by the most skilled in contrast to any amateur effort I might put into it if that makes sense.

I also am fairly sure once we have some hard data on it, some fairly simple math would clearly define performance expectations based on heat transfer surface alone.

paul wheaton wrote:
Wouldn't the RMH effectively have the same short term output (per unit of propane) due to the steel barrel?  So all of the gains that you get with the mass would be from the stuff that was gonna shoot out the chimney anyway?

No... and that was the point of the whole exercise. He set it up with both being 100% efficient and still had the stove with mass being more effective. He is giving you stuff you can argue with and win. Saying that one sends more out the flue than the other is laughable to anyone who has studied these things because you need heat to power the flue... even with a RMH if you are feeding a 30ft vertical flue... maybe not as much, but not enough of a difference to account for 1/8th the amount of fuel. With a horizontal exhaust, the RMH might be 10% better, but not 85% better.

With the RMH, despite the temp of the barrel at the bottom, the gas going to the mass is still pretty hot. The gas in the middle of the stream moves a lot faster and maintains it's temp, but the gas close to the barrel surface moves slow and gives up it's heat to the barrel quicker and therefore is measured cooler. The masonry heater guys who test their stuff pretty rigorously, measure both... and find the probe in the middle of a pipe shows way higher temperatures.

Len wrote:
He set it up with both being 100% efficient and still had the stove with mass being more effective.

Now THAT is exactly how you say it, hands down I love it.

MORE EFFECTIVE

That is the key absolute and because it is more effective it discourages improper operation, you do not feel like you need to load it back up as soon as the fire goes out because the temp does not start dropping. You do not feel like you have to move your furniture away from it so one side of your body feels like it is cooking, you do not throw three longs in to get it roaring to warm your hands after tending to something outdoors.

The CWS pushes folks around on the peaks and valleys of its operation and in doing so it causes the structure to lose far more heat than it should or will when well balanced!

I do not know if you can find good refrence information on "Manual J" by the ACCA but I think the community would find a great deal of information to help them out by learning about heat load studies on homes.

Professor Rich wrote:
Now THAT is exactly how you say it, hands down I love it.

I do believe the good doctor is becoming rather enthusiastic about the device he is studying

brice Moss wrote:
I do believe the good doctor is becoming rather enthusiastic about the device he is studying

Let me explain a couple of things that can give insight when dealing with a grumpy old fart.

But first, lose the doctor stuff as it does not apply at any level and I take a great deal of pride in not misrepresenting. You are speaking to a dying breed.

As a teacher one has a responsibility to evaluate students, some rely on testing and cross compare lecture series and modify on information missed by most etc. Early on in my teaching career I developed a significant distaste for using that method alone. It bothered me because it is after the fact and from my point of view, the student paid me to get it, not improve how the next student was taught. In my quest I discovered a very simple and accurate gauge I could use to determine interest.

Students ask 3 types of questions, what, how, and why and that really determines it by itself, a product of human behavior.

What- this is a question from a student only interested in gaining the knowledge required to pass a test, you have not reached them yet.

How- this comes from a student whom is satisfied they are ok for the test and are at least engaged enough to wonder how it works.

Why- this is time to turn up the heat, they seek a deeper understanding of why it works that way, they are hungry for it and it is time to feed them and foster the hell out of that curiosity.

We have this from Paul:

paul wheaton wrote:

If neither of these is accurate, then I would very much like to understand why they are not accurate.  That might help me to get onto a different path.

I certainly did not enter into this thread with the intent of "teaching" but if you look at the flow of it, it followed that exact path, this is WHAT it does, this is HOW it does it, and this is WHY it does it that way so to speak.

You and others may not understand obsessive compulsive behaviors like I have, but I tend to capitalize on them as opposed to allowing them to hinder me. Once Paul asked why, it is all over, I no longer have a choice and I must simply find a path to simplify the complex in such a manner I help them figure it out. I can assure you I did not peck out 3k words because I did not want to sleep, I had no choice, I had to try and gt him a path to WHY.

While Len's comments might seem benign to many, it showed that my point had gotten through and someone better at choosing words has made it perfectly clear. Some have said thank you or they appreciate the post etc and that's all good, nothing trips my trigger more than hearing someone sum up and completely grasp my point I was trying to drive. It is why I teach.

