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

mekennedy1313 wrote:, the "house" is compromised in very cold climates.  Floors buckle, plumbing bursts, frost heaving destroys foundations etc    Cold, by itself can be very damaging because of the differential rates of contraction of different materials including woods of different species. 

In theory perhaps different rates of contracting could be detrimental but my personal experiences do not illustrate any great problems. I have a cabin that sits at about 9000 feet in a western mountain region. It sits unheated dropping to whatever the local temperatures are, below zero F much of the time between Christmas and the Ides of March. Every two weeks I heat the interior up to over 70 degrees for 4 or 5 days. Then I leave and return 10 days later to go through the process again. The interior walls have a lot of drywall, some pine wood boards with tongue and groove joints. The floor is porcelain tile. The ceiling is wood boards like some of the walls. There are no drywall cracks, the windows and doors move freely summer and winter, the floors have not buckled, there is not even a hint of any cracks in tiles or grout lines. All that because the structure was properly built. Cold is not automatically going to cause problems with a structure. There are right ways and wrong ways to do everything. The wrong way will invariably cause problems at some time.

The pipes don't freeze because there is provision for simple draining. The foundation was built to handle the expected freezing conditions and is no problem. On the other hand I also have two small shed structures. One was hurredly built and does not have what I consider a proper foundation. It's door sticks a little after the ground freezes. The other has no issues at all, but it has a proper cold climate foundation.

So don't be afraid of the cold, simply build for it.

A post over at HT got me thinking.  And then I came back to this thread and re-read a lot of stuff. 

Here is where I am now:

1)  The most efficient conventional woods stoves are about 75%.  And that is the best efficiency that one could get under optimal conditions in a lab by experts. 

2)  That 75% is actually based on something where 100% does not account for the heat needed to get the smoke to go up the chimney - which has a standard of 15%.  So it is actually 75% of 85% which is 64%

3)  A person that is pretty savvy about how to get good efficiency out of a wood stove is probably going to get about half the efficiency that an expert is able to get in a lab.  So we go from 64% to 32%.

4) An average person that burns only hot fires is probably going to get half that. 16%.

5)  An average person that does a lot of slow burns, a lot of wet wood, or a lot of cold fires, is probably going to be under 5%.

6)  Due to the exhaust of a rocket mass heater being near room temperature, I think it is safe to say that a rocket mass heater is 95% efficient - maybe more. 

7)  95% (6) is 19 times greater than 5% (5).  So it is plausible that some people might heat their home with 1/20 of the wood.

And then there is the one point that Donkey made that I want to expand into two points:

A)  conductive heat is far more efficient than convective heat.  A person can be sitting in a room where the air temp is 40, and the seat temp is 100 and they are perfectly comfortable.

B)  radiant heat is far more efficient than convective heat.  A parabolic radiant heater can be directed at a person and they feel perfectly warm even though the air temp is 40.

So when we take point 7, and then mix in A and B, we end up with somebody being able to be comfortable all winter with, perhaps 99% less wood burned. 

True?

Anything I left out?

paul wheaton wrote:
A)  conductive heat is far more efficient than convective heat.  A person can be sitting in a room where the air temp is 40, and the seat temp is 100 and they are perfectly comfortable.

B)  radiant heat is far more efficient than convective heat.  A parabolic radiant heater can be directed at a person and they feel perfectly warm even though the air temp is 40.

might be picking nits here, but critics will probably be worse: convective heat transfer is just conductive heat transfer + fluid movement in gravity.  it isn't really a third mode of heat transfer.

maybe your A and B could be stated more simply as "heating people directly uses less energy than heating air to heat people".  maybe not.

paul wheaton wrote:
6)  Due to the exhaust of a rocket mass heater being near room temperature, I think it is safe to say that a rocket mass heater is 95% efficient - maybe more.

some of the lost energy in conventional wood stoves is involved in moving flue gases up and out the chimney.  those issues don't go away entirely in a rocket mass heater.  the energy required is almost certainly significantly less than in a conventional stove, especially if the vertical exit is left out, but how much less?  maybe 95% efficiency is a reasonable guess, but I wouldn't call it "safe."

paul wheaton wrote:
So when we take point 7, and then mix in A and B, we end up with somebody being able to be comfortable all winter with, perhaps 99% less wood burned. 

True?

seems like you're doing most of your maths with less than solid numbers.  it could be right on, but 99% reduction in wood burned seems a bit optimistic.

and the building and operation of these things is also subject to user variables, just like the operation of conventional wood stoves.  comparing a hypothetical conventional wood stove user with bad wood-burning habits to a hypothetical rocket mass heater user with impeccable wood-burning habits doesn't seem useful.  hot burns are sort of built into the design of the rocket mass heaters, but using properly cured wood certainly isn't.  and neither is making sure the mass doesn't cool off too much and cool down the burn (cold starts can be very dirty).  there is still room for bad habits with a rocket mass heater.  and there are plenty of ways to screw up the construction of a rocket mass heater.

what's the biggest reduction you've actually heard of?  I'm guessing it's the titular 1/8 the wood, or 87.5% reduction.  biggest I've heard of is 1/5, or 80%.  those are really, really encouraging numbers and backed up by users' experience.  I haven't heard of anybody using these things burning only 1% of the wood they would have burned with a conventional wood stove, though I would be pleasantly surprised to.

paul wheaton wrote:
Anything I left out?

the shape of the radiator.  wood stoves heat folks through radiation, too, but only close by because they're relatively small.  the energy from larger, flatter radiators (mass benches, floors, walls) doesn't fall off as quickly with distance.  can't give you a number for this, though, and it might be insignificant, so this probably won't fit in to your calculations.

I really think that our use of the word "efficiency" is leading us out into the dark.. That word should NEVER be trusted when left, out there standing naked in the field by itself. It should ALWAYS come well clothed in qualifiers!
Efficient at WHAT and under which conditions? It wouldn't be TOO hard to build a stove that achieved 80% efficiency at burning the fuel (at x ambient temp y humidity and z% wood moisture), 50% efficiency at capturing heat into storage (when bench is w degrees), 30% efficiency at returning the heat from storage in a useful way (when room and storage temps are different by n degrees) and finally, 5% efficiency in heating the people (when they sit here)..

