Rocket Mass Heater Fact or Fiction SHOW ME

Satamax Antone wrote: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.

Oh Yeah. I am sure they work. Like if I go out in the yard and put up 67 bricks a barrel and a pipe. It will work. But I doing my research here before I go to the Big Mac. I do not want to end up with frustration like Danny Zeigler has with "Is my barrel too small".
I do not want to reinvent the wheel here. I want to learn from all the posts here,before I build the thing. I want to use the best parts of the system that will work in a three bedroom home. For me the Mass on the main floor of a wood floor over basement is a lot of weight and loss of space in the room. I also worry about open fill area in any stove. I have had wood pop and send a hot ember shoot 6 feet into a room onto a carpet and start to burn. I am going for a Basement with concrete floor system this fall. Letting the heat rise into the home vented and naturally rise. second combustion chamber (Barrel above the actual wood fire)is very interesting to me.

I truly appreciate all the posts by all members who puts positive and negative experience of their build in an effort to help everyone. Pictures are Fantastic all the time.

For an existing home, I highly recommend looking into replacing the thermal mass with circulating water. It is no big deal to add a boiler heater system to a house, the fact that it is wood fired makes it no different. Since you mentioned a basement, you should be able to do a in floor hydronic heating system quite easily. If you have existing baseboards, they should be able to be modified to run off circulating water with a little creativity. If you have a central AC, you can also get a heat exchanger and use your central blower to distribute heat.

I've been doing this for two years with great success, the thermal mass system was simply not an option for me since I live in a doublewide. I may not get quite as good efficiency out of my system, but an exhaust temperature of 250 degrees is nothing compared to that of a traditional wood furnace. The reason I keep it that high is to prevent water from condensing in the unit. A comparable commercial unit with secondary burn and heating limited amounts of water for better efficiency cost upwards of $7500 + hooking up to home. With all the screw ups I've made on mine, I'm in $1200 all together. I could do it for half that or less now that I know what I'm doing.

There's a lot of ways to collect and use heat from a rocket stove, you just have to decide which one works best for you.

Jeremiah, if i say build one, it's because you will have a learning curve. Build one outside, just to see. Thoses things don't throw embers. I have a grid around mine, and i'm not much woried. It's in a wardrobe in my basement workshop. Woodworking workshop that is. A good mass with a half barrel system (to hold the mass, see donkey32) weights about 800kg per square meter. Just above what comercial building joists should cope with in europe. But by looking at your floors in the us, your floor could cope with that much i think. Tho, if you're removing your actual floor and puting concrete in, use that as your mass! Please! Insulate underneath and the perimeter, and you have a lovely floor heating system. Put the top of the feed tube flush with the slab, then when you brush the tiles you've put on top, you can brush directly into the mouth of the rocket stove. I want to do this in my future workshop and run pipe in the slab. Insulated on the perimeter and underneath by 10cm of styrofoam. Talk about toasty

If you want to make an omelet you have to break some eggs.

A bit to address the issue of feeding fuel for a longer period of time. These peeps have an auto-fed pellet Rocket stove design. Heats greenhouse and aquaponics ponds.

Great Video Ryan.
Thanks

Anyone who thinks thermal mass rocket stoves are full of sh!t just might be right. In fact, I've got photographic evidence. This horse poop (see picture) was the primary ingredient for the finish plaster on this stove (see other picture).

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

they work.

The corollary to this states, "A plant does not have to believe in photosynthesis for it to occur"

August Brooks wrote: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.

I want to believe. However, let's do the math. A BTU is roughly a kJ. You say that 1lbs of wood gives 8600BTUs. This may be the case in oven dry wood, so I'll give that to you, even though it's unrealistic in air dried wood. This 8600BTU figure is about as good as you could possibly get, as far as I can tell. Any more

I'll convert that figure to SI units, as that's what I'm familiar with. 1lbs of wood = 8600BTUs = 9073kJ. 15 pounds is 129,000BTU = 136,102kJ. If this is supposed to heat a house for 24 hours, we can work out the average power. P(average) = W/T = 136,102/(24*3600) = 1.57kJ/s = 1.57kW. Now 1.57kW is a fairly pitiful amount of power.