I enjoy playing mad scientist so fooling with the RMH is a given ad it is driven by my curiosity.

Enthusiastic is because some folks are showing true interest in a subject I am interested in and they are allowing me to share it and I get to watch as they figure it out which is really cool to me, there are not a lot of subjects I know much about. I only wish we had a room and a white board so the communication was real time both ways and included voice tone, inflection and body language so I could determine if my communications were being effective at the time of delivery.

What i have learned from your post is that rmh is 1/8 the effort .

How is by btu diffusion through thermal mass.

Why Thermodynamics.

What i want to know is.Could it be set up to auto tune.Is cob the best medium.
Would the thermal mass work better vertical. 

charles johnson "carbonout" wrote:
What i have learned from your post is that rmh is 1/8 the effort .

depending on.... insulation of building, volume of room, a persons comfort level... etc. Your mileage may vary.

How is by btu diffusion through thermal mass.

Why Thermodynamics.

What i want to know is.Could it be set up to auto tune.Is cob the best medium.
Would the thermal mass work better vertical. 

Auto tune? Use fuel of consistent size, shape, moisture, embedded energy... if you mean what I think you do. But it really doesn't matter, this is a manual load device that is fired once in a while... You fill the mass up with heat and it radiates it over time. It does not depend on the room temp and so automation to add fuel is not easy... as the device would have to be able to ignite the fuel as well.

Is cob the best mass?... depends... on what your meaning of best is. Able to get a permit/insurance? Use a masonry heater. It can even use rocket tech to work, but it would have to have engineered drawings and maybe a mason to install it. Materials close at hand? Cob is great. Easy? cob is great. Cheap? cob is great. It depends.

Vertical? For that warm feeling, I think horizontal is best... vertical takes less floor space.

charles johnson "carbonout" wrote:
What i have learned from your post is that rmh is 1/8 the effort .

How is by btu diffusion through thermal mass.

Why Thermodynamics.

What i want to know is.Could it be set up to auto tune.Is cob the best medium.
Would the thermal mass work better vertical. 

Well I am not stating that it is a 1/8, I am stating that the better matched balance of heat loss to heat gain results in far less heat escaping the space which results in folks not over firing their unit. In the case of Ernie and Erica (hope I got that right this time) they have in fact used 1/8 of the fuel they did in the past and this was simply an explanation of what is the largest factor that impacted that in contrast to efficiency losses as some thought those were the source.

Btu diffusion by thermal mass is simply taking energy that is over produced at one point, storing it for release at another point. It is not an uncommon practice at all and has in fact been used for years. Many campers have used it, warm rocks by the fire and then bury them under the space they are going to sleep. Some large buildings will run the air conditioning equipment on high overnight and make ice, then use the ice the next day to supplement cooling needs.

I do not understand the “why thermodynamics” question at all. Thermodynamics is the science of heat energy doing work. In this case it is working by heating the space, in the case of a steam engine it is converted to mechanical etc. I guess if I understood the question better I might be able to answer it, if your just asking me why I picked that, well it tripped my trigger and anything that does I become an extremist in pursuit of understanding.

Auto tune, well to some degree it already does that by itself and is governed by the size dimensions of the rocket assembly itself. As far as being able to tune it to differing outputs, that is not something I believe to be possible to the degree it is with other furnace types, I think we have some room for expansion of the range, but it will remain limited to some degree.

Is cob the best medium? When I get asked such questions I become frustrated because they are impossible to answer. The feedback from owners of the units and the choice of the words they use describing them indicate that the cob is a very good match for balancing the heat distribution, I suppose Occam would say it is the “best” as it seems the simplest.

There is no “work better” in altering the thermal mass, if it has the same mass and surface area, its heat distribution would be the same. Altering where that heat is distributed would however change the way the structure lost heat and how the persons within felt as you have taken some of the heat distribution and moved it up. Since heat rises from all practical points of view, the lower the “better” is what I would call best practice. There are photos of a system where the floor is a large part of the mass, while I am somewhat critical of it for other reasons, it is absolutely hands down the best potential heat distribution method for comfort heating.

Could part of the overheating factor involve how some parts of the room end up much hotter than others?

paul wheaton wrote:
Could part of the overheating factor involve how some parts of the room end up much hotter than others?