After all that, you gotta consider equivalency and what it is that we're measuring. The HVAC industry measures how much air can be heated and what rate. (Basically, efficiency at heating a substance that is better used for insulation.. ) Us Hipnecks measure at the seat of the pants (so to speak) by whether it's ok to take the socks off or not, how many cords of wood did I cut and haul this year and so on.
The two are completely different languages and lobbing a word like efficiency over the net between them can lead to pretty deep confusion and leave us rocket stove folks banished way out in fringie land with the other nuts.

..Anyhow, I'd say that 1/20th of the wood burnt (all else being equal) may be a bit of a stretch, though not a HUGE stretch.. I'd LOVE to see this one proved either way.

Oh.. One more thing. Wood being stored solar energy... Only about 1% of the energy going into wood becomes wood. Perhaps if we busied ourselves with more efficiently capturing heat from the original source, we wouldn't need to fool around so much with it's byproducts..
Passive solar heating rocks..

Donkey wrote:Passive solar heating rocks..

I like to passively solar heat water, but heating rocks is useful, too.

and you're right on about the e word.

paul wheaton wrote:

Anything I left out?

The thing you left out is the amount of time each one is likely to burn. A mass heater runs fast and hard for a few hours and heats for many hours. An iron stove is burning as long as heat is needed. So even if an iron stove was the same efficiency as a mass heater, it is running all the time. A fire that is going 6 times longer may not use 6 times the wood, but it will use a lot more (and a two hour burn to heat 12 hours is about the minimum I have heard, 16 to 18 hours is common). While you have sort of covered this with the heating people instead of air... I think less running time is easier to understand.... lets people who bought "90%" stoves still feel they have not been ripped off... political stuff.

paul wheaton wrote:
True?

Anything I left out?

Well true is a debate I would rather not get into as it tends to provoke arguments.

Put a pound of wood in aka 8,000 btus of fuel, heat the space, how much heat was added to the space is the "efficiency" and how much goes up the flue or the type of heat output is irrelevant.

Though I believe your motives to be pure, your estimations on the efficiencies of other stove designs appears to be as skewed as a marketer trying to press the 95% efficient cat stove, smoke and mirrors.

Like I said, I do not view you as that type but IMHO in your efforts to describe the merits of the RMH you are falling into the trap of tearing down competitive designs, not a strategy I tend to employ myself nor one I think you intended, however that is indeed the view you gave me.

Efficiency is really a simple concept but because the word is used in different ways by different people it gets confusing.  Heating efficiency is simply (how much heat gets into my house /how much heat is in the fuel)x100 to get a percent.  It doesn't matter what form the heat takes, radiant, conductive or convective, as long as it stays in the house.  Burn efficiency is (how much heat is released from the fuel/how much heat is in the fuel)x100 with no regard for where that heat goes.  If 90% goes up the flue I still have a 90% efficient combustion device.  This latter is unfortunately the number quoted almost exclusively by the manufacturers. 

That being said there is a significant difference in the effectiveness of the heat type.  In that I would agree that radiant is more effective in that I can be comfortable at a lower measurable temperature, for example with a radiant floor.  Thus an RMH, using radiant heat, is a much more effective heater than a cast iron wood stove as well as being highly efficient.  Thus if both are 90% efficient you can be more comfortable with the RMH.  As we are creatures of comfort this leads to reduced fuel use and better effective efficiency of the system.  As someone above said "efficiency" always needs to qualified.

In any comparison, one has to use apple to apple comparison, or oranges to oranges,
regardless of the percentages or the words used,


in the beginning the statement RMH used 1/8 the wood,  and from the understanding I have is some person took out there old wood stove and put in a RMH and now uses 1/8 the wood he used before,

and that is the comparison right there one stove used 8 units of wood and one uses 1 unit of wood to heat the same area and building,

if all is the same, building and no upgrades, then the RMH is 8 times more efficient than the one taken out,

since we do not know what the old unit was or if it was even a conceptual air tight unit,

does not mean in all instances if you put in a RMH it will lessen your wood usage by 8 times.
My SIL bought a new cast iron box heater, and use nearly the same amount of wood in it to heat a small 12 x15 foot room. he has little control over the burn of the fire, (mostly by varying the size and amount of wood in the stove),  air control is nearly use less, on it, to control the burn so he either has a very hot fire that is going out the chimney at nearly 1000 degrees, (pipe glowing dull red),  to no fire at all,
a very efficient burn, but very poor recovery of the heat that is produced,  in my stove, I have a very good control of the fire, (I can put out the fire by closing it down),  and can pack it full and have a nice burn all night long, with a very stable heat out put that I can efferently recover, (no the burn of the fire is not as efficient as his but I can much more efficiently recover the heat produced),

now from what I under stand the RMH burns the wood efficiently (hot), and recovers the heat efficiently (keeps it in the room or building),

now if one goes from a poor wood stove that dumps the heat to one that recovers the heat you going to use less wood,  and if one can do that and burn the wood more efficiently as well, you will use less wood yet, and apparently the RMH fits the bill on both of those areas.

(in our last cold snap, below 0 for number of days, my SIL with his barrel stoves and box stove and 1940's cook stove (yes three stoves burning) could not keep the temperates in the house on the main floor, above 40, basement was good and bed room where there box stove was good) but the kitchen and main living area got below freezing at night,  they went through a estimated 500 pounds of wood that day, and I went though about 120 and my house was at 70 on the main floor and about 55 up stairs, and one stove, (yes the houses are different in there insulation qualities, size is much the same,  but he had three stoves going and only a few comfortable locations, and I had one going and the whole house was basically comfortable,  there are differences of efficiencies, of the stove out there and the buildings there in,

he told me about a piece of yellow pine he threw in the box heater, said it was a 2 x12 about 14" long, and he said it weight about 14 pounds, he put it in the box stove and said it turned the little heater to a dull red,  and the stove pipe to a dull red,  (he said he got his fire extinguisher and keep it ready as he could not control the burn rate of the fire in the box stove,)
now if that piece would have been in a RMH, most likely it would have heated the room for many hours, but under the conditions of his stove 90% of the heat mostly went up the flue, and out side, and in return they only got an hour or so out of  usable heat out of that wood, where if they could have recovered that heat and stored it for some time they may have recovers over 90% of the heat instead of only about 10% of the heat that was produced, 
that would easily result in a 8 to 1 reduction of wood usage,

Birdman wrote:

and that is the comparison right there one stove used 8 units of wood and one uses 1 unit of wood to heat the same area and building,

if all is the same, building and no upgrades, then the RMH is 8 times more efficient than the one taken out,

This is not an accurate statement, you can not ignore the complete operational difference.