I've seen some estimates of daily wood requirements for other rocket stoves. For example, 35lbs/day in the worst of winter, a winter that gets to well below freezing. That equates to 3.6kW. That is still not a lot of heat unless your house is very well insulated. For example, my house has some insulation in the roof but none in the walls. It is 1200 square feet in area. Our heat pump's maximum heat output is 8.5kW. On a night that was no colder than 8 degrees Celsius, (46F) this heat pump was working its hardest and was heating the house barely adequately if the doors to the kitchen and dining room were open as well. Presumably with a rocket stove the heating would have either been insufficient or we would have had to use more wood.

August Brooks wrote:(E). Due to the hot rock and second burn you will actually use less wood ex. 1 cord vs 8 cord

It is specifically this that I am dubious about. Now, I absolutely hate smoke from wood fires, I hate that it causes asthma, and I hate that in areas that do not have sufficient government regulation the whole of a region must suffer because of the selfishness of those who pollute the neighbourhood. However, there are relatively clean commercial stoves out there these days and I'm finding it hard to reconcile the claims of using 1/5 the wood as one of these (being generous), all else being equal. Does anyone care to comment?

Frank R.: While there are stoves that can be run nearly as efficiently as a rocket stove, and new ones being ''rushed to market'', the new owner soon teaches themselves
how to 'load that dirty bastard up.' ' to let it burn all the day without taking up any more of my time ' ! ! i.e. burning dirty and inefficiently, like my neighbors ! The worst
ones are the ones that deliver domestic hot water, they smolder along 24-7, 52 weeks of the year to guarantee a hot shower at will 24-7, 52 weeks of the year !

I will make a General statement that while the Rocket Stove has a major draw back, that it needs frequent attention to run at its most efficient, that this is the only way
that the Rocket Stove can be run! This is also its biggest asset, forcing you to burn it in its most efficient manner, or get smoked out of house, or left with a cold house !

Living with a Rocket Stove and its hungry Dragon is a life style commitment! If you are not willing to live this way, the Rocket Stove is not for you! If someone has to be
'out of the house' to be 'the Bread Winner', then they most spend 6-8 hrs prepared to and attending on their Rocket Stove To get 20-25 hrs of heat stored inside their
Thermal Battery,or have a Partner willing to assume that job ,or deal with a cold house !

For the good of the Craft ! Be safe, keep warm ! PYRO Logically, BIG AL ! - As always, your comments and questions are solicited and welcome !

allen lumley wrote: Frank R.: While there are stoves that can be run nearly as efficiently as a rocket stove, and new ones being ''rushed to market'', the new owner soon teaches themselves
how to 'load that dirty bastard up.' ' to let it burn all the day without taking up any more of my time ' ! ! i.e. burning dirty and inefficiently, like my neighbors ! The worst
ones are the ones that deliver domestic hot water, they smolder along 24-7, 52 weeks of the year to guarantee a hot shower at will 24-7, 52 weeks of the year !

I will make a General statement that while the Rocket Stove has a major draw back, that it needs frequent attention to run at its most efficient, that this is the only way
that the Rocket Stove can be run! This is also its biggest asset, forcing you to burn it in its most efficient manner, or get smoked out of house, or left with a cold house !

Living with a Rocket Stove and its hungry Dragon is a life style commitment! If you are not willing to live this way, the Rocket Stove is not for you! If someone has to be
'out of the house' to be 'the Bread Winner', then they most spend 6-8 hrs prepared to and attending on their Rocket Stove To get 20-25 hrs of heat stored inside their
Thermal Battery,or have a Partner willing to assume that job ,or deal with a cold house !

Some good points. Thanks for your comments.

Frank R. : There are to many hungry, abused, Rocket Stove Dragons out there already without creating more. Yes, I believe completely in Rocket Stoves.
I am trying to reduce, not increase the number of mis-matches out there, in many ways this Forum Thread is a powerful tool adding me in a sacred cause .

The use of such seemingly negative comments as Lizard Rock ( Which I am not attributing to you or anyone ) only increases the frequency with which the
bellow of Wounded pride and Self-indignant outrage is heard, a very good way of identifying those people who are in a happy denial of their own inability
to serve properly and with due reverence their Rocket Stove Dragon.

One only needs listen to identify the homes of happy, contented, Dragons by their throaty Roar! Finding the Unhappy Dragon is much harder, in such circum
stances we use the tools available to us! - And Muggle-types say "politics'' makes strange bed-fellows !

As always, your questions and comments are encouraged and are Welcome !

For the Good of the Craft/its Dragons ! Be safe, and warm ! PYRO Magically ! Allen Lumley, R.S.P.C.Draco, A.S.P.C.Draco

A rocket stove is for cooking outdoors.

A rocket mass heater is used for heating your home.