Without any doubt what so ever and is in fact a very large factor in what we are talking about.

The huge level of radiant heat along with the convection heat that travel straight up from a CWS is going to create a very large hot spot in the ceiling above. Since radiant heat is reduced at the square of the distance, if we call the ceiling flat, there could be a very significant difference from one side of the room to the other. When we are talking about our target temp of 50, I would not be at all surprised to see one side of the room ceiling at 45-50 and the area directly above the stove 100-150. The wall behind the stove also very hot while the one farthest away from the stove much much cooler.

This is the reason that many placed the stove directly in the middle of a cabin, an attempt to even out the heating of the room to get better balance. I will post a photo of a cooling set up and how the solar radiant heat impacts it, while the opposite of what we are talking about, simply pretend the sun is the stove and the cool space is outside.

Radiant heat is not “insulated” to contain it, it is reflected and different materials have different emissivity. In order to contain radiant heat one must couple a reflector with insulation to keep the heat where it is desired. With some of what I call “huts” where the RMH have been installed, a CWS would be blasting heat through the canvas materials almost as if they were not even there and the heat going to warm persons would be limited to what radiant hit them and the natural convection generated by buoyancy changes in the air.

1 is a radiant graphic I spoke of

2 is a thermal image of a house, red is high loss, blue is low. Notice one part of the roof does well and another doesn't.

Interesting.. In the thermal image, to the right where the roof is the hot spot, the windows are noticeably cooler (loosing less heat) than at the left, where the roof is loosing less heat than the windows and the effect appears reversed.
What do you suppose has caused that, Prof?

Very hard to know for sure without inspecting, but my hypothesis would be a vaulted ceiling on the right hand side of the home with the curtains pulled closed and a standard 8’ ceiling on the left hand side with the curtains wide open.

The vaulted ceiling likely has r-11 insulation and a vapor barrier but no radiant barrier, the standard ceiling on the other side likely has r-20 or better blown in insulation with no vapor barrier or radiant barrier.

The windows are most likely a cheap builders grade that test well in a lab but lose the inert insulating gas and more or less are just two pieces of 1/8 thick glass staked in front of each other.

They are also dumping a lot of heat into that garage on the right, a fairly common cheap way is to not bother putting insulation in the walls that touch the garage or if a bathroom backs up to it not to insulate the plumbing chase which might be 4’wide by the 8’ tall and lots of heat would dump in to the air leaky garage.

These folks could save a ton of money with some improvements, unfortunately they likely would need the services of a contractor to complete it which would push the payback beyond 5 years. If they have skills to do the work, they could likely recover their full cost and begin saving some time in the second year.

Since it is winter, interested parties can see this when you get a 1 to 2” snow, look at the roofs and the patterns of the snow melt, you can spot who needs more insulation very quickly.

Best guess I have on it Donkey, could be wrong.

Sounds reasonable to me..

So, correct me if I'm wrong..

You seem to be placing the lion's share of the success of Rocket Stoves on the mass, slow regulated movement of balanced heat into the space. (perhaps I can intuit that you would correct "balanced heat" into "matched heat". Meaning matched to the particular heating requirement (and feedback trained for proper firing, etc.) I've seen rocket stoves that work GREAT but don't entirely match the needs of the space.)

What then of the (I think) unique burn characteristics of the rocket stove? Doesn't the "burn full out, high temperature all the time" characteristic almost insure a more "efficient" burn? Certainly we're gaining THERE from rocket stoves as well? Wouldn't a complete investigation of the question START there, and what do you think would be found?

Thanks Professor Rich for taking all the time to explain things. I understand your excitement when people ask why and seek to understand. I get wound up as well when my co-workers start asking similar questions.

I don't see the RMH as a gasification stove. However images I've seen and the lack of significant clean out indicate it burns thoroughly. Is there a gassification stage, or just a high enough temperature to burn completely?

Donkey wrote:
You seem to be placing the lion's share of the success of Rocket Stoves on the mass, slow regulated movement of balanced heat into the space. (perhaps I can intuit that you would correct "balanced heat" into "matched heat". Meaning matched to the particular heating requirement (and feedback trained for proper firing, etc.) I've seen rocket stoves that work GREAT but don't entirely match the needs of the space.)

