are rocket mass heaters "appropriate technology" given the availability of efficient woodstoves?

I want to thank the OP for asking a great question.
I myself will not put my RMH in my house,due to insurance/inspection/safety issues.
$900.00 dollars would be great for a working wood burning heater.
But it wouldn't be $900.00 after installation. And if I installed it myself,it would probably not pass the insurance/inspection/safety test.

My van isn't worth more than 2000 dollars. That is generous,it has three busted doors and a broken wind shield.
I really need to fix the wind shield.
My mortgage is under 400 dollars.
I live in the ghetto.
Things are tight,my heating bills are a place to cut costs,which is why I am considering wood heat in the first place.
Thousands of dollars for a stove + installation might as well be ten's of thousands for someone like me.

A RMH might work for me. Might not.
But I can try it,working my way up to a full install a bit at a time,as time and material come my way.


The SHTF everyday around my house. It isn't an abstract possibility,it's life as we know it.
Sometimes we have only ourselves to blame,sometimes it's the wolf at the door,but either way technology isn't appropriate if it is simply out of reach.

Glenn Herbert wrote:You may remember that they talk about their wood use per day, and figure about a cord for four months of deep winter in the mountains near the Canadian border. This is for their pre-existing small house, construction standards unknown but probably standard stick-framing at best.

This reduction in fuel is an interesting claim. if we can simplify things and simply multiply the %efficiency * fuel btu per unit weight. Now if we have 100% efficiency then we should have the full btu value/unit mass which means we have our maximum theoretical value for a fuel. But no matter the fuel that btu value is limited. So no matter the technology or the fuel there is a limit to the amount of heat produced. Because of that I have doubts about people being able to heat their home with 1/8 the wood when it seems that it is unlikely that for a given home that they would have a fuel that produced enough heat, even at 100% efficiency.  Now I haven't run the numbers but even with very good insulation and high btu fuel I doubt that a situation where 7 cords were burned before can heat a home with 1/8*7 cords. Anyone else wonder about this?

Hi Lawence
I dont quite understand your argument .
I burn about six units of wood a year in my home here in France . I have a conventional home conventional stove  . If I get a RMH and I build it my self I use less wood than I do at the moment I still heat my house . May be a tenth  may be a quarter of what I burned before . The important word is LESS the rest is just detail .

David  

If the previous system wasted 7/8 or more of the fuel value up the chimney, then a fully efficient system would put the same amount of useful heat into the house with 1/8 the wood.

David Livingston wrote:Hi Lawence
I dont quite understand your argument .
I burn about six units of wood a year in my home here in France . I have a conventional home conventional stove  . If I get a RMH and I build it my self I use less wood than I do at the moment I still heat my house . May be a tenth  may be a quarter of what I burned before . The important word is LESS the rest is just detail .

David  

What is a "unit" of wood?  Here in the US we measure wood in cords, which is 128 cubic feet (3.6 cubic meters).  How many "units" is that for you?

Lawrence Wood wrote:

Glenn Herbert wrote:You may remember that they talk about their wood use per day, and figure about a cord for four months of deep winter in the mountains near the Canadian border. This is for their pre-existing small house, construction standards unknown but probably standard stick-framing at best.

This reduction in fuel is an interesting claim. if we can simplify things and simply multiply the %efficiency * fuel btu per unit weight. Now if we have 100% efficiency then we should have the full btu value/unit mass which means we have our maximum theoretical value for a fuel. But no matter the fuel that btu value is limited. So no matter the technology or the fuel there is a limit to the amount of heat produced. Because of that I have doubts about people being able to heat their home with 1/8 the wood when it seems that it is unlikely that for a given home that they would have a fuel that produced enough heat, even at 100% efficiency.  Now I haven't run the numbers but even with very good insulation and high btu fuel I doubt that a situation where 7 cords were burned before can heat a home with 1/8*7 cords. Anyone else wonder about this?

I'm in the same boat as you.  On hearth.com a typical user reports burning 4 cords of wood in their stove per winter.  If we take the 1/8 factor at face value, am I to assume that a typical RMH burner would heat that same home with 4* ( 1 / 8 ) = 1/2 cord of wood per winter?  Half a cord of wood is what fits in the bed of a Ford F150 pickup truck.  

RMH burners - do you heat your home on less than 1 F150 pickup bed worth of wood per winter?

here in france we have steres  as for how much not sure in m3 not sure I just know I count on six. I will find out .
As for folks saying they used an eighth of what they used before unless you actually ask them what they used before comparing it to a self selected group of "experts " with the latest kit seems a bit simplistic to me . I have a two year old fire and I dont expect to use an eigth of what I used before . My folks used to have a fire from the 1970's that burned logs like no ones biz I expect them to use an eigth

David

Lawrence Wood wrote:
This reduction in fuel is an interesting claim. if we can simplify things and simply multiply the %efficiency * fuel btu per unit weight. Now if we have 100% efficiency then we should have the full btu value/unit mass which means we have our maximum theoretical value for a fuel. But no matter the fuel that btu value is limited. So no matter the technology or the fuel there is a limit to the amount of heat produced. Because of that I have doubts about people being able to heat their home with 1/8 the wood when it seems that it is unlikely that for a given home that they would have a fuel that produced enough heat, even at 100% efficiency.  Now I haven't run the numbers but even with very good insulation and high btu fuel I doubt that a situation where 7 cords were burned before can heat a home with 1/8*7 cords. Anyone else wonder about this?

Here is a very important thing:

If there is a long list of people that are heating their home with one tenth the wood, doesn't that make you wonder about the "math" used to claim that another wood stove is "75% efficient"?

Another important thing:

The evidence that a rocket mass heater works as claimed is overwhelming.  The information provided in this thread alone is just a taste and is very powerful.   You are welcome to choose to remain skeptical.  In the mean time, hundreds of thousands of others have penciled out the math and will move forward.   The benefits are just far too massive.  Perhaps you will be less skeptical in a few more years when the body of evidence is even larger.

Davis Tyler wrote: On hearth.com a typical user reports burning 4 cords of wood in their stove per winter.  If we take the 1/8 factor at face value, am I to assume that a typical RMH burner would heat that same home with 4* ( 1 / 8 ) = 1/2 cord of wood per winter?  Half a cord of wood is what fits in the bed of a Ford F150 pickup truck.  

RMH burners - do you heat your home on less than 1 F150 pickup bed worth of wood per winter?

It seems like we are at the end of the road here.  If this is your last concern, then we have passed the gauntlet with flying colors.  

I think you will find several mentions through this thread of using about half a cord of wood per winter.  And there is even a video posted that is just over a minute long where it was reported that 4 cords were used, every winter, with a conventional wood stove, and with a rocket  mass heater it is about a half cord of wood.  And it was more comfortable with the rocket mass heater.  

paul wheaton wrote:

Davis Tyler wrote: On hearth.com a typical user reports burning 4 cords of wood in their stove per winter.  If we take the 1/8 factor at face value, am I to assume that a typical RMH burner would heat that same home with 4* ( 1 / 8 ) = 1/2 cord of wood per winter?  Half a cord of wood is what fits in the bed of a Ford F150 pickup truck.  

RMH burners - do you heat your home on less than 1 F150 pickup bed worth of wood per winter?

It seems like we are at the end of the road here.  If this is your last concern, then we have passed the gauntlet with flying colors.  

I think you will find several mentions through this thread of using about half a cord of wood per winter.  And there is even a video posted that is just over a minute long where it was reported that 4 cords were used, every winter, with a conventional wood stove, and with a rocket  mass heater it is about a half cord of wood.  And it was more comfortable with the rocket mass heater.  

I just re-read the thread to make sure I didn't miss anything.  The only reference to 1/2-cord per winter was referring to Ernie and Erica's video.  That is a single (valuable) data point.  I have not read a single first-hand account, in this thread, of someone saying "I heat MY house with 1/2 cord of wood using a RMH".  Maybe I should start a separate thread to compile those first-hand responses from current RMH users (unless it already exists?).

So far in this thread, we have people who have never tried a RMH doubting the claims of how little fuel it uses.  

We have people who have tried just modern high-efficiency woodstoves who know that these stoves do not live up the hype.

We have people who have tried both RMH and woodstoves who have personal experience as to how little wood Rocket Mass Heaters use.  

We have people who have done the numbers and found a way to blast away the equivocation and compare modern woodstove 'efficiency' with actual efficiency - RMH are the clear winner.

We do not have anyone who has tried both a modern woodstove and a RMH claiming that RMH's do not live up to the hype.

This last point is a very strong one when it comes to deciding if I will include a RMH in my life.

I suspect that the house I am living in right now ....  that we use about a half cord of wood per winter.   I was just visiting with jocelyn and we have a small, rough book shelf that we fill with firewood.   We talked at length about how often we fill it ...   there were a lot of variables (we usually don't put anything on the bottom shelf ... a few times we brought wood in without filling the shelf) ...  but we think that we end up at about half a cord of wood per year.  

But, to be really certain about this, I would want to get a few more winters under my belt.  At this point, I am pretty confident that a half cord per winter is where we will probably end up.   The real test will be with a really cold winter.  

And I live in montana.

R Ranson wrote:So far in this thread, we have people who have never tried a RMH doubting the claims of how little fuel it uses.  

We have people who have tried just modern high-efficiency woodstoves who know that these stoves do not live up the hype.

We have people who have tried both RMH and woodstoves who have personal experience as to how little wood Rocket Mass Heaters use.  

We have people who have done the numbers and found a way to blast away the equivocation and compare modern woodstove 'efficiency' with actual efficiency - RMH are the clear winner.

We do not have anyone who has tried both a modern woodstove and a RMH claiming that RMH's do not live up to the hype.

This last point is a very strong one when it comes to deciding if I will include a RMH in my life.

Totally agree.  That's why I was so bummed to see that Dragon Heaters dropped out of the Wood Stove Decathalon contest : http://www.forgreenheat.org/stovedesign.html

It would have been a great opportunity to quantify the relative merits of a RMH vs other "best in class" wood-burning technologies on a level playing field

R Ranson wrote:
We do not have anyone who has tried both a modern woodstove and a RMH claiming that RMH's do not live up to the hype.

Excellent point R.   Apples for you.

paul wheaton wrote:I suspect that the house I am living in right now ....  that we use about a half cord of wood per winter.   I was just visiting with jocelyn and we have a small, rough book shelf that we fill with firewood.   We talked at length about how often we fill it ...   there were a lot of variables (we usually don't put anything on the bottom shelf ... a few times we brought wood in without filling the shelf) ...  but we think that we end up at about half a cord of wood per year.  

But, to be really certain about this, I would want to get a few more winters under my belt.  At this point, I am pretty confident that a half cord per winter is where we will probably end up.   The real test will be with a really cold winter.  

And I live in montana.

That's seriously impressive.  This is the double-wide?  How many square feet is your place?

Davis Tyler wrote:
That's seriously impressive.  This is the double-wide?  How many square feet is your place?

Double wide.  I would guess it is 1400 square feet.  

paul wheaton wrote:

Davis Tyler wrote:
That's seriously impressive.  This is the double-wide?  How many square feet is your place?

Double wide.  I would guess it is 1400 square feet.  

Are you able to tolerate a lower thermostat temperature with the same comfort level, due to the radiant heat of the mass bench?  I wonder if that's where the disconnect is: people heating their homes with convective means require 70F for comfort, but those with a radiant RMH bench could tolerate 60F air temperature because it is radiating heat to the person.

Davis Tyler wrote:

paul wheaton wrote:

Davis Tyler wrote:
That's seriously impressive.  This is the double-wide?  How many square feet is your place?

Double wide.  I would guess it is 1400 square feet.  

Are you able to tolerate a lower thermostat temperature with the same comfort level, due to the radiant heat of the mass bench?  I wonder if that's where the disconnect is: people heating their homes with convective means require 70F for comfort, but those with a radiant RMH bench could tolerate 60F air temperature because it is radiating heat to the person.

Could you explain this a bit more?
Why would radiant heat meal cooler air temperature?
If I was sitting on the bench, I could see less need for warm air, but I don't spend much of my life sitting down.  If one wanted warmer air, wouldn't they just re-light the fire in the RMH?

That could have something to do with it, though it wouldn't account for whatever the comfort level is in areas out of sight of the heater mass.

If the air temperature can be lower, then there will be less heat lost through the walls and air changes, which would also act to improve total efficiency.

I put underfloor radiant heat in my house 16 years ago. It makes the space comfortable at 65F or less, partly because it warms my feet (conduction) and partly because the radiation warms nearby objects without creating hot air at the ceiling to accelerate heat loss.

Isn't a woodstove radiant heat also?  I get the impression RMH provides a more steady heat due to the mass than many woodstoves do.  Maybe these super fancy (expensive) modern woodstoves also put out steady heat for a long period, but our mid-90s-era woodstove does not.  As not-great as it is, is is fabulous compared to the forced air electric furnace which came with this cheap house.  That was seriously horridly awful and expensive to run.

R Ranson wrote:

Davis Tyler wrote:

paul wheaton wrote:

Davis Tyler wrote:
That's seriously impressive.  This is the double-wide?  How many square feet is your place?

Double wide.  I would guess it is 1400 square feet.  

Are you able to tolerate a lower thermostat temperature with the same comfort level, due to the radiant heat of the mass bench?  I wonder if that's where the disconnect is: people heating their homes with convective means require 70F for comfort, but those with a radiant RMH bench could tolerate 60F air temperature because it is radiating heat to the person.

Could you explain this a bit more?
Why would radiant heat meal cooler air temperature?
If I was sitting on the bench, I could see less need for warm air, but I don't spend much of my life sitting down.  If one wanted warmer air, wouldn't they just re-light the fire in the RMH?

The same reason you can be outside on  a 40-degree day in the high desert with bright sunshine and be quite comfortable in a t-shirt.  The sun is heating your body radiantly even though the air temperature is way below your comfort level.  That same 40-degree day in overcast Buffalo NY, you will be wrapped up in multiple layers to achieve that same comfort level

I rarely sit on the bench.  But when I do, it is usually when the fire has been out a while.   And I mostly do it just because it feels so .... luxuriant.  If we had a cat, the cat would probably spend all winter on it.

When the fire is running, sometimes l like to sit near the barrel for a bit - get extra toasty.  

We have a thermometer around the corner from the rmh.   It seems like we stop putting wood into it when we get to 75.  The next morning, the temperature is usually about .... 71?  We usually fire it up every other day, so on the second morning it is usually about 66 or so.   I get pretty uncomfortable at 65, so we fire it up again.  

As I mentioned earlier in this thread, the wood feed is 7.25 inches by 4.75 inches.   So it holds four pretty small sticks at a time.  We burn it for about an hour and a half every other day.  So I think you can see how a half cord of wood is about right.

So, when it comes to "what is the most luxuriant form of wood heat" I cannot imaging anything coming close to a rocket mass heater.   I once stayed a week in a house where the floor was a chimney for a fire.  The rocks in the floor became warm.  That was pretty nice.   But sitting on the warmth is even more luxuriant.  My bench does not facilitate lounging as well as most rocket mass heaters.  If I could do it over again, I would have something that facilitated lounging better.

A woodstove with its generally high surface temperature does radiate significantly, but also heats a plume of air that rises strongly to the ceiling. The lower surface temperature of the RMH mass does not generate the rising plume of hot air.

Tyler Ludens wrote:Isn't a woodstove radiant heat also?  I get the impression RMH provides a more steady heat due to the mass than many woodstoves do.  Maybe these super fancy (expensive) modern woodstoves also put out steady heat for a long period, but our mid-90s-era woodstove does not.  As not-great as it is, is is fabulous compared to the forced air electric furnace which came with this cheap house.  That was seriously horridly awful and expensive to run.

Yes we're certainly way down the path of good>better>best

Woodstoves are a combination of radiation and convection.  Many of them have fans to distribute the heat around the room/house, especially the insert models that sit back in a masonry fireplace

Glenn Herbert wrote:That could have something to do with it, though it wouldn't account for whatever the comfort level is in areas out of sight of the heater mass.

If the air temperature can be lower, then there will be less heat lost through the walls and air changes, which would also act to improve total efficiency.

I put underfloor radiant heat in my house 16 years ago. It makes the space comfortable at 65F or less, partly because it warms my feet (conduction) and partly because the radiation warms nearby objects without creating hot air at the ceiling to accelerate heat loss.

So radiant heat makes you feel more comfortable at a lower air temperature - which uses less fuel/resources to heat an area?

That sounds like another reason why RMH would be more efficient than a wood stove.

I've never been keen on the idea of a wood heater that requires electricity to work.

Interesting thread. I'll pass along some of the things that prompted me to build my Rocket Mass Heater, and I've since retired my high efficiency box stove, i.e. the design that has secondary air-supply tubes and refractory insulated ceiling and chamber etc.

RMH

* Burns less wood because it burns super efficiently and completely. Gathering wood is labor intensive, and I'm all for saving my labor for more interesting things than processing firewood.

* No smoke after initial start-up. In reality, on mine it is difficult to detect any smoke even on startup when using dry tender. But most importantly, I can't smell any smoke outdoors at any point of the burn, and that allows me to do outdoor chores without having to dodge smoky areas like I was forced to do when running the metal box stove.

* There's no need for a fire through the night, a big selling point for me. The masonry mass continues heating the space well into the next day or two. During really cold winter nights here in the Powell Valley, just south of the Cumberland Gap, the air is often dead still, no movement at all. Thus any overnight fire, or even an early morning burn, in the steel box stove resulted in smoke accumulating around the house and finding its way back inside.

* No creosote. Both smoke and creosote are fuel, and I'm not into wasting fuel because it means wasted energy in gathering firewood. With the metal box stove I was always having to chimney sweep, annually and sometimes biannually. So no more getting up on the roof to clean the 15 ft. worth of Class-A chimney. Another BIG (+) for my RMH.

Masonry heaters may not be the perfect solution for all, however. They do best in open floor plans. I've knocked out some walls to open up the space of my 1350 sq. ft. humble abode.

Davis Tyler wrote:
I just re-read the thread to make sure I didn't miss anything.  The only reference to 1/2-cord per winter was referring to Ernie and Erica's video.  That is a single (valuable) data point.  I have not read a single first-hand account, in this thread, of someone saying "I heat MY house with 1/2 cord of wood using a RMH".  Maybe I should start a separate thread to compile those first-hand responses from current RMH users (unless it already exists?).

Why the focus on 1/2 cord of wood?  Houses are different, people's comfort levels are different, climates are different.  The amount of wood it takes to heat the teepee at Wheaton's to a comfortable level would be different than wood utilized in the RMH used as back-up for occupants of a passive solar building in the western Colorado desert.  And it would be different in the same dwelling for different occupants.  It would also vary depending on the moisture level in and temperature of the wood when added to the fire.

Why the focus on precise quantification of percentage of reduction of wood use?  I can't say where the data is available, but the wood stove industry has skewed numbers on their efficiency ratings.  The term "85% efficient" applying to a modern high tech state of the art wood stove does not mean it can deliver as heat to the living area, 85% of the BTUs produced through complete combustion of a given weight of wood.  They have a fudge factor in there.  The estimated amount of  heat a "perfect" stove would deliver is used as the 100% figure and it is no where near 100% of heat produced by the combustion of a given weight of wood.

I suggest that at this point enough information has been provided, that a truly curious person would begin to pursue real life evidence and experience.  A truly curious person would look at the standards by which conventional modern wood stoves are rated, and find the percent of available heat from wood combustion that is rated to be "100%" for modern wood stoves.  Then a person could have a coefficient to multiply by a conventional stove rating to discover what the true percentage of efficiency is.  So, the most telling bit of information, IMO, would be the fudge factor the wood stove industry utilizes when making claims of 75, or any other percentage of efficiency rating.  That is the amount they are just willing to "throw away" because it makes their industry look good, and prevents further research on more efficient means of utilizing wood heat.

A person wanting to learn more could attend a workshop and help build, could find and visit a demonstration site, could find and visit an existing RMH, and begin to find out why people with functional RMHs are so satisfied.

Because it is outside the realm of our (USA) regulated and engineer sanctioned culture, it is problematic to try to bring engineering standards to alternative paradigms and technologies.  When a new paradigm challenges an established paradigm, then there is often deeply entrenched resistance to it.  It challenges assumptions the existing paradigm has asserted as fact.  It challenges the established power that has assimilated around the old paradigm.  One of the strategies utilized to suppress  a new paradigm is to disqualify and or discredit any and all reports of the benefits of the new paradigm, and create doubts around the reports of those reporting success.

One of the problems with trying to quantify the decrease in wood use is that many rocket stoves are not retrofitted into existing conventional buildings.  Another is the fact that in many jurisdictions in the USA they are illegal, and drawing the attention of regulatory agencies to one's own illegal activity is not something most people do.

Being as the gathering, cutting splitting, curing and storage of wood are all labor and resource intensive, then would all these satisfied users of RMH be claiming satisfaction out of contrariness or stupidity?  Isn't it possible that one measure of wood efficiency is their satisfaction itself!?  Surely if there were a way to heat with less wood, they would do it.  In part it is the desire to spend less time and resources on fuel wood that brings people to RMHs, and if there were no benefits to the use of RMHs to warm them through long and cold winters, they would not claim otherwise, thereby inviting Doubting Thomases to come make a mockery of them, highlight their ignorance and stupidity, and cast aspersions on their intelligence, their integrity, their honesty.  

There is an opportunity for an open minded member of the culture built around "established" beliefs related to conventionally produced modern wood stoves, to undertake a thorough investigation of the possibly unsubstantiatable claims of so many satisfied users of RMHs.  A curious person could seek more information, where a person with an interest in the established paradigm would only seek to discredit any new paradigm that challenges established theories and conventions.

If you were to start a new thread to compile first hand responses it might yield more information, and more usable information to query users satisfaction, and convenience of use.  And secondarily ask about levels of wood consumption, also collecting information about climate, type of construction and size of dwelling.  Also ask about how they utilize the conductive and radiant heat available, and are they able to use the RMH for any additional functions such as cooking, heating water, drying food or clothing, or functions you have not thought to query.  Also ask about the use of zoning in their homes, keeping some areas warmer than others, and because they are not relying on convection, and heating the air that is blown around the house, query rate of respiratory illness through the "cold and flu season" and the rate of accumulation of dust around the house, which is generally the impetus for cleaning activities such as dusting, sweeping and vacuuming.  And as you analyze your data, remember to subtract the utilization of other energy sources for such things as cooking, heating water, baking a pizza or bread in an attached oven, subtract energy if they vacuum less frequently, etc.

Maybe there are several new threads here, one about wood use, one about the many ways folks utilize their RMH, how it changed the way they live, what surprized them most when they began using their RMH.

paul wheaton wrote:I suspect that the house I am living in right now ....  that we use about a half cord of wood per winter.   I was just visiting with jocelyn and we have a small, rough book shelf that we fill with firewood.   We talked at length about how often we fill it ...   there were a lot of variables (we usually don't put anything on the bottom shelf ... a few times we brought wood in without filling the shelf) ...  but we think that we end up at about half a cord of wood per year.  

But, to be really certain about this, I would want to get a few more winters under my belt.  At this point, I am pretty confident that a half cord per winter is where we will probably end up.   The real test will be with a really cold winter.  

And I live in montana.

I've been in their house, and some of the time it was too hot for my comfort

R Ranson wrote:

So radiant heat makes you feel more comfortable at a lower air temperature - which uses less fuel/resources to heat an area?

That sounds like another reason why RMH would be more efficient than a wood stove.

Here is a little investigation I did into the radiant heat side of RMH's. https://permies.com/t/55993/rocket-stoves/Bell-hidden-advantage

I was compelled to investigate this side after living with my (first and so far only) RMH built I guess six months ago now. The most immediate difference we all noticed was the enveloping sense of warmth that was never present with the (hot water) radiators, which work by heating circulating air. One thing we all noticed was how warm our feet were in comparison. When the heat is delivered by heating the air, well your feet are always cold!

So any of the illustrations already given about how you can 'stand outside in the snow on a sunny day with only a t-shirt and feel warm' are very true, the radiant heat is what is warming you. Once the sun goes behind the cloud you suddenly feel cold again, the ambient air temperature at work.

I found that the temp of the radiant heat source has a lot to do with it. For sure, a hot metal body of a standard heater radiates heat, but perhaps NOT at the right temperature (tho to be honest I did not run those calcs, but a perusal of my 'thesis' will show what I mean). The mass in a RMH is no way the same temperature of the metal body of a standard heater, it is much lower....but far more in tune with our bodies. (see the data given in the link) The fact that that temp is much lower is then coupled with another point, the surface area of that blackbody radiant source. It is vastly larger than the surface area of the metal heater. That natural consequence of how the RMHs are built perfectly fits into how blackbodies radiate heat, as the temp of the radiating body is much lower, to get the amount of power into the room requires a much larger surface area.

It all somehow 'magically' all fits together! The metal heater has a small surface area at much higher temps (so the power radiated to the room is the 'same' in this argument), but we find the frequency of that higher temp is not correct to heat our bodies) and the surface area of the RMH is much larger but lower temp (ie same power to the room) yet falls exactly into the bandwidth that heats water (ie our bodies) most efficiently.

Again, quite magical.

When it is the air that is heated and then heats us, apart from 'hot air rises' (explaining the cold feet) it was always a mad scramble to 'shut that bloody door quickly!' because every time you opened the door, the warmth (ie the heated air) was lost immediately. It was 'just gone, lost totally' and we then had to heat all of that cold air that rushed into the room before you felt warm again. When a lot of the heating value comes from radiant heat (sure, of course the air is heated too but it is far from the primary heating mechanism) then not only is less heat lost from air exchange but the radiant warmth just carries on without a beat if the door is opened.

It IS the radiant heat that is giving warmth 'x' hours after the fire has gone out.

I also very strongly feel that the radiant side of how RMHs work has been overlooked a lot, and is far more important than previously suspected.

David Livingston wrote:Hi Lawence
I dont quite understand your argument .
I burn about six units of wood a year in my home here in France . I have a conventional home conventional stove  . If I get a RMH and I build it my self I use less wood than I do at the moment I still heat my house . May be a tenth  may be a quarter of what I burned before . The important word is LESS the rest is just detail .

David  

I don't doubt at all that an RMH would use less fuel. Of course my question lies in how much less but I'll get to that in a moment. I'm a fan of RMH, don't get me wrong. Heres how I see it:
1. Typical wood stoves suck.
    A. Dirty
    B. Not efficient
    C. Overrated efficiency numbers
    D. Some need expensive catalysts
    E. Reburn chambers are good but add complications to design
    F. Heat capture is too low
    G. Heat transfer is too low
    H. Expensive, comparatively
2. Rocket Mass Heaters are impressive
    A. Efficient
    B. Cost effective
    C. Less fuel needed
    D. Flexible design

Now back to my question. Amount of fuel needed.  So far we have the following pertinent points:
1. Estimates are fuel usage is 1/8 to 1/10 that of a conventional wood stove or as one example in this thread from 4 cords down to 1/2 cord.
2. Per paul there are thousands of these heaters built and they are consistent with the 1/2 cord claim

So heres my question, restated from another direction.
Does 1/2 cord of firewood contain enough btus to heat a house for an entire winter?

From what I have been able to determine:
Not adjusted for any particular climate, or insulation factor and assuming the heat duration is 3 months at full heating, 4 months of half heating, 4 months at 1/4 heating
1. A 1400 sq ft house takes approx 24,000 btu / hour,
2. Estimated usage: 3 months at full heating, 4 months of half heating, 4 months at 1/4 heating
Using the following calculations and 100% efficiency:
Months Days Hours/Day Hours
3        90 24         2160
4        120 12         1440
4        120 6          720
 Total        4320

Total BTU 103,680,000
BTU/Pound 10800
Total Pounds 9,600
3. Most common species of wood weigh between 3000 - 5000 pounds when green. Estimate 1000 - 3000 pounds when dry.