The math on how they are so much more efficient is spelled out at http://www.richsoil.com/rocket-stove-mass-heater.jsp

The great thing about a rocket mass heater is that not only does it heat your home with 1/10th of the wood, but it requires far less fooling with than a conventional wood stove. If nothing else, you have 1/10th the wood to fool with. But even better is that you can burn a fast hot fire in the evening and then you're done. The house stays warm through the night with no further fire. You wake up in the morning and the house is warm - no need to start a morning fire.

Keep in mind that a rocket mass heater leans heavily on radiant heat and conductive heat - which are both far more efficient than convective heat. Also keep in mind that most people run their conventional wood stoves in a way to try to make the fire last longer. So while their stove might be rated as 75% efficient, they might be getting only 10% efficiency. Or worse.

Paul W.: You are right,Sir ! We have gotten so loose-y goose-y, with our name calling that to paraphrase the Cheshire Cat, Me means Rocket Stove,
Rocket Stove Means Me !

I will try to modify the way I speak of the Rocket Stove Family on a day to day way, and if asked I will say that ;

1) The Rocket Stove Is for Cooking and Feeding yourself !

2) The Rocket Mass heater Is for Heating your Home and Feeding your Dragon !

3) The Pocket Rocket Is for Heating all outdoors, Picket lines, Preppers, and Ice Fishers

There, easy to say and easy to remember, I hope to see the day When A New Rocket Mass Heater Builder (a little difficult to say) will ask just a few
neighbors to find a 55gal drum top adapted to make a Pocket Rocket so that he can burn off/out his new barrel for his new Rocket Mass Heater !

For the Good of the Craft ! Be safe, Keep Warm ! PYRO Logically Big AL ! As always questions/comments are encouraged and Welcome ! A. L.

paul wheaton wrote:A rocket stove is for cooking outdoors.

A rocket mass heater is used for heating your home.

Sure. I'll accept your nomenclature. I suppose that we might call the rocket mass heaters some have made without the mass "rocket heaters", even though that is probably more confusing. If that's the case, the CWS is better named the CWH.

paul wheaton wrote:The math on how they are so much more efficient is spelled out at http://www.richsoil.com/rocket-stove-mass-heater.jsp

The great thing about a rocket mass heater is that not only does it heat your home with 1/10th of the wood, but it requires far less fooling with than a conventional wood stove. If nothing else, you have 1/10th the wood to fool with. But even better is that you can burn a fast hot fire in the evening and then you're done. The house stays warm through the night with no further fire. You wake up in the morning and the house is warm - no need to start a morning fire.

Keep in mind that a rocket mass heater leans heavily on radiant heat and conductive heat - which are both far more efficient than convective heat. Also keep in mind that most people run their conventional wood stoves in a way to try to make the fire last longer. So while their stove might be rated as 75% efficient, they might be getting only 10% efficiency. Or worse.

I've read your page several times already, before I even registered for Permies. It's part of what got me so interested in these rocket heaters to begin with. And what I want to reiterate is that the secondary burn concept, using insulation to get the gases to combustion temperature before attempting to extract the heat, is an excellent one.

I've also posted here on the subject.

As an engineer, like professor Rich I think it's better to attempt to quantify what the stove is doing thermodynamically. I haven't noticed that conductive or radiant heat is any better than convective heat. I don't get any hotter from a 1000W radiator than I do from an electric 1000W bar radiator vs a 1000W fan radiator vs a 1000W oil filled radiator (which is primarily a convector I believe, which is why it is designed like a passive heat sink with widely spaced fins). I will get as warm from a 70W electric blanket if I stay in bed but that's a big proviso. And the only reason that works is because both above and below the electric blanket there are several inches of insulation (blankets) to trap the heat in, where my body is. It's not because conduction is more efficient. If I took any of the other heaters under the covers I would soon be scorched because it is the insulation that is providing the "force multiplier" so to speak. (I am not referring "force" in "force multiplier" as in physics, I'm using the term as is typically used, to get more from your investment.)

From what I can see, there are some unstated provisos that are used to get the 1/5 or 1/8 or 1/10 the wood use required that I have seen. Namely, that in order to achieve those benefits you either need some combination of:
a) Have some serious insulation in your house (e.g. R30 or more)
b) Have a small house
c) Not live in a very cold climate
d) Are willing to put up with a (much) colder house except the room you choose to heat
e) Are willing to constrain yourself to being on/near the rocket mass in much the same way as you need to be proximate to an electric blanket or water bottle in order for it to be useful.

What could also be made explicit is the amount of wood that is deemed necessary to heat a house of particular specification (size and insulation level, outside air temperature) and for how long in order to heat it effectively so that the claims might be evaluated. From what I can see from the figures typically bandied about, 35-45lbs of wood worst case, we are not looking at big power levels even if the heat is liberated with 100% efficiency. I don't think that the comparison is apples to apples with the CWS in order to get the 1/5-1/10 figures of wood used.

In order for me to address everything you mention here, it would take two days of typing.

To save myself time I am simply going to state:

Thousands of people have replaced a conventional wood stove with a rocket mass heater. They now require 80% to 90% less wood to be just as comfortable as they were before. If your math does not account for that, there are two possibilities:

1) Your math has a bug in it somewhere.

2) I'm a liar, and all those other people that are reporting this here on permies and other places are also liars. In which case you should drop the whole thing so that you don't get mixed up with a bunch of liars.

I know that a lot of people create some sort of freak show of flaming death and call that "a rocket mass heater" - and i think it is important to not include those in our anecdotal sample set.

Frank R. - You are very correct in stating that if you need X btus to maintain a comfortable room temperature, you will have to burn a specific amount of wood at 100% efficiency to achieve that. For example, red oak has around 6190 btu/lb. If you need 30000 btu to maintain a comfortable temperature differential in the cold of winter, you will have to burn nearly 5 lb/hr to achieve that. There is no way around that. In a perfect world with a 100% efficient stove and heat transfer, you will burn a cord of seasoned red oak wood in 29.75 days.

I use a slightly different type of rocket heater than many here do. Mine heats water, which in turn heats my house and provides household hot water. Assuming an internal temperature at the hottest point being 1500 degrees (which judging from flame and coal colors is a low estimate), I've measured the output at being 250 degrees, yielding 83% efficiency. An impressive number considering I'm a backyard engineer with less than $1500 in the whole system. I'm going to have to burn 6.02 lb of wood at that efficiency to get my 30000 btu needed, and more if I need extra hot water. Wood is fairly low in energy density and you will have to burn a good amount of it to keep the typical household comfortable in the dead of winter.

Mine is gravity fed, needing a refill every 6 to 8 hours. Compared to my neighbor's Earth wood furnace, it makes no smoke and goes through a good deal less wood. I think this is because my neighbor runs his fire on a smolder to minimize reloads. Mine runs red hot and full bore to maximize every btu possible in the wood. I adjust my burn rate by the size of the fuel fed. I run through a cord of wood every month or two, since Missouri weather changes so much. My neighbor goes through three or four in the same amount of time.

That is my real world experience as well as my understanding of the theory behind it all. I agree with your assessment to use 1/5 to 1/10 of the wood in a rocket stove would require significant house changes as well as changes to lifestyle. I hope you find this information useful.

Paul, I'm not calling you a liar. There is a third possibility, that the parameters of a system in which someone burns 1/5 to 1/10 the wood of a CWS are not nailed down properly, that it is not an apples to apples comparison.

I'm open to the possibility that my math, physics or chemistry is incorrect, but that would require some discussion. Physically, a RMH is not magical. It can only convert the chemical energy in the wood to heat energy, and since there is no fission or fusion going on, this is all the energy that is available. Now, it is possible to pump the heat from outside the building to inside the building with a heat pump (this is how the electric heat pump is able to do 4 or so times as much heating as the electricity going in, through the Coefficient Of Performance - COP), but to do this with a RMH would require situating something like a Stirling engine on the RMH barrel and hooking it up to a conventional heat pump with compressor situated outside.

Because none of that is going on, there is an upper limit to the energy that is available from any wood heater, and that is the amount of chemical energy liberated from a given quantity of oven dried wood (i.e. 0% Moisture Content) when it is burned completely. We are looking at something like 20,000kJ/kg or 8600 BTU/lbs. I have seen 35lbs-45lbs/day as a worst case, coldest day figure used for the 8 inch RMH systems. If the 45lbs figure is used, that equates to 38,7000 BTU/day, or 16,125 BTU/hour. Alternatively, that equates to 409,000kJ/day, or 4.73kW average. In reality few people are burning oven dried wood or the highest energy density wood, the exhaust temperature of the RMH will be significantly above the outside temperature, and this will have risen by the end of the burn. As a result, the heat available to heat your house will be somewhat less than this ideal figure.