What then of the (I think) unique burn characteristics of the rocket stove? Doesn't the "burn full out, high temperature all the time" characteristic almost insure a more "efficient" burn? Certainly we're gaining THERE from rocket stoves as well? Wouldn't a complete investigation of the question START there, and what do you think would be found?

It is not so much that the RMH is a match, it is more what a mismatch a fully loaded CWS is more than anything else. Because of the limited loading capacity of a RMH and the significantly larger burn area of a CWS its variable capacity makes it a flexible unit, however most pretty much run them way over demand and losses result.

I think a RMH will win the emissions when it comes to a CWS by a little, I think the RMH will fall behind a catalytic stove significantly, but and it is a big but, because of the lower "typical" consumption, the lower emissions quantity would be a lower total. In other words higher by percent but lower overall.

This of course assumes I am correct about the first two theories of operation and that I am wrong about the third hypothesis I disclosed to you. If I am actually correct about that, the rules change significantly and it gets really interesting really fast.

Here is Paul's podcast interviewing Ernie and Erica on rocket mass heaters: http://www.richsoil.com/permaculture/182-rocket-mass-heaters-permaculture-podcast-019/

Has anyone mentioned that RMH stoves are for heating people not air?  Typicaly we heat air in a house not the objects in the house that people touch or sit on or sleep on etc.  Its simple you sit on the heater and your body is warmed no matter what the air temp.  Ianto at cob cottage told me they use about 2 cords a winter down there for the 2 or 3 residents at cobville in a normal winter.  Lindas House about 400 sqf Iantos old house, jaba the hut 80 sqf and the myrtle about 200 sqf.  finnish masonry heaters also work on the principal of mass, they just dont burn at 2000 degrees.  my 2cents

Professor Rich wrote:It is not so much that the RMH is a match, it is more what a mismatch a fully loaded CWS is more than anything else. Because of the limited loading capacity of a RMH and the significantly larger burn area of a CWS its variable capacity makes it a flexible unit, however most pretty much run them way over demand and losses result.

I've just been reading this and another thread. Thanks for your contribution. It took me a lot longer than I should have to figure out what was going on because I kept getting caught up on this "1/8 the wood business". I didn't realize that in order to benefit from that you had to virtually be on the cob bed a lot of the time. In much the same way, I could sit in bed with an electric blanket and consume 50W or so of electricity - very efficient. A hot water bottle is much the same idea. However, neither compare to being able to use the whole house in comfort. The comparison with a CWS and RMH is not apples to apples.

At the end of the day, if there are other engineers reading this, wood has only about 20,000kJ/kg, best case. If you want, you can work it back to the heat of combustion of burning cellulose (which is similar enough to glucose that we can use that instead). So 180g of C6H12O6 produces 2560kJ, and hence 1kg produces about 14,000kJ. Maybe the 20,000kJ/kg comes from tars, resins etc. that may have higher heating values. Anyway, this provides an upper limit to the amount of heat you can extract to heat your home. The only way to get something from nothing is through the use of a Stirling Engine sitting on top of your barrel, running a heat pump with the electricity produced, so that you might get a Coefficient Of Performance (COP) effect. So please don't make the same mistake I did and think that an RMH is doing exactly the same job as a CWS only with 1/8 the fuel - because it isn't.

Now, I know there is some talk about how with convection and a CWS, you get a hot spot and a lot of energy transferred through the ceiling. This may be true, however, I've found that with a heat pump there is not a lot of difference between the output of that and the ambient air in terms of temperature. You will not get the same losses. And yet the 8.5kW heat pump we have is barely enough to heat the home, if rooms are blocked off. If we run the CWS, and it's not a large one, it definitely heats the house up better. It gets comfortably warm. So the idea that this thing (which is almost certainly less than 20kW) is only 10% efficient or something in reality seems unlikely to me.

It does seem however that there may be some areas in which a CWS might be designed to be more effective. I would need to study the state of the art to see where the deficiencies lie. From my research, it seems that there are some problems with the EPA models. It seems to me that what would be ideal would be to be able to light the stove, load it up with enough wood to burn either 12 or 24 hours, and then have it both be damped down and with an efficient secondary burn, capable of both an efficient high maximum output AND an efficient damped state. Correct me if I'm wrong, but it seems that nothing like this exists.