The standard wood stove puts heat into the space very fast, the RMH puts heat into the space very slowly.

If the standard stove burns 1 lb per hour and puts 7500 btu's per hour it is operating at 93% efficiency. if the RMH is burning 1/8th at the same efficiency it is burning at a rate of 1,000 btu's per hour and putting 930 into the space.

There is a HUGE difference in the amount of heat a RMH puts into a space and there is a HUGE difference in the expectation on performance.

While a RMH may well satisfy many folks whom enjoy relaxing on the arming bench and have adapted themselves to cooler air temps by CHOICE, that does NOT define the RMH as more efficient than a conventional stove, though I do believe it is SOMEWHAT more efficient, it is no where NEAR the numbers that keep getting tossed around and folks do not get a firm grasp on that, it will continue to be totally discounted as much of what gets stated is not only impossible, it is far enough outside standard safety practices that it brings more concern than it ever could through its efficiencies.

The flue of a conventional stove requires approximately 16% of the thermal heat content to power the exhaust, even if it was 25% and the RMH was 100% (which it is no where near) the usage rates would vary no more than the 5 difference.

Cord of Oak 4k pounds, 32000000 BTU's of heat, @ 75% efficiency = 24,000,000 BTU's of heat into the space. At 1/4 the burn rate, 1k pounds you have 8,000,000 even if we claim the RMH is 99% we have less than 1/3 as much heat entering the house without any regard for any opinions to the contrary, our max potential was 8 million.

The RMH does not defy the second law of thermodynamics which simply put is energy can be neither created nor destroyed. All 8k btu per pound enter each stove and all 8k come out, the end, if you put 1/8 as much in you will not get more out.

The truth is, folks have found that with the use of the RMH and especially with the associated warming benches they do not need the inside temps nearly as warm to be comfortable which is great and very cost effective no matter the type of heat used, the lower you can live with, the less expensive it will be, the less fuel used of any type to do the job.

Until folks begin to stick with the positives, focusing on the adaptation and ease of instead of trying to compare significantly different heating needs, they are simply inviting dismissal by anyone whom knows much about it at all.

The reports are pretty universal.  People are heating their homes with much less wood.  And then we have another group of people who are saying that it cannot be so.

The 14% to 16% figure is an allowance when measuring optimal performance.  But with a cool fire, it is possible that 90% of the heat could be going up the chimney and the stove is not performing at the 75% efficiency that it could do if used in a different way. 

Are there efficiency numbers available for stoves where the damper is low, or wet wood is used?

paul wheaton wrote:
The reports are pretty universal.  People are heating their homes with much less wood.  And then we have another group of people who are saying that it cannot be so.


Are there efficiency numbers available for stoves where the damper is low, or wet wood is used?

I would personally rephrase that to say  "people are adapting to heating their home less and are significantly reducing their consumption of wood through this change and high performance equipment to do so"

Your own experiment with electricity proves this, you are using far far less electrical power than ever before, however the efficiency of your furnace did not change one tiny bit. If you had installed a RMH in no way what so ever would it "prove" the RMH was efficient to any level.

Q = U * A * TD is the universal heat transfer law, not theory, law.

If the same structure is used then from practical stand points U and A do not change.  Using simple numbers instead of calculating the U factor for your walls and having you measure everything, since they are the same, algebraically lets assign them values of 1.

It is 35 degrees outside, you want it 70, that makes TD = 35 so for this equation we have 35 btu's leaving your house.

Now lets install a RMH with a nice warming bench, you find that you are more than comfy sitting on that bench reading along on the internet when it is only 60 in your house. That gives us a new TD of 25 aka 25 btu's of heat leaving your house aka a  29% reduction in the rate heat is leaving.

You can not confuse "creature comfort" with thermal efficiencies and this is the apples and oranges part that keeps getting skewed. Because the RMH system brings the heat to the space at a slower RATE the TD remains lower for longer periods of time an the duality of the RMH having the barrel and the cob allows one to locate themselves near the barrel to gain a lot of fast heat if they become uncomfortable for that fast heat buzz if you will. When you fire the conventional stove it starts dumping massive amounts of heat into the space at a much higher RATE which warms it quickly and typically to a much higher temp as well so the occupants are comfortable.

Folks DO use less fuel with the RMH system, I have very little doubt about that at all and am certainly not calling anyone whom sates as much a fibber or anything of the sort, but it has far far more to do with how they have adapted to living than it does with the fuel burning capacities of either of the stoves and the stoves are no where near as far apart on efficiencies as gets implied which is why I keep ranting about that being a detractor to anyone with a firm understanding of heat transfer.

If I set both stoves in the same spot in my home and maintained the same temperature at a point far away from both stove locations I guarantee you it would NOT be an 8 to 1 ratio. That in no way means I could not use 1/8 the wood and be perfectly comfortable even perhaps a bit too warm sitting on a cob bench typing this note, it just means the air temp in the room 75 feet away and down the hallway would not be where I opted to play Yahtzee with the kids tonight.

I posted up somewhere here or on Donkey's site some psychrometric charts that sort of define the creature comfort zone, a huge part of what folks are talking about as efficiencies are actually in reality linear relationships on opposite corners of that zone. A conventional stove will produce a very hot and dry environment, a RMH will produce a warm and more humid environment.

Without spending a huge amount of time talking about how the human person is cooled through evaporation and controlling that evaporation rate through the total heat content and enthalpy of the air, it is very hard for me to effectively communicate how little impact the efficiency difference of these two systems has to do with keeping folks comfortable.

That is in fact how it SHOULD be said, "a RMH uses 1/8 as much wood to keep people warm and comfortable." as that may be absolutely dead on but "a RMH is 8 times as efficient as a standard wood stove" is flat out false and is instantly identifiable to any science type.

I hope I did not just waste a lot of words and I certainly did not mean to offend anyone, if so please know there was no ill intent.

And herein may lay the MOST important point:
Efficiency, scientific language, btu's, heat transfer law and all the rest DOESN'T DESCRIBE HUMAN COMFORT.
It's all very useful for some purposes, tools that when used in proper context define a lot, though in the final analysis, inappropriate for describing and/or defining the bottom line, at last coming to the salient point.. What is it that makes me comfortable?

Donkey wrote:
What is it that makes me comfortable?

Not as hard to answer as you might think. Unfortunately there are a lot of huge misunderstandings of simple definitions. I know that sounds condescending but it really applies. One of the broadest misunderstandings is the word temperature. Many folks put way way too much into temperature and think it means something when it comes to heat content, it doesn't and actually means virtually NOTHING.

This is some what hard as I do not have the classroom format to gain the knowledge of participants thoughts and misunderstandings to develop a plan to correct them and put them on the track of linear thinking. Let me give you some insight. The Marines make a soldier by tearing a man down completely and then rebuilding him programming his body, muscle reaction both gross and fine motor skills and even thought process to exactly what they want in a soldier. When I get a student I must tear down their preconceived thoughts and get them to open their mind to the simplicity of many thinking patterns and if they seek to understand thermal energies and heat transfer, they HAVE to employ very simplistic linear thought and avoid creativity as much as possible as most of what is involved is counter intuitive.

Usually within the first 4 weeks I have a fairly good handle on the mean average thinking patterns and since I firmly believe I am unable to teach anyone anything, I prefer to say I help them figure things out for themselves. I will deliver the harsh news, there is not such thing as hot or cold, they are descriptors for perceptions and I of course get significant objections and then am forced to prove that hot nor cold actually exist. It is simply done and highly effective in the effort to guide them to change their own thinking and I offer it to you now.

It was twice as cold yesterday as it is today, today it is 0 degrees F, what was yesterday's temperature in degrees F? I then reverse it and exchange "cold" for the word hot. I then tell them I will not answer it until the following day, (yes perhaps I am sadistic) but it is very effective in opening the door of their mind and they do it themselves, I only give them the path.

That said, the average skin temp is 86.6 degrees, your comfort zone is going to typically fall about ten degrees below that as long as the dew point remains approximately 10-20 below that point. Altering your perception is simply manipulation of  the dry bulb and wet bulb temps of the air surrounding your person. Changing those in proportion to your activity level (hence heat generation) of your person provides your comfort, if you are slinging cob building a project, you may find 30 below your skin temp dry bulb and 50 wet bulb make it a pleasant experience, above that you are "hot" and below it much you are "cold".

This does vary significantly across persons and "average" is not really a good barometer, for instance Paul's electric thread, I was under the impression that Paul shared a trait with me but as his experiment has come to a close I find it interesting that my thoughts were so far off. Paul has now listed that he pretty much did not find it pleasing when it was lower than 65, I myself prefer temps below 62 and in fact almost never wear a coat until it is into single digits, far below the norm so to speak. I also an really unhappy when it is above 75 and will seek cooler environments if possible. I actually when I lived alone would not close the windows or doors until it was in the 30's i carry the nick name of "polar bear" at work due to the simple fact I never wear a coat and visit with others outdoors in the 20's in a polo shirt.

So to answer your question, it varies, but some general rules can apply and allow us to manipulate the environment we stay in to provide general comfort. We can change our habits and maintain the same level of pleasure, however few are prone to doing so and recognizing this in order to present a potential product requires that we effectively communicate that the alternative is highly pleasurable for some and not so interesting to others instead of falling into the marketing trap of claiming buzz words like efficiencies etc.

Tagging on to the last post, two points.

1. I know a lady, the sheriff of the county whom has the exact opposite pleasure temps as myself and in fact desires her office at 90-95 year round. I shook my head repeatedly as I installed base board heating in her office and blocked off all air conditioning systems in mid July a few years ago.

2 that twice as cold question, well enough info exist within this thread to solve it, it is a matter of putting it all together and since I am a sadist lol, I will not explain until Monday!

I understand the part about being comfortable with eight times less wood vs. the stove being eight times more efficient. 

In the meantime, I feel I am frequently in the position of attempting to persuade people who are .... horribly resistant to new information.  So i try to think of how i can present the information in terms that might be easier for them to digest. 

The challenge appears to be that they have a 75% efficient stove, therefore, cutting your wood use by 10% is something they can grok, but 90% is like talking about little green men from mars.

So, in an attempt to come up with something that could move this from the "impossible" to the plausible, i present my next feeble attempt:

step 1)  A 75% efficient stove is actually a 64% efficient stove because of the ~15% industry fudge factor for heat up the chimney.

step 2)  64% is what you get with first class wood and the best of all worlds at just the right moment.  Experts are lucky to get 50% on a regular basis.  Average folks are probably going to be closer to 30% to 40%.  People burning wet wood, or closing the dampers a lot might get as low as 5% to 10%.

step 3)  radiant heat is more efficient than convective heat.  So if you are in the room while the wood stove is burning, you can feel the difference between the radiant heat and the convective heat, by having somebody stand between you and the stove.  The same thing goes for the mass in the rocket mass heater.  If you have a clear line of sight to the mass, it will continue to warm you even when the fire is out and the air temperature is lower. 

----

better?

Paul, let ME sell YOU a RMH. I think your enthusiasm gets into your way just a bit and your focus still appears to be to tear the other product down instead of showcasing your own. I know your intent is not to "sell" these units, however if you seek people to buy into them, you have to pay attention to the process.

Here goes:

Paul, you have got to check this out, it is a really interesting design in an old technology, well actually it is a combination of a couple of technologies that has produced some astounding results. It is called a "rocket mass heater" but it really is not a rocket the open types just sound like a small rocket.

These things produce heat by burning wood just like your wood stove but that is pretty much where the similarities end. They burn the wood in a completely different way that is similar to the gasification burners but not exactly. It is kind of hard to explain but let me show you. (At this point I produce two wooden matches, I strike them both and hold one horizontal and the other one vertical.) You see how the horizontal one rapidly lite all across the surface and the other one burned down very slowly?