Even the best case level of heat with the specified 45lbs of wood for 24 hours is insufficient to heat my home adequately. I know this because my heat pump will output 8.5kW and it barely keeps up at maximum output. I live in a temperate climate where snow might happen once in ten years. i.e. it rarely gets colder than freezing temperature overnight, and usually not to that level.

Geo,

Thanks for your comment. I've read your web page and a few of your posts and it seems that you have had a similar idea to me. One of the first things that occurred to me when thinking about RMH technology was to create a water heater, storage and radiator system as you have done. Well done on your implementation.

I keep thinking of how to make the system safer, e.g. by making an insulated fire box rather than using a feed tube. Obviously if you do that, you have to incorporate some secondary air into the system for secondary combustion. This system then starts to look a lot like the EPA non-catalytic stoves, since they have somewhat insulated fireboxes and preheated secondary air.

Another thing I was considering was to have a 180 degree turn at the top of the heat riser for the heat exchanger to follow, so that you could have a counterflow heat exchange (ideally you want the cold water contacting the coolest part of the exhaust where it can extract the most heat), which may also permit a thermosyphon effect as the water is travelling in at the bottom, cold, and out of the top, hot. This might also help with pumping the exhaust, as it is doing what the barrel does in the conventional RMH. In effect a reverse thermosyphon I believe, as the hot air goes in at the top and exits as (relatively) cool.

If this concept was executed as a 2 tonne mass heater (the mass is vertical within a 2 metre tall, 1mx1m footprint), I was thinking that you could just route a bunch of vertical pipes straight down through the liquid you are trying to warm, which carries exhaust. Added up, the CSA of these tubes would be the same as your flue diameter. One would have to have a way of accessing these tubes from the manifold so as to be able to clean these with a pipe cleaner. The walls of the mass heater would do the radiate the heat and convect it. The mass container might be filled with either paraffin wax or water. Water is much cheaper, and the high initial heat transfer and exponential lowering over time would work quite well with typical heating needs.

There are some issues that would make this impractical for me though - I don't think my landlord would approve. Also you need a floor capable of holding 2 tonnes of water. Perhaps outside. Nice work though. Consider making plans and selling them.

Frank
Since you have stated that your current system cannot heat your house and it is the same size as a proposed rocket mass system may I respectfully suggest that maybe insolation should be your priority . Then you can heat your current properrty at much less cost even if you dont build a Rocket mass heater .
I have a friend who heats his house for less than 200 € a year and its a big house must be close on 200msq most of the "heating " is due to a big mass he heats using passive solar in the summer and he puts his stove on for only three months ;Its not a rocket stove I havnt told him about them yet

David

that it is not an apples to apples comparison

Same house. Same insulation. Same person chopping the wood, drying the wood and carrying the wood. The only thing that has changed is that a rocket mass heater is used instead of a conventional wood stove.

Apples to apples.

I think the path you need to follow is to either build it and see for yourself, or write this off as some sort of crazy voodoo and steer clear of it. Because it would seem your math has failed you (hint: you are exploring the BTUs without considering the BTUs that go out the chimney).

Frank Rasmussen wrote:Geo,

Thanks for your comment. I've read your web page and a few of your posts and it seems that you have had a similar idea to me. One of the first things that occurred to me when thinking about RMH technology was to create a water heater, storage and radiator system as you have done. Well done on your implementation.

I keep thinking of how to make the system safer, e.g. by making an insulated fire box rather than using a feed tube. Obviously if you do that, you have to incorporate some secondary air into the system for secondary combustion. This system then starts to look a lot like the EPA non-catalytic stoves, since they have somewhat insulated fireboxes and preheated secondary air.

Thanks for the feedback. The firebox and heat riser is actually heavily insulated, about 8" thick refractory. The feed tube in my more recent videos simply guides new fuel into the fire, no gasification or combustion occurs there. Waste heat that travels up the bottom portion of the feed tube is used to preheat combustion air and the fuel, adding to efficiency. My feed tube and no jam method of bundling the fuel is the secret behind not having to tend the fire constantly. In fact, with the weather websites that show hourly temperatures, you can add wood sized to meet temperature demands automatically.

Another benefit of the feed tube is that you do not need a thermal mass to store the heat produced, simply create heat as you need it. This works really well where I live where night time temperatures can be quite cold, while the day is very mild. If I had a large thermal mass radiating heat, I'd have to open my windows and let it out during the day, wasting fuel.