If you want the power output of a CWS you can achieve this one of two ways. You can either use a thermal battery (I was thinking paraffin wax), or use the fuel itself as the store and figure out a way to do what I said in the previous paragraph. If you use paraffin wax, you need about 1000kg for every 5kW in average power, if a burn was instituted once in 24 hours. Of course, feeding such a device like a rocket stove would be a PITA and it would be better off making it run like a masonry heater, with an insulated fire box etc.

And the other option is build your house to Passivhaus spec, with all that entails. Certainly something to consider - supreme energy efficiency AND be comfortable everywhere in your house.

Prof Rich: This should be a simple question, you made a Post on December 21st, 2010, where you seemed to make your point that there was a requirement to -
'keep the flue (gas temperature?) above the dew point' stating that that required heat energy in the Flue !

Why are we, or how are we, enjoined against being allowed to gain the Latent Heat of Evaporation, though I am willing to agree that much of its loss can/must
occur within the Exterior Chimney, I do believe that we 'get back' much of that Occult heat within the interior piping of our Thermal Mass, While I KNOW that this
is not 'free energy' it feels that way and I for one want to celebrate it !

For The Good of the Craft ! Be safe, keep warm ! PYRO Magically BigAL ! - As always your comments/questions are encouraged and are Welcome ! A. L.

allen lumley wrote:Prof Rich: This should be a simple question, you made a Post on December 21st, 2010, where you seemed to make your point that there was a requirement to -
'keep the flue (gas temperature?) above the dew point' stating that that required heat energy in the Flue !

Why are we, or how are we, enjoined against being allowed to gain the Latent Heat of Evaporation, though I am willing to agree that much of its loss can/must
occur within the Exterior Chimney, I do believe that we 'get back' much of that Occult heat within the interior piping of our Thermal Mass, While I KNOW that this
is not 'free energy' it feels that way and I for one want to celebrate it !

For The Good of the Craft ! Be safe, keep warm ! PYRO Magically BigAL ! - As always your comments/questions are encouraged and are Welcome ! A. L.

I'm not professor Rich, but I'd like to quantify the latent heat of vaporisation in this case. I'm an electrical engineer, FWIW. If we use glucose as an approximate, here are the possible gains (from a spreadsheet I made). The last figure is the calculated heat that goes into making the H2O a gas (40.68kJ/mol). The H20 on the left side is equivalent to the amount of water in the wood. Air quantity is stoich.

Oven Dried Wood
1 C6 H12 O6 + 6 O2 + 24 N2 + 0 H2 O -> 6 C O2 + 6 H2 O + 24 N2 + 0 O2 + 2560kJ + -244.08kJ
Loss due to uncondensed water: =244/2560 = 10%

Firewood dried to 25%
1 C6 H12 O6 + 6 O2 + 24 N2 + 2.5 H2 O -> 6 C O2 + 8.5 H2 O + 24 N2 + 0 O2 + 2560kJ + -345.78kJ
Loss due to uncondensed water: =345/2560 = 13.5%

Moisture content 100% (i.e. green wood, note that a few types of wood green are more than 100%, most are less. So this is more an upper limit on the potential efficiency gain of a RMH in such a worst case scenario.)
1 C6 H12 O6 + 6 O2 + 24 N2 + 10 H2 O -> 6 C O2 + 16 H2 O + 24 N2 + 0 O2 + 2560kJ + -650.88kJ
Loss due to uncondensed water: =651/2560 = 25%

Note that in this last case, in a poorly designed CWS (which may be most of them?), the temperature does not get to a sufficient temperature to completely combust the fuel. As to how complete the combustion is, that's a question I can't answer. If it's smouldering with no visible flame, the answer is "not very".

Also one thing that needs to be mentioned is that a RMH may be more efficient, but it is not 100% efficient. Not all of that water is condensed, and if the temperature of the exhaust rises towards the end of the burn, the efficiency has dipped down.

Edit: edited to put the correct numerator when calculating the percentages.

Thanks Frank, I'm bookmarking this ! - Big Al - For the good of the Craft !

We took some time from the rocket mass heater workshop jamboree to ask the global experts about this very topic: