Rocket Mass Heater Fact or Fiction SHOW ME

I have been reading all this stuff and looking at Videos of Mass Heaters. Well, I have to sey that I can not believe the statements. I have heated my place with a regular stove for years. I can only keep a little stick (kindling) fire going for a few moments. I need solid wood and hardwood at that to last all night.
It is very difficult for me to believe that I can heat my house with small sticks. Go to bed and 8-12 hours later it is "Toasty" still in the house.

Seems like I am being told that I can get 65 mpg on a 1970 v-8 F-150

I have not seen any info on how often the stove need to be refilled. How long does wood last? I seems like a nice design but I would want the bench level with the floor and last all night.
Any comments with facts or videos on these points?
Thanks

I am no expert but the RMH does not claim to keep 50lbs of wood burning longer if anything it burns it in a shorter period of time.
What it does however do is:
1)Burn the wood/fuel completely so you dont get un-burnt chimney buildup or high levels of carbon monoxide, etc.
2)The exhaust of a regular chimney can be hot (500F?) but in a RMH it is old (90F), so it is extracting more "heat", wasting less.
3)RMH does not heat the air aka " forced air" style instead it heats soil objects. (radiation vs convection)
4)Even though the wood burns fast the heat is held and release slowly bu the "bench" thermal mass. So what really heats your house is the bench not the wood.

Jeremiah wales wrote:Seems like I am being told that I can get 65 mpg on a 1970 v-8 F-150

If you could take all the heat and energy that a 70 F150 wastes or doesn't combust then I bet 65 MPG would be conservative. If you keep reasearching you will figure out why they are what they say they are. Insulated high temp burn chamber extracts the maximum heat energy from the fuel/gases, radiant heat quickly warms the room and the thermal battery of the bench that extracts almost every last ounce of heat out of the exhaust keeps the heat going long term. I had a hard time with the concept at first too, seems like if this was the best way it would be the most mainstream.

I have not seen any info on how often the stove need to be refilled. How long does wood last? I seems like a nice design but I would want the bench level with the floor and last all night.
Any comments with facts or videos on these points?
I presently have a 1000 sq ft lower level of home I want to heat. But dont want to have set my bed on top of the Bench to stay warm.
Toasty to me is 60 to 75 inside when it is 0 outside. I can do that with a regular wood stove and if I use Oak or Hard Maple I can last until 6am. Looking for something more effective.

I would start here http://www.richsoil.com/rocket-stove-mass-heater.jsp , Paul Wheaton and others have invested a great deal of time writing articles and videoing rocketmass heaters in an effort to explain the concepts and functionality of the device. After watching all the videos and reading the article @ the previous link I had a great understanding and need no more proof.

The main point is that its not the wood that is doing most of the heating its the "bench" thermal battery that is doing the heating hours/days after the fire goes out.

The RMH wood/bench heating combo will keep your 1000sqft home heated 5-8 times longer.
So if you currently use "30lbs" of oak wood to keep warm for 8 hours.
That same "30lbs" of oak wood will keep you warm for 48hrs.
You just use "15lbs" of wood in two burning/heating session.
If the 30lbs of wood normally burn for 8hrs non-stop. In the RMH look for it to burn out in just 6hrs(two 3hrs session).

Jeremiah wales wrote:I have not seen any info on how often the stove need to be refilled. How long does wood last? I seems like a nice design but I would want the bench level with the floor and last all night.
Any comments with facts or videos on these points?
I presently have a 1000 sq ft lower level of home I want to heat. But dont want to have set my bed on top of the Bench to stay warm.
Toasty to me is 60 to 75 inside when it is 0 outside. I can do that with a regular wood stove and if I use Oak or Hard Maple I can last until 6am. Looking for something more effective.

I presently have a 1000 sq ft lower level of home (not a one room Hobbit House),that I want to heat, and I dont want to have set my bed on top of the Bench to stay warm.
Toasty to me is 60 to 75 inside when it is 0 outside. I can do that with a regular wood stove and if I use Oak or Hard Maple I can last until 6am. Looking for something more effective. One of the worst thing on a wood stove in winter is your fire going out and getting the house temperature back up again. Everything in my place needs to heat up, so I understand how the mass can keep the place hot.
But filling the wood on a mass stove every three hours is a bit much. I wish i could see one actually working somewhere in winter. Im not a believer yet, so that is why I am still asking for Real Houses that use some kind of Mass Rocket Heater in their place.
Thanks