That is what the RMH does! When you put a log into your stove, the bottom is lit from the embers but in very short order the entire surface of the log is burning and you have a big hot fire. That is great for warming your hands on a cold winters night, but burning all that fuel at once is often a lot more heat than you actually need, especially if your cabin is already warm! That extra fire needs more air too so it is going to pull more air from the outside to feed the fire so the extra heat is kind of a loss or goes on up the chimney.

The RMH is like this other match, it burns the tip. Now I know what you are thinking, how would you put wood into the stove from the bottom. Well you don't do that at all, but you do load it vertical. Instead of burning at the top like this match, the wood burns at the bottom, but the draft comes in through the top and blows the flame and heat AWAY from the log and into the burn tunnel. In doing so it significantly slows down the burn rate of the log while still burning at a high enough temperature to turn the fuels in wood to gas which then travel to a heat riser that oxidizes the unburnt fuel very rapidly at a very high temp like a catalytic stove only instead of just cleaning the flue gas and shooting that extra hot exhaust right up the chimney, it all happens BEFORE the heat exchanger so the RMH gets to use that additional heat for the space instead of driving the exhaust.

The whole key to the success of the RMH is that it slows down the rate the wood burns at, it is a much closer match to what folks need to maintain a warm comfortable space. You do not get the temperature swings like you do with a CWS (conventional wood stove) and most find that very pleasing, you also do not burn nearly as much wood because you are not overheating the space, you are not drawing in huge amounts of outside air to feed the huge fire inside, and because the RMH stores the heat not radiated immediately, you do not have to move your favorite chair to the other side of the room in fact some folks have made really cool chairs and benches to lay down on for those times they just want to be extra toasty!

Now Paul I have to tell you straight up, this is not going to be a device for a hunting cabin where you come in and it is 20 inside, you throw some paper, kindling and a couple of logs in and an hour later your cabin is 85, it is a device that would take that same amount of wood and maintain the temp in your cabin for perhaps a couple of days! They warm up slowly, but if you were going there for a couple of weeks, take the electric blanket, fire up the RMH and keep it stoked and in a few hours it will be all warmed up and will begin maintaining your cabin using much less wood than your CWS. The key is matching the burn rate much closer to your needs by burning the wood just as hot, but slower as you need it.

Sales switch off:

I guess this is likely the key misunderstanding, the RMH does not convert more heat, it converts significantly less and introduces it to the space at a much slower rate, that is why there has been little to no success in using them to heat water.

Take an aluminum pot, fill it with water and set it on a fire and it will boil in short order with the large quantity of heat at 400 degrees from that fire. Take that same pot and apply a #1 oxy acetylene torch to it where you have a 5,000 degree flame, the water will never boil, you might actually burn a small hole in the bottom of the pot.

I think the terms "efficiencies" has no place in the discussion nor does "cleaner exhaust" until those factors are set in stone to exist. They cloud the issue of a better matched burn rate to a persons needs and open a can of worms due to significant differences in what such words mean to different persons.

If by chance the RMH is 2.5% less efficient than a catalytic stove and has a 1% higher pollution rate does that suddenly make it a poor choice? From your perspective I would think not. If I burn 10 lbs of wood and release .1 lb of pollution and you burn 2 pounds of wood and release .75, you are indeed per pound releasing MORE than I am, however, you are doing far less of it. IMHO this is likely to show more merit than the thoughts that the exhaust is nearly as clean as some think.

I just prefer to avoid the pitfalls that can come with claims not only from a factual standpoint, but that perception view as well since such numbers always get twisted to meet the audience instead of raw data.

Professor Rich, I just want to say thanks for the explanations, and I really wish I had been able to take classes from you back in college!

Little success in heating water.. But NOT none!
I've got a VERY simple model here at home that works VERY well.
15 - 30 minuets to hot and with a little slack between showers (like 5 min or less) we've blown the safety valve WHILE running 10+showers back-to-back.

The secret??
A small tank (10-15 gallon) of water placed directly in the flow of the heat in a bell stove like arrangement. The tank sets BESIDE the heat riser and the heat is exhausted below the water tank.
No coils, no central flue, no over-tall installations and it's built out of recycled bits and mud.

Donkey wrote:
Little success in heating water.. But NOT none!

10 gallons of water taken from 60 to 120 takes 4800 btu's of heat or about 5/8 of a pound of wood burned. Since no where near all the heat goes into the water it is fair to say that likely takes 1 pound to get it done.

I am fairly sure we could rob from peter to pay paul with one and have a fairly productive unit but there are better methods for water heating.

Professor Rich wrote:
10 gallons of water taken from 60 to 120 takes 4800 btu's of heat or about 5/8 of a pound of wood burned. Since no where near all the heat goes into the water it is fair to say that likely takes 1 pound to get it done.

I am fairly sure we could rob from peter to pay paul with one and have a fairly productive unit but there are better methods for water heating.

True, not ALL the heat..
But when built in bell stove fashion with the tank in the bell and the chimney at the bottom, it's probably one of the finest wood heated hot water system I've seen yet.
I live in the heart of what was the "back to the land" movement in California during the late 60's early 70's. The remnants of every half-baked hippiefied backwoods water heating contraption EVER contrived litter the hills here and some of them (even) still work. This IS the home town of the Blazing Showers company (check out old original Mother Earth Catalog they're in there) after all..
Anyhow, I've got quite a bit of experience with wood hot water contraptions and this one (IMHO) sits on the top of the heap. I think it might surprise even you, Prof..

Donkey wrote:
True, not ALL the heat..
But when built in bell stove fashion with the tank in the bell and the chimney at the bottom, it's probably one of the finest wood heated hot water system I've seen yet.
I live in the heart of what was the "back to the land" movement in California during the late 60's early 70's. The remnants of every half-baked hippiefied backwoods water heating contraption EVER contrived litter the hills here and some of them (even) still work. This IS the home town of the Blazing Showers company (check out old original Mother Earth Catalog they're in there) after all..
Anyhow, I've got quite a bit of experience with wood hot water contraptions and this one (IMHO) sits on the top of the heap. I think it might surprise even you, Prof..

I can't imagine I'm the only person here who would appreciate a thread elaborating your design a little bit.  if you've got a thread about it on your forum, I suppose a link would do if you don't fancy repeating yourself.

Donkey wrote:
I think it might surprise even you, Prof..