The only time I ever tend the fire is to fill the feed tube every 6 to 8 hours or if I need extra hot water for bathing or cleaning. In that case, I simply add a piece of wood horizontally into the firebox, giving the extra boost needed. This is where I came up with the dual plane rocket stove name.

This upcoming winter will be my third year using this system, each year I improve on it based off my experiences the year before. Once I get all the kinks worked out (hopefully after this winter), I intend to distribute detailed construction plans for those interested.

Jeremiah wales
I think you are missing a very important point and that is :there are a lot of people in this world and the educated people are beginning to understand that to ensure the world will last longer every one of us needs to do what we can as soon as we can
The question you should be asking yourself is Why do you want to keep a 1000ft lower level of home TOASTY or maybe you have 20 people living there?
What do you call TOASTY most people in the RMH videos are wearing jumpers and clothes not running around naked. my central heating is never higher than
16deg C but i know friend's who have theirs set to 25deg C and still complain
So these Rocket Mass Heaters that heat your home/workplace and could cook your food, heat your water.
Use a sustainable fuel and considerably less than most other types of similar heaters
Produce less pollution than almost any other source of heating/cooking
Can be built using easily obtainable items some locally sourced
They are safe with no risk of explosion/nuclear radiation or environmental pollution

So what was the reason you have not built one oh yea You cant be bothered to put a jumper on and don't want to tend to it a few times a day
I just wish I could go home and build my own ASAP

calen kennett wrote: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.

That seems impressive to heat a such a large place. In addition to researching this technology for my own future use I am writing up an essay on renewable energy (and proposing a heating system) so I would be really grateful if someone could answer a few questions:

1.In theory, could a RMH easily heat a 2300 square foot building (one level, 10 rooms in total) with maximum space heating need of 25kW?
2.Would there be issues with the heat dispersing into the various rooms, or is it likely that in a reasonably well insulated building that the heat disperse across the rooms with doors left open?
3. As a guess, in a typical UK climate, approximately how man cords of wood, would be needed over the year?

thanks

Frank Rasmussen wrote:

August Brooks wrote: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.

I want to believe. However, let's do the math. A BTU is roughly a kJ. You say that 1lbs of wood gives 8600BTUs. This may be the case in oven dry wood, so I'll give that to you, even though it's unrealistic in air dried wood. This 8600BTU figure is about as good as you could possibly get, as far as I can tell. Any more

I'll convert that figure to SI units, as that's what I'm familiar with. 1lbs of wood = 8600BTUs = 9073kJ. 15 pounds is 129,000BTU = 136,102kJ. If this is supposed to heat a house for 24 hours, we can work out the average power. P(average) = W/T = 136,102/(24*3600) = 1.57kJ/s = 1.57kW. Now 1.57kW is a fairly pitiful amount of power.

I've seen some estimates of daily wood requirements for other rocket stoves. For example, 35lbs/day in the worst of winter, a winter that gets to well below freezing. That equates to 3.6kW. That is still not a lot of heat unless your house is very well insulated. For example, my house has some insulation in the roof but none in the walls. It is 1200 square feet in area. Our heat pump's maximum heat output is 8.5kW. On a night that was no colder than 8 degrees Celsius, (46F) this heat pump was working its hardest and was heating the house barely adequately if the doors to the kitchen and dining room were open as well. Presumably with a rocket stove the heating would have either been insufficient or we would have had to use more wood.

August Brooks wrote:(E). Due to the hot rock and second burn you will actually use less wood ex. 1 cord vs 8 cord

It is specifically this that I am dubious about. Now, I absolutely hate smoke from wood fires, I hate that it causes asthma, and I hate that in areas that do not have sufficient government regulation the whole of a region must suffer because of the selfishness of those who pollute the neighbourhood. However, there are relatively clean commercial stoves out there these days and I'm finding it hard to reconcile the claims of using 1/5 the wood as one of these (being generous), all else being equal. Does anyone care to comment?

Actually, that number is correct. It is the avg btu output of woods available in north america and is the avg for all wood here, softwood and hardwood. The bottom of the spectrum begins at around 8k btu's and high end, which is produced by highly resinous hardwoods is 9.7k. I believe the numbers are for 1 year old "seasoned" wood. Meaning, air dried wood as kiln dried wood would not be economically sound to use as firewood. It's the resin content of the wood that effects these numbers more so than drying methods.

This thread is ten years old.  I've forgotten all about it, except that I read the dailyish a moment ago and monica mentions it.  

Here is me heating my 3-bedroom home in montana with 0.60 cords of wood for a very cold winter