Some time back, I did some research on this subject, and found some answers. People were asking how they could expect a 50% to 80% reduction in their fuel use when their stoves were already 78% efficient - wouldn't that imply an efficiency of 156% to 360%? Two things I did were research on the efficiency ratings given wood stoves by the DOE, and the characteristics of heat passing from the stove and flue into a building, according to system.
The first thing I discovered was that the DOE ratings have nearly nothing whatever to do with the amount of heat available in fuel that is actually put into the building. What they measure is how well the fuel is combusted. If you think about this, it makes sense. You can measure a stove's output, but you cannot measure the flue's, so in rating a stove; there is a lot of variation in efficiency in stoves depending on how they are installed.
The next thing I did was to do a simple comparison of burn systems. One writer said that he was able to measure a 1000 C temperature in the firebox of a rocket mass heater, and a 30 C temperature in the output of his flue. This compared with a 300 C temperature in the output of his older stove, at the top of the chimney.
It sounds like the rocket mass heater extracted the heat from 1000 to 30, for a total of 970 degrees, and the older stove was from 1000 to 300, for a total of 700 degrees, which would make the rocket mass heater about 138% as efficient as the older stove in extracting heat from fuel. This, however, is not the case, because the flue gas from the rocket mass heater is undergoing a phase conversion at 100 C, where the water starts to condense.
I did some work on this and discovered two things. First, any residual water in the rocket mass heater's fuel will boil, reducing heat of the burn, just as it does in any other stove. The difference is that it delivers most of that heat to the thermal mass, making the rocket mass heater potentially more efficient in extracting heat from incompletely dried wood.
The second thing is far more important. When wood burns, the products of combustion are carbon dioxide and water. The water is in the form of very hot steam. In a normal wood-burning system, that steam is lost to the environment without extracting the heat of phase conversion. In a rocket mass heater, however, the heat is captured. This means that hte 138% efficiency mentioned earlier becomes a good deal higher. My recollection is that it was over 180%. This would correspond with a reduction in fuel use of between 45% and 50%. I would point out that it is not necessarily the only efficiency improvement in the rocket mass heater, so greater reductions in fuel use may be explained by other factors.
The rocket mass heater has a fire that burns for a short while and most of the heat produced is captured in the thermal mass. It is only necessary to burn fuel until the thermal mass heats up, at which point the fire can be allowed to go out, and the heat for the house is extracted slowly over the following period of many hours.
A friend and I have been working on improving the efficiency of a rocket mass heater by using more efficient thermal storage. I will write this up on this forum when I have more information to provide. For now, I can confirm that we had a very hot fire, leading into a four inch flue system twenty feet long. The gas coming out ranged from barely warm - I would guess about 60 degrees F, to somewhat warm, possibly 90 degrees F, depending on how the system was configured. There was a substantial amount of condensation in the flue, and since it ended with a slight rise in the pipe, it had to be emptied frequently between burns, a fact that will need to be addressed as we develop the system. I mention our experiments because they confirm the efficiency gain of the rocket mass heater, and also indicate greater efficiency may be available.
The bottom line here is that regardless of how well older stoves are constructed, they are nearly never installed in such a way that as much as half the available energy of combustion is delivered to the building. The advantage of the rocket mass heater is the huge increase in the efficiency of capturing and storing the available heat.

This is going to catch me some trouble here, but here is a article I wrote on the subject yesterday: http://greenenergyexperimenter.com/wp/?p=240. Keep in mind that I am using a rocket stove boiler that I built to heat my house and water, so I am biased toward rocket stoves. In comparing notes with my friends and neighbors that have traditional wood stoves or wood furnaces, my system does use less wood to accomplish more than theirs do.

You cannot escape the fact that wood has very limited energy density. If your home needs 40000 BTU/hr of heat to maintain a comfortable temperature differential, you are going to have to burn at least 5lbs of the best seasoned hardwoods per hour with 100% efficiency to do it. The answer is to downsize and better insulate our homes to realize true energy savings. Very few of us need a 1000+ sq ft home. I plan to downsize when I get this redneck mansion of mine paid off.

Jeremiah wales wrote:I have not seen any info on how often the stove need to be refilled. How long does wood last? I seems like a nice design but I would want the bench level with the floor and last all night.
Any comments with facts or videos on these points?
I presently have a 1000 sq ft lower level of home I want to heat. But dont want to have set my bed on top of the Bench to stay warm.
Toasty to me is 60 to 75 inside when it is 0 outside. I can do that with a regular wood stove and if I use Oak or Hard Maple I can last until 6am. Looking for something more effective.

it doesn't show the finished product, but this video shows there is some precedent for in-floor or sub-floor systems rather than benches. the stove in the video appears to be a 6" system. for a larger space (the 1000 square feet in question), you could size it up to 8" or perhaps a bit more. planned carefully, I don't think there should be any problem heating that space with a rocket mass heater. as Geo points out, it will obviously use more fuel than a smaller space, but far less than a conventional wood stove.

Jeremiah,

Why not just build yourself an outside rocket mass heater, just a temporary contraption. Few fire bricks, a salvaged barrel, two pieces of tube, and mud, and you're sorted. Then you could see for yourself.

You will not have to add wood to the RMH every 3 hours,8 times a day. You will add wood the same about of times per day as you are doing right now. In fact it will be less The main point to remember is that when the wood/fire goes out. And stop giving off heat. you dont have to light another fire. The thermal battery "the bench" will then take over and provide heat. It is only after the battery "dies" that you have to light the fire to recharge it up again. Just think of a solar PV cell charging a battery for night usage, same battery concept.

Jeremiah wales wrote:I presently have a 1000 sq ft lower level of home (not a one room Hobbit House),that I want to heat, and I dont want to have set my bed on top of the Bench to stay warm.
Toasty to me is 60 to 75 inside when it is 0 outside. I can do that with a regular wood stove and if I use Oak or Hard Maple I can last until 6am. Looking for something more effective. One of the worst thing on a wood stove in winter is your fire going out and getting the house temperature back up again. Everything in my place needs to heat up, so I understand how the mass can keep the place hot.
But filling the wood on a mass stove every three hours is a bit much. I wish i could see one actually working somewhere in winter. Im not a believer yet, so that is why I am still asking for Real Houses that use some kind of Mass Rocket Heater in their place.
Thanks

Jeremiah,

Think of these two key points:

1) When you're burning your wood stove, no matter how efficient it may be, you are shooting a ton of heat and fuel out the chimney. An RMH not only burns the fuel you're throwing away, but it traps nearly all of the heat you're already losing plus the heat produced by the previously unburnt fuel. So, even if you burn the exact same amount of wood you are burning now (and with a well designed RMH, you will burn less), you would be getting back nearly all of the heat (energy) your system is currently wasting.

2) When the fire goes out in your wood stove, it doesn't take long before it cools off to the point at which it is no longer heating the living space. What, couple hours, maybe? An RMH can produce consistent radiant heat for anywhere from 8 to 24 hours, depending on the size of the mass in the thermal battery.

With just these two points alone (not to even take into consideration the factors of far less pollution and the ability to efficiently extract heat from deadfall branches instead of cutting down whole trees), I would expect you to seriously reconsider the possibility of building an RMH.

Also, if you currently heat that space with a wood stove (or are planning to), I don't see how an RMH would be much different as far as heat distribution is concerned. You don't have your bed next to the wood stove, do you? An RMH gives you the latitude to design a radiant floor mass or a mass that is in two or three rooms.

Jeremiah wales wrote:I have been reading all this stuff and looking at Videos of Mass Heaters. Well, I have to sey that I can not believe the statements. I have heated my place with a regular stove for years. I can only keep a little stick (kindling) fire going for a few moments. I need solid wood and hardwood at that to last all night.
It is very difficult for me to believe that I can heat my house with small sticks. Go to bed and 8-12 hours later it is "Toasty" still in the house.

Seems like I am being told that I can get 65 mpg on a 1970 v-8 F-150

I have not seen any info on how often the stove need to be refilled. How long does wood last? I seems like a nice design but I would want the bench level with the floor and last all night.
Any comments with facts or videos on these points?
Thanks

RMH is a hippy-dippy re-branding of the masonry stoves that are common in the colder parts of Europe. Yes, it's hard to believe, but you can use the exhaust from a short hot burn to heat the masonry of the stove.

The key features of masonry stoves are, according to my research,

1. Small "airtight" firebox that gives you control over the burn rate via air input instead of blocking the exhaust
2. Fire is built on a grill to hasten the burning of coals
3. Complete combustion of wood gases prior to exhaust entering the flue
4. Long and convoluted flue acts as a heat exchanger but is sized so that there is still an adequate draft

The enthusiasm of the various folk that hang around this part of the interwebs tend to get excited about cob, burning small sticks, a certain shape of firebox, the seeming simplicity of the "RMH", etc, but those are all distractions. The fast hot smoke-free burn is key. In a traditional stove, you stuff a bunch of large logs into the stove, turn down the damper, and go to bed. Problem with these stoves is that the surface/volume characteristics of the wood determines the rate of burn. You can kind-of control the burn by turning down the draft, but you end up with a lot of un-burned wood gases (smoke) going up the flue. The outcome is a dirty chimney, fly ash, and smoke in your neighborhood. Poorly operated wood stoves give a bad name to a good sustainable way of heating. A masonry heater is a great solution because it combines an airtight firebox with the heat capacity of stone. The stone is a poor conductor of heat, so you need a long enough flue that the heat exchange is sufficient. And because heat moves through the stone slowly, it is released over the course of hours, which is perfect for residential heating.

Paul does a lot of re-branding and that's important because it allows great ideas to reach new ears. But for me, videos of hippys on the interwebs isn't enough to convince me to build a huge poo-colored mound of mud in my living room. And I could never get a 2/3rds majority in the Ways and Mean Committee for that kind of thing, even if it did appeal to me. For me, the history and physics/chemistry of the process are critical for believing that it works and for convincing The Wife that it's a good idea. There is a long tradition of masonry heaters in Europe that extends back hundreds of years and was a response to reduced availability of wood. There is a research article on the web that indicate 80% efficiency is possible [1] although I have seen another credible source that indicated over 90% efficiency was possible.

Oh, and you can make one of these things that conforms to more mainstream sense of aesthetics [2], if you're into that sort of thing.


1. http://pages.uoregon.edu/hof/W09HOF/21MasonryHeater_ppr.pdf
2. http://mha-net.org/html/gallery.htm

Ok George.... 70% is the burning efficiency, which does not mean that those stoves are efficient for keeping the heat...

BUT some non rocket are also mass heaters!
What's about their heating efficiency?
The concept of RMH is not completely new of course.

I do not know if a topic has been made about RMH drawbacks,
such as the need to be there and refill the small mouth of the dragon,
or taking care of the vertical feeding when using crooked wood.

It would not be a good idea for a place that is not heated on a regular basis either.
And, if you want to keep the heat long, then the heat goes up slowly after you have lighted the fire.
Of course you can get some radiance from the barrel, but this is not made to warm you up quickly.

@nelfson So you would stick your nose in the exhaust of a full roaring masonry heater for more than half a second?

K Nelfson wrote:RMH is a hippy-dippy re-branding of the masonry stoves that are common in the colder parts of Europe.
Yes, it's hard to believe, but you can use the exhaust from a short hot burn to heat the masonry of the stove.

The key features of masonry stoves are, according to my research,

1. Small "airtight" firebox that gives you control over the burn rate via air input instead of blocking the exhaust
2. Fire is built on a grill to hasten the burning of coals
3. Complete combustion of wood gases prior to exhaust entering the flue
4. Long and convoluted flue acts as a heat exchanger but is sized so that there is still an adequate draft

The fast hot smoke-free burn is key.

I wrote lastly before without seeing this, and this is true.
Also both Romans and Koreans had some stoves that heated the floor the way a rocket does.

BUT, some masonry heaters you can buy, are very expansive, and I understand the way things went to re-invent the mass heater, in a way it costs little...

Another question. On the exhaust from the Barrel. Can you do down below the barrel with the Exhaust? If so how far? I see some people seem to go down 12 " and some Just come out of the Barrel and it is Horizontal and then thru and out of the Mass.
Is it possible to have the Combustion chamber on the floor of a unit and then the mass in the crawlspace. Like 30" Below it and heat the floors with the mass?

Jeremiah wales wrote:Another question. On the exhaust from the Barrel. Can you do down below the barrel with the Exhaust? If so how far? I see some people seem to go down 12 " and some Just come out of the Barrel and it is Horizontal and then thru and out of the Mass.
Is it possible to have the Combustion chamber on the floor of a unit and then the mass in the crawlspace. Like 30" Below it and heat the floors with the mass?

yes.

the better insulated the J-tube is, and the more heat the outer barrel can shed, the stronger the draft. 30" down should not be a problem.

That would be interesting to list out all the differences
between a Masonry mass heater and a rocket mass heater...

I can see the price : ++ advantage to the RMH!
Big fire place with a fire to look at : ++ to the MMH!

Xisca Nicolas wrote:Ok George.... 70% is the burning efficiency, which does not mean that those stoves are efficient for keeping the heat...

It would not be a good idea for a place that is not heated on a regular basis either.
And, if you want to keep the heat long, then the heat goes up slowly after you have lighted the fire.
Of course you can get some radiance from the barrel, but this is not made to warm you up quickly.

I've seen designs where a section of metal flue (exaggerated S-shaped) is used to give a quick blast of heat during firing. It's a nice compromise between mass and instant gratification.

"you believing it is not a requirement for it being true." - US Army

they work.

Xisca Nicolas wrote:That would be interesting to list out all the differences
between a Masonry mass heater and a rocket mass heater...

I can see the price : ++ advantage to the RMH!
Big fire place with a fire to look at : ++ to the MMH!

You could build a horizontal-feed style L-tube if you want a fire to look at... -or- install a quartz window on the front of a J-tube, or a mica window...

I've thought of putting a vertical series of little mica inspection windows at the top of the barrel-- it would allow you to see when you were getting a good re-burn at the top of the riser and the quality of the flow dynamics at the gap.

Chris Burge wrote:

Xisca Nicolas wrote:That would be interesting to list out all the differences
between a Masonry mass heater and a rocket mass heater...

I can see the price : ++ advantage to the RMH!
Big fire place with a fire to look at : ++ to the MMH!

You could build a horizontal-feed style L-tube if you want a fire to look at... -or- install a quartz window on the front of a J-tube, or a mica window...

I've thought of putting a vertical series of little mica inspection windows at the top of the barrel-- it would allow you to see when you were getting a good re-burn at the top of the riser and the quality of the flow dynamics at the gap.

Windows will fog/smoke if they are placed incorrectly. The swirling wood gases in the firebox will condense if they come in contact with a cool surface. Some commercial units have the window positioned so that the air intake vents clean air across the window. Otherwise, a hot fire or a multi-pane window might help minimize fog/smoke. If nothing else, you can scrape the window clean with a razor blade from time to time.

After all of the information I have seen. Rocket Mass Heaters seem to work best in very small homes. Even one room Homes. What is created is a Lizard Rock that people put in the one room and are able to keep warm as the most important part of the one room home is the Lizard Rock.
Please dont take it wrong or be offended, Just see so many people focus on laying on the Mass or sleeping on the mass. It works like the Hot Rock I kept in my Snake Tank years ago.

Rich Pasto wrote:"you believing it is not a requirement for it being true." - US Army

they work.

I love this, I am going to have to use it more often.

It works just like your hot rock in your lizard tank...except that when it is running it has a 3000 degree core that will glow a barrel red hot and if you run it too long without fans to move the air around would probably heat you out of whatever room you decided to put it in. I think it's true with any point heating device that if you don't circulate the air to the rooms that need heat, they will simply not get warm.

: Jeremiah, the extended heat capability of an RMH is obtained by using it in a mass heating mode, be it water or masonry. In addition to the cleanliness of burn and the expediency of available heat, the RMH is compact and lends itself readily to hydronic or convection apps. I am about 90% finished with a unit I built to produce both steam and hot water, either individually, separately, or simultaneously (2 coils). My fuel chamber measures 6" by 8" by 12", and I have gotten as much as a 3-hour productive burn out of it, depending on the fuel I use. It now will burn 5" X 10" firewood, wood pellets, or biomass briquettes. So far, I have been pleased and amazed at the results. I plan to extend the fuel chamber height to 16". The combustion air can enter either thru the bottom load, or through the fuel chamber lid, or both, as needed. With both coils in operation hydronically pumped by a small circulator, I have achieved a 120 degree rise in 5 g of water, in 20 minutes.

In our tests, our stove will burn about 1/2 cubic foot of wood in 2 hours. That's roughly 15 pounds of wood. If one pound of wood gives off 8600 BTU's, then the total BTU's produced by 15 pounds of wood is approximately 129,000 BTU's in 2 hours. Take into consideration the wasted heat from the exhaust which was 130 degrees F, (for demonstration purposes) we'll call it 10% BTU loss. That means that our stove alone (without a thermal mass), is giving off 116,100 BTU's over 2 hours.

Now, when adding a thermal mass to this equation, the heat loss goes down considerably and the total BTU's utilized is a much higher percentage of total heat produced. Conventional wood stoves can't even come close to this percentage because they send most of their BTU's out the exhaust.

Which part of the hype do you not believe in.

(A). That if you and I both have to burn 8 cord/8000lbs of wood for the winter I will pollute the air less.
(B). That everything being equal the RMH is mostly sourced locally and self-build
(C). That even if we both use 8 cord of wood I can use twigs because the hot rock will condense and store the twig heat.
(D). Due to the unique second burn feature you have less cleaning to do.
(E). Due to the hot rock and second burn you will actually use less wood ex. 1 cord vs 8 cord

If you can tell me which ones you find unrealistic then I might research and learn something.

Jeremiah wales wrote:After all of the information I have seen. Rocket Mass Heaters seem to work best in very small homes. Even one room Homes. What is created is a Lizard Rock that people put in the one room and are able to keep warm as the most important part of the one room home is the Lizard Rock.
Please dont take it wrong or be offended, Just see so many people focus on laying on the Mass or sleeping on the mass. It works like the Hot Rock I kept in my Snake Tank years ago.

This is where you are misunderstanding...

Just because you see pictures of people sitting or lying on the thermal mass, does not mean that the only way that heat is transferred out of the mass is through conduction. The mass is radiating heat, just as a wood stove's primary method of delivering heat is through radiant energy-- it's just that a wood stove has a limited amount of surface area and operates at very high surface temperature, so there is no way you would take a seat on one.

A thermal mass is also a radiant heat source, just with a larger surface area and a lower, but consistent, temperature. People sit on them simply because they can, not because the have to.

Chris Burge,
I am just Average Joe. I have looked at all of the Videos and pictures of this Rocket Mass Heater, and they tell part of the story. There are Thousands of people like me who are only seeing part of the story. As many people here. it is hard to understand how it is possible for them to work. If this information was shared with General Public I bet they would go mainstream.
Seems like you have all this Information stored up inside of you. Why not do several presentations on Youtube and here to get out the Whole Story.
Here is your Calling Chris.
Good Luck... I am sure so many people would appreciate it.

I can understand the science behind these heaters. However I do see a couple potential problems.
1. If the gases can cool to a point where there can be condensation inside the flu before the gases exit the chimney, couldn't this cause the flue pipes to rust out in time? Perhaps this isn't a problem?
2. Could this be eliminated by placing a T right where the flue starts up the chimney to allow condensation to drain down underneath to a bed of gravel under the floor?
3. Is there a certain length of pipe that would be optimum for this type of heater that might keep the gasses say at 100 degrees C at the chimney so the gases would exit as steam and not cool too much?
4. Is there any limit to the length of pipe you can use say 20 ft./30ft/ 60ft as long as you have a clean out every so far?
5. What size of flu pipe is the best to use, 4",6", 8"? Does it matter?

I would love to see the internal paths of the flu pipes on some of those masonry heaters that KNelfson shared on that site. Thanks.

I'm working on a video about building these stoves and the stove we build heats a 3,000+ sf home with 3 stories. It is the exclusive source of heat for the house. I have spent a couple of months in the house at different times of the year since we built the stove a couple years back and I tell you, it is awesome! Everyone comments on it and everyone wants to sit on it when its hot. It has been their exclusive source of heat for a couple years and they still remark about how much they love it. They do crank it up for a couple hours in the evening to keep the house warm overnight, but it does work, and it works really well. I've seen it.

Jeremiah,

It sounds like you have some new questions. It would be great that if you take those questions to other threads. Preferably threads that already exist. Since you are asking new questions, one possible interpretation is that your original question has been answered. If that is the case, please say so.

I think I have worked extremely hard in putting together exceptionally persuasive information in this space and made that information available for absolutely free.

If nothing else this video makes an excellent "show me":

Roy Emerson wrote: : Jeremiah, the extended heat capability of an RMH is obtained by using it in a mass heating mode, be it water or masonry. In addition to the cleanliness of burn and the expediency of available heat, the RMH is compact and lends itself readily to hydronic or convection apps. I am about 90% finished with a unit I built to produce both steam and hot water, either individually, separately, or simultaneously (2 coils). My fuel chamber measures 6" by 8" by 12", and I have gotten as much as a 3-hour productive burn out of it, depending on the fuel I use. It now will burn 5" X 10" firewood, wood pellets, or biomass briquettes. So far, I have been pleased and amazed at the results. I plan to extend the fuel chamber height to 16". The combustion air can enter either thru the bottom load, or through the fuel chamber lid, or both, as needed. With both coils in operation hydronically pumped by a small circulator, I have achieved a 120 degree rise in 5 g of water, in 20 minutes.

Odds are extremely good that those who never make mistakes, never DO anything...engineering is 80% experimentation, 10% research,5% ability, and 5% effort.

Jeremiah, i'll say it again, build yourself one. Then you'll see for yourself. There is no magic behind RMH. And you can make one for free or pretty much.