Heating water is not tough, it just has a lot of problems in doing so that create long term problems, the problem with doing it with wood is maintaining control of those problems.

It is also far more dangerous than most folks understand. It would be far safer to use a heat transfer fluid into the water tank and valve the fluid to stop the heating of the water than it is to direct fire it in the amateur world. A standard hot water heater sized tank can build thousands of horsepower in steam pressure very quickly when direct fired and the results of that instant release usually takes a code spec house to the ground.

The pressure build up is almost exponential actually and the specific heat of steam is half of liquid water so it heats twice as fast. This is why the coil units perform poorly, they get pockets of superheated steam and over pressure before all of the water is even hot.

I have no doubt what so ever the rocket heater can service water systems, but at the same time I know it is not going to be done safe and easy with scrap materials.

That's why I don't generally do coil systems anymore.
I'm also religious about relief valves.. It's the only thing I won't consider getting used or short-cutting around.

Heat transfer fluids are, for the small scale tinkerer, more hassle than they're worth. They also take us back into some of the (non-dangerous) drawbacks of coil systems.

Here's the basic idea, somewhat simplified..

Donkey wrote:

Heat transfer fluids are, for the small scale tinkerer, more hassle than they're worth. They also take us back into some of the (non-dangerous) drawbacks of coil systems.

You wil have a real hard time convincing someone whom is familiar with 120,000,000 btu per hour systems that he was indeed tinkering with something small scale.

The design in your picture requires fire control to try and maintain water temp, with a secondary medium that you control the flow of, you can perform the same goal with increased safety and better temperature control of the water side.

Since calcium and magnesium both precipitate out of water and heat dramatically increases that process, significant scale may build in that tank if you have high tds water or hardness above 10 grains and that scale can render relief valves ineffective and those results are off the chain.

The main problem with direct firing it from the rocket unit is the total lack of automation in the rocket, there is nothing to shut it down if over temp is approaching. The relief valves are a redundant safety secondary to the fuel cut off devices employed in most water heating systems. you in fact have the thermostat as first level, upper limit as second level and the relief valve as your third redundant safety. By using uncontrolled wood fired flame, you are two levels short of what is considered the minimum.

In the case of water heating, it is not so much the difficulty that drives the concern, it is the result if you have a failure that drives the redundancy requirements.

I am not sure mechanical controls are still produced to make direct fired systems a good idea for a hot water type system, a secondary fluid system can put the mechanical controls of today into place to do it.

what if the vent is left open to the outside through a stand pipe of 15 feet or more?

Perfessor:  I have now read your post three times.  On the previous two times, I read it and thought "I don't get it.  Maybe I'm tired and should try again when my brain might be working better."  This time I'm thinking that I understand everything you are saying, but it doesn't help me solve the problem.

Exibit A)  the video of ernie.  4 cords before, 1/2 cord after.  Add to that the dozens of people with similar reports. 

Exibit B)  The people that seem to be saying that everything in exibit A is a lie because the worst wood stoves are 50% efficient, so the best that could happen would be to go from 4 cords to 2 cords.  Any claim beyond that is a lie.

I do not think that Ernie, or the others, are liars.  If nothing else, I don't see any motivation to lie about it.

The purpose of this thread is come up with the words to persuade the people of exibit B that this stuff is true.  I think the way to do it is to convey information to them in the terms that they seem to be currently understanding.  The basis for their position is on their interpretation of "efficiency". 

It seems like there is a really easy way to do this from two different angles:

angle 1)  measure the output.  a large volume leaving at 400 degrees vs. a small volume leaving at 100 degrees.

angle 2)  embrace their "efficency" math:  start at 50%, add in the bit about the 15% chimney fudge factor, then add in the part about how a slow fire might be only 5% to 10% efficient (with the rest going up the chimney).

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.

paul wheaton wrote:
a lie.

I do not think that Ernie, or the others, are liars.  If nothing else, I don't see any motivation to lie about it.

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.

No one is a liar and being incorrect is not lying. It will take far more time than I have to respond so it will have to come later, but the long and short of it, they used to overheat their space which wasted a lot of heat, now they do not and have a much better balanced system in place that gives them a better matched result.

I just can't get enough. please post more
im data hungry
nedreck, professor rich, donkey,mt goat and last but not least paul are my favorite posters . rich you may need more tact but data is never offensive 

p.s. everyone on this forum seems to be smarter and im glad to be here with you all

Umm....
Seems like the prof. kinda hit it on the head.
RMH's provide 2 main advantages:
1) The burn IS extremely efficient. Yes, less wood is burned and therefore less BTU generated over-all, but of the wood that IS burned, there is little to no waste of fuel AND heat generated. Also as a side effect, less replacement air is drawn into the house.
2) Measured, slow release of the heat over time, in a convenient, accessible place. The output is spread over more time, is better BALANCED towards comfort and you can place your body parts ON the heat storage medium, so it's kinda snuggly and tactile too. 

Maybe the "hook to sell" is BALANCE...    ??

Very very close to what I was saying Donkey.

If we burn 1,000 btu's of fuel in each, the output is the same, however, the time is not.

Better yet, forget the fuel completely because it is clouding the issue!

I will graph something out when I  get time later today

I think I get it!

To get consistent heat from a CWS once a room is warm, people will keep a fire going and close the dampers.  So now, you have an inefficient fire: a lot of heat is going up the chimney.

There are no dampers on a RMH.  So you always burn a hot, efficient fire.  The mass acts as a heat buffer.

A pocket rocket would burn more efficiently than a CWS because a pocket rocket doesn't have dampers either.  But then you could struggle with getting too hot and too cold. 

Right..
You can actually get a VERY HOT, efficient burn out of a potbelly stove.. It's just gonna be unsatisfactory in every other way.. Too hot during the burn and when the fire goes out, too darn cold, ALSO the chimney will continue to cycle heat up the chimney after the fire is out.

I think that the barrel, bench, chimney arrangement of rocket stoves, where the bench piping is below the fire can act like a thermal lock after the fire goes out..  It seems to work that way with my water heater at least. Seems to keep the chimney from continuing to work when the fire goes out.

Wadd'ya think Prof??

Good information here.
Does any one know what insurance companies think of these designs?

ytram wrote:
Does any one know what insurance companies think of these designs?