So to get the btus we need 9,600 pounds and dividing that by 2000 pounds average dry weight we get shy of 5 cords. Another way to look at it is 1/2 cord is 10,800,000 btu about 1/10 of the btu needed for a years heating.  So while I have no doubts whatsoever about the efficiency of the RMH i just don't see how you can get enough heat out of that amount of fuel regardless of how it is burned.

Lawrence
I think you may be missing my point .
I doubt not your math
Some people find they use an eighth of what they used previously BECAUSE they had a crap fire to begin with !
If they did not have a crap fire to begin with they they wont I dont expect to use an eighth less but I do expect to use a lot less .

David  

Lawrence, is it possible the baseline your working from is heating the air of the space in those calculations? RMH waste only a small portion of their energy heating gasses, most of it is radiated directly.

Then there's also the question of whether you're using the raw energy to raise heat values, or extrapolating needs based on wood stove knowledge.

How many BTUs do you need to heat a house with convective heat?   With radiant heat directed at the occupants?  With conductive heat and the occupants are touching the heat source?

How many BTUs do you need for an even mix of all three?  How about for every possible combination of all three?

Here is a 700 square foot house that is 100% electric baseboard heat.   You can probably calculate the BTUs for that.  But we added in some low wattage radiant and conductive heat.  That reduced the heat bill 87%.    So, Lawrence, how many BTUs were used here?





When I first saw a rocket mass heater, I asked why they don't get the information out more, or put some of this on youtube.  None of them were interested.   So I did it.  

"Why don't you guys have a comprehensive article?" - so I had to do that too.

"What if you made it all in a wood box?" - so I did that too.

there are now a lot of youtube videos that are showing the wrong stuff.  And there are too many bits and bobs.  We need a whole workshop on a DVD.  Me again.

And on, and on, and on....

So, lawrence, if you feel there is something missing, it sounds to me like you are volunteering to fill that gap that only you feel a need for filling.

Is this an accurate description of the issues at work when comparing RMHs to wood stoves and masonry stoves?

1) Combustion efficiency: what percent of the chemical energy in the fuel is converted to heat?  The claim is that RMHs combust more efficiently than wood stoves (and maybe more than at least some traditional masonry stoves?)  Evidence for this includes the formation of (energy-dense) creosote in chimneys using wood stoves but not RMHs.  If the waste products of combustion are flammable, then the combustion wasn't as efficient as it could be.

2) Heat retention efficiency: what percent of the heat stays in the house?  If the exhaust gases are hot, that heat is lost.  RMHs apparently have relatively cool exhaust.  The air sealing and insulation of the building, and the temperature gradient between the inside and outside also matter, of course, but those don't depend on the stove type.

3) Type of heating: RMHs (and masonry stoves) heat mostly by radiation, and wood stoves by a mix of radiation and convection.  Less energy seems to be needed to heat people by radiation than by convection.

3a) Convection heating tends to lead to dry air, radiative heating does not.  Dry air is unpleasant and probably not great for health.

4) Temperature of radiating surfaces: there is likely an optimal temperature for radiating surfaces to get them to produce enough heat to keep people warm but not scorch them.  RMHs and masonry stoves seem to get closer to this ideal.

All of the above relate to overall heating efficiency: how much wood does it take to make people comfortable.

The next factors have to do with which heating technology is appropriate for your particular space:

5) Suitability of radiation vs. convection for heating a given space: if you have a one-story, open-layout space, a radiating source in the middle works well.  If you have a multi-story dwelling and/or one where there are walls between the stove and some of the living spaces, you might need to resort to convective heating.

6) Mass and structure: if the structure of your building does not support a massive stove in the living room, you might not be able to use an RMH or masonry stove.

7) Construction cost: RMHs seem to generally be the cheapest option.

8) DIY repair: RMHs are best here, followed by masonry stoves, followed by iron stoves (especially those with exotic catalyst materials).

9) Durability: RMHs are new enough it's hard to know.  Masonry stoves probably win on this one, based on the centuries of use in northern Europe.

10) Safety: Any of them can be safe or unsafe depending on how they are built, installed, and maintained.  RMHs have less history behind them so a direct comparison is difficult.

11) Complexity: RMHs win here.  Masonry stoves are tricky to get right, by many accounts.

12) Building code and resale: RMHs are not well known so the building inspector (and potential buyers) may balk.  I suspect this is true to a lesser degree for masonry stoves.

David Livingston wrote:Lawrence
I think you may be missing my point .
I doubt not your math
Some people find they use an eighth of what they used previously BECAUSE they had a crap fire to begin with !
If they did not have a crap fire to begin with they they wont I dont expect to use an eighth less but I do expect to use a lot less .

David  

Actually your point is the one I think I get:-) and actually I quite agree. I think there are a lot of really inefficient wood stoves out there that are going through fuel like mad and have a lot of heat going right up the chimney.  I'm also wondering about how many are backing their stoves with some other kind of heat like electric etc. With that situation they would easily be using less wood fuel and not making for good comparisons.

Kyrt Ryder wrote:Lawrence, is it possible the baseline your working from is heating the air of the space in those calculations? RMH waste only a small portion of their energy heating gasses, most of it is radiated directly.

Then there's also the question of whether you're using the raw energy to raise heat values, or extrapolating needs based on wood stove knowledge.

Actually I'm using industry HVAC heating values so this would be forced air heat.  Really I think this is ultimately where the disconnect is. So I've read Paul's comments, looked at the video, did some other research and what i've come up with is the following:
1. RMH are used differently. More on this in a moment.
2. Comparing RMH to wood stoves, oil heat, electric (in other words radiant/convection/conduction) is pointless
3. Adopting an RMH system is a change of lifestyle not just switching out your heater

So what we have is a situation where if you adopt an RMH that you are also buying into changing how you think about heat and how you are willing to change your lifestyle to adapt to those changes. You have to redefine what you consider to be heating your home. With forced air heat you heat the air and move it around. You as a person live within that heated air which has a relative degree of uniformity.  With an RMH you have a mass of heated material that slowly radiates the heat into the space in a much more localized manner. I don't doubt that you can blow a fan across it to move the air around and heat things more uniformly.  If you are one who likes a uniform heat throughout your house I doubt an RMH would fit your desires. if however you can adapt to having heat in one or two localized places then this is a potentially a good option. Note in the video paul posted the heat is extremely localized and the room is cold. The subject feels fine. She had to wait a bit for her hands to warm up on top etc but overall she felt good  What you notice is the positioning of the RMH in locations to place them close to where you will be so you will be heated.

I compare RMHs to a high efficiency boiler like a Garn (www.garn.com) that is also highly efficient but considerably more flexible since you can put radiators all over and or have radiant floor heating. They have their downsides for sure since a Garn or similar unit is considerable more expensive, requires electricity and could leak like any other water based unit. Now I spent a few decades in Cascade county MT. It was cold as hell at times but on the bright side it was low humidity. Today, as a resident of Western WA I have to deal with high humidity. As a result I have had a few instances where lack of air circulation has allowed cold wet spots to promote the growth of mold. In my quest to research RMH heaters for deployment in a workshop, house and possibly a chicken coup i have concluded that i need to account for localized heat and while I would love to use as little heat as possible like the video demonstrated, that would be insanity in this humidity. I had originally thought that all I needed was dehumidifiers to lower that humidity but then I learned about dead spots and had to rip out some moldy drywall and carpet. Ironically the air drying effect of a standard wood stove is quite helpful in my situation.  At present the only combination that is effective is some heat, dehumidifiers and air circulation. Unfortunately with the layout of my current house the locations of the RMH compared to where I would need to have heat are incompatible. As such a hybrid scenario can be employed where the RMH would heat the main area and some more localized heating for other areas.

In my original question I wondered how 1/2 cord of wood would be enough to heat a structure. it makes sense when you consider you are not heating the air as a forced air unit does. You are not heating the house per se you are heating those areas where you are living and from what I can tell, to a lesser extent. In my case I doubt I could get by with so little fuel but i think I can reduce the consumption considerably.

I think you are getting RMH figured out.  For folks who want the air in the whole house heated, who are used to "central heating" and the convenience of it, then RMH requires w paradigm shift.  I was raised by people who thought a 70 degree house was stifling, who are aghast at the way houses are sealed up with tyvek so there is no air communication with the outdoors.  So the RMH was no paradigm shift for me.

The humidity where you are sure sounds like a challenge.  Some people do duct the heat to other parts of the house, or get heat circulation throughout by natural convection, floor vents, and cool air return ducts.  One thing I have seen is a duct that draws up (very slow moving fan in a duct) the very coldest air in the house and delivers it to the ceiling above the heat source, a conventional wood stove.   Just that one cool air return made a huge difference in the whole house.

It has been mentioned that RMH is best suited to an "open" floor plan.  With the humidity challenges you face, perhaps RMH is not the best option for your situation.  But, possibly there will be some discussion now about dead spots and mold and such.

Anyway, good luck with your heating dilemma.  As thorough as you are in your information gathering, you are sure to come up with viable options, and most likely choose the one that will fit your situation best..