Well, they probably don't care about the "1/8 of the wood" thing - perhaps you could start a new thread?

ytram wrote:
Good information here.
Does any one know what insurance companies think of these designs?

I can just about guarantee that insurance companies won't even consider these things... Might even drop you or raise your rates for having one.

They haven't passed an inspection for permitting yet anywhere that I know of, until they do (maybe even then), forget insurance.

Ok, lets see if we can drive a couple of points home solidly.



Disregard everything you know about the stoves, blank your mind completely as too many are still focused on unimportant details that have far less impact than one thinks, especially flue losses.



Bear with me and allow the flexibility in accurate information to be disregarded simply because the math would indeed change, but that change would be linear in nature and equal for both units. Now we are going to take a propane torch, light it and put it inside the stove. The one pound bottles the torch is attached to has 2,000 btu’s of heat, the torch heads will be IDENTICAL and both will burn for exactly one hour.

Both rooms start at 0 degrees and it is 0 outside. Propane burns at 2,000 degrees. I believe this is all the ground work we need to develop the fundamental concept, if I change my mind going through it I will edit as need be.

The single line on the CWS represents the stove wall, the thick black line on RMH drawing represents the cob wall, 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.

Now we light the torches and throw them into the stove and put the cover on/close the door. Now there is 0 doubt that both units have 2k in fuel inside, they are going to quit at exactly the same time 1 hour later, since the stoves are both inside the space there is no doubt we have indeed released all the heat inside the structure. Now while not completely accurate as I do not have time to check tables and calculate the information to exacts we are going to give heat transfer values to the materials that are geared to easy math. For this exercise we will assign a conductivity rating of 1 to steel, .125 to cob, .125 to the exterior walls of the structure.

So now looking at the pictures supplied we can examine some rates by using the universal heat transfer equation. Q = U * A * TD and take a look at the whole picture. For this exercise we are going to give even surface area to both for easy math purposes only and will use 10 square feet of surface for the stoves and 500 feet for the exterior of the space. While it remains a focus, flue losses are smaller than believed so instead of estimating them, we are all going to choke to death and have 0 exhaust from our propane burners to drive the point.

Stove to space would be 2,000 btu/hr inside 0 outside so 2000/UA=TD

CWS 2k/1*10=200 degrees

RMH 2k/.125*10=1,600 degrees

Now before you take pause at that, more comes into it than just these numbers but for a lack of better terms this is the driving force and those extras are near equal for both units so they are some what moot, out numbers are just slightly skewed by not considering them. While that might seem like “bizarre” information, simply think about it, if the mass releases the heat slowly then it absorbs it slowly and since we are using identical burner systems we are creating it at the exact same rate so while there is no difference in the rate, holding it in the fire box longer as it is absorbed slower means we have a higher concentration of heat in that firebox aka more intensity aka more temp but the same amount of heat entering.  Remember you intentionally insulate the riser don’t you? That is a key to slowing the process down and by definition is exactly what you are doing, insulating is to slow down the heat transfer process.

I say this because those issues DO count, however flow patterns, filming effects, stratification and all of the others added together do not amount to a hill of beans on the scale we are speaking to at all. When you are working with less than 100k per hour input the very small percentage increases gained through correction of those issues is practically moot. At 100k a 90k output vs a 96k output is less than 2 degrees F in mean temperature in the space and humans are not sensitive enough to notice 2 degrees.

Now since you are transferring that heat to the space, as the temperature of that space begins to rise from 0 your TD is going to change and it is somewhat proportional but those other factors I have referenced get in the way too. Lets keep it linear for now as we explore what is happening. There is absolutely a curve to the process and it is not unlike an electrical sine wave where the peak voltage is not the same as the mean (usable) voltage, because of the peaks and valleys there is an average of the curve.  The TD changes as the steel and cob warm up and the closer they become to the same, the slower the heat transfers between them.

So now we have explored the “air” in the firebox warming the materials of construction, now they must warm the air outside, we are still keeping this all the same size for practicality.

CWS will transfer heat out of it and into the air along the same curve it absorbed the heat and with steel and its speed of transfer, thin composition it is going to remain fast, fast enough that if the internal temp is 200 for practical purposes its outer surface is also 200 so it is going to transfer the heat out at pretty much the same rate that it enters it. Most folks would agree, you fire up the newspaper kindling and wood, within a few minutes of lighting it, it is going to be hot enough to not want to touch it in less than 10 minutes. That is not going to be nearly the same with the cob, especially if it has cooled to 0 before firing it up.

Since we are not controlling the flame at all, the CWS dumping heat into the room at a very high rate aka 500 every 15 minutes it is going to warm up fast, as soon as it does, the room is going to start dumping heat through the wall and to the outside. So lets take a quick look at how that impacts us.

For sake of discussion lets say that 500 btus raise the temp of the space 25 degrees. We said the wall has a conductivity of .125 and 500 sq foot so we are going to have .125*500*25=Q or 1562.5 heat loss per hour. By the second 15 minutes we have added another 500 btu’s to the space and in that same 15 minutes we know 390 have left through the outer walls but because we added more than we lost the temp rises, not as fast as it did the first 15 minutes but it still rises, 110 btus above loss aka 22.5% would give us an additional 5.6 degrees rise so now from a practical view 31 degrees.

Now the RMH is not going to be performing at that level, in fact it may not have affected the temp in the room at all yet because it is still warming the cob and while it has a much higher combustion box temp, that heat has to move inch by inch through the cob and get it above the room temp.  Now we set the U factor for cob at .125 but that would be for one square foot at 1” thick, if we have it piled on 6” thick, we have a lot more waiting before that heat hits the outer surface and begins warming the space. Because the heat will be dispersing itself through the cob the entire time the TD on the fireside will remain fairly high aka readily absorb the heat from the fire similar to how a battery takes a charge.

So here we are at the half hour point, the CWS is getting the space near temp, but at the same time the space is already losing heat. If we say the cob is just now starting to add heat to the space slowly, it has only just began to lose heat from the space.

Now we have paul (had to pick on someone) feeling fairly comfy in the space with the CWS but the temp has kind of leveled out and in the RMH space it is still cooler than he would like so he moves himself to the warming bench which is getting sort of warm and settles in with a book.

Paul knows that throwing another propane torch into the CWS will make it warm up, he also knows that doing the same in the RMH will not have much of an effect. What choice might he make? Lets assume none and follow through. At the end of the hour the space has been losing heat for 45 minutes on the CWS and because the space is warmer it has lost a LOT more heat than the RMH system because none of it was stored so it had a fast rising curve and as soon as the fire is turned off, it will have a fast falling curve. The RMH on the other hand has a much slower rising curve and falling curve as well.

As we saw with the first 15 minutes our losses were already 75% of our total input with the CWS while the RMH had yet to have any losses as it was still warming our heat battery. At the end of that hour, exact same amount of fuel burned the RMH is still warming up the space and the CWS space is dropping. As a human can indeed feel this process with a CWS your going to throw another propane bottle in to “keep the heat going” and on the RMH your going to be saying “its feeling warmer” and while you might or might not throw another bottle in at that moment, by the end of the second hour you are not going to be nearly as prone to do so as you would with the CWS because with it, the space temp is once again dropping but with the RMH it is likely little to no change at that point and in fact might not be but perhaps 1 or 2 degrees cooler in another hour and you are more likely tossing the blanket aside as the warming bench is no very toasty if not hot and you ain’t done with your book yet.

I have drawn some graphics of what is described here, one is performance of the first hour, the second is what a typical day might look like. I pulled the 50 degree temp out of my nose so to speak simply because one must have a target. During times of overshoot the CWS is not going to waste a lot of energy, but a boat load of un-needed heat will enter the space and in doing so will dramatically increase the rate the heat will leave the space. If our desired set point is 50, the additional 10 degrees of heat to get it 60 would represent a 20% increase in loss. Now on the 8 to 1 savings on wood, well a cord is about 4,000 pounds 8 cords 32k pounds, save 20% 6400 or 1.6 of the cords literally go out the roof because of thermal overshoot of the target temp.

Now if you go back to wood instead of the controlled flame of a propane cylinder and we can have even more over shoot issues because the log is burning horizontal aka a much larger flame and faster burn time over the vertical burning methods employed by the RMH. This is going to bring in significantly larger amounts of 0 degree outside air. This has yet to be confirmed via testing, but simple burning area and required air to do so would indicate at least 100% more air would feed a CWS over a RMH. If that is indeed the case, then it is not without limits as only so much air will indeed travel through an 8” flue pipe and that max is about 750CFM which may not sound huge to some, a 20x20 space with a 10’ ceiling only has 4,000 cubic foot total in it. At full flow that would pull all of the air out and trade it every five minutes. The reality is likely much closer to about 75 cubic foot per hour on the RMH and 150 for the CWS. That equates to about 1 full change out of the air per day with the CWS and ½ of that with the RMH so all of the energy required for the CWS to warm the space from 0 to the target point is required which appears to be approximately 1 hour of burn time as it has no buffer. The RMH on the other hand has only ½ an hours worth coming in and while that is still energy lost, since it would be about 200 BTU’s for the CWS aka 100 for the RMH we are not talking lots of impact, however, over the entire season it adds up. With the wood stove however, by not adjusting the damper system properly, this loss can be significantly more than is here, but over all, it indicates how little the flue loss actually is if tuned proper.

About ½ lb of wood per day, call it a 120 day season, 60 pounds of wood that is 1.5% of a cord at 50% for the RMH that would equate to .75% Even if you triple that, it is still fairly minor against the whole number. More of an adder than a primary concern if that makes sense.

Now another human element, stoking the fire. Did Ernie and Erica stoke the CWS only to the amount they thought it would burn in the next hour or did they do like a vast majority of folks and fill the box? What about over filling at night so you still have some coals left in the morning to re-fire with? The RMH has no such requirement and over filling the firebox is not really an option in fact it is self training in that if you over stuff it, it fails and smokes back, a CWS does not do that, it burns, albeit poorly, it still burns. It also has often got a fire box of about 16x16x24 significantly larger than the 8x8x8 that the RMH allows which is what makes it far more prone to overshooting target temps for the space.

With all due respect to all parties, I would offer that the greatest fuel savings account for the RMH using 1/8 of the wood of a CWS has nothing to do or better said very little to do with efficiency differences and a lot more to do with the unit forcing proper operation and the better balance that operation meets the actual heating needs of the spaces they are employed. The cob itself acts as a thermal buffer that smooth’s out the over/undershoots and creates a more desirable average space temp.

A thermostat for comfort air heating has a device in it known as a heat anticipator, it is a variable resistor that adds heat to the thermostat sensing element it warms it a little to prevent overshoot. Heat pumps are known to be efficient however some folks do not enjoy them because they never get that large heat rush they feel when using gas heat. This is because a heat pump puts out more air at a lower temp aka a better balance to maintain the space temp closer to the target set point. I think one could say a RMH employs BOTH strategies, the barrel being a high temp radiator give one the opportunity to feel that high heat as desired and the cob battery provides a very even balanced temperature match to maintain the space.

Anyway I hope I have given clear enough information to indicate why I feel the efficiency claims should be avoided, they simply lack merit as far as the unit itself is concerned. If the RMH is indeed even 10% more efficient, it is not the strong point and it is not an arguable point that carries a lot of weight, if a CWS is 89% and a RMH is 98% that is not enough to make a difference as that claim is not going to be provable to the average Joe, look at how long we have been discussing it and working back and forth through the misunderstandings and this is a subject we all have interest in ourselves.

I do not know if I have reached everyone in the right manner, this is a lecture that would have drawn a lot of questions that would have allowed for item by item clarifications, my white board would have been filled and erased perhaps 100 times. I could say lots more, however, I am rapidly approaching 3k words and many will become weary and disinterested long before they read these last lines.

Try and cut and paste only the part you have a question on with your questions and I will try and clear it up or correct any errors in communications.

having trouble getting the graphics to load up

First one is an hour long graph of theoretical operation.

The second is to show the effects of loading the stoves and a theoretical graphic indicating the reasons behind an 8 to 1 consumption rate driven by human behaviors.

I get your point about overheating using a CWS - and the hotsy-coldsy that comes with the CWS package.

I'm still re-reading and trying to digest a lot of what you 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?