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

paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15631
Location: missoula, montana (zone 4)
    ∞
When i talk about rocket mass heaters, the most common question is that how can it use five to ten times less wood to heat a home, if the existing wood stove is 75% efficient.

And by "question" I mean that there is usually a long list of my obvious mental deficiency and how I must be selling snake oil. 

To me the answer is obvious:  smoke leaving the house of a 75% efficient stove is really hot.  Usually 300 to 600 degrees.  It has to be to get the smoke out.   Exhaust leaving a well built rocket mass heater is leaving the house at 70 to 100 degrees.   

That's it.   

Am I overlooking anything?

[edit - changed "90%" to "75%"]




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Joined: Nov 20, 2010
Posts: 140
I don't see how the calculation that arrived at the 90% efficiency figure could possibly be correct.  I believe that smoke from an efficient stove will be much cooler that that.  Creosote condenses below 240 degrees and efficient stoves tend to accumulate lots of creosote.
Joel Hollingsworth
volunteer

Joined: Jul 01, 2009
Posts: 2103
Location: Oakland, CA
Same here: I'd like to see what defines 100%.

Also:

A stove is there for the comfort of the person using it. You can't comfortably sit on a woodstove, because the temperature is too high. This reduces the efficiency of heat transfer from the stove to the person. This means a RMH-heated home can remain comfortable at a lower temperature.

Because the temperature of a typical surface is lower, and the heating system is not designed to encourage radiation or convection, the home's insulation can function more efficiently: much more of the heat that is generated, can stay in the home.


"the qualities of these bacteria, like the heat of the sun, electricity, or the qualities of metals, are part of the storehouse of knowledge of all men.  They are manifestations of the laws of nature, free to all men and reserved exclusively to none." SCOTUS, Funk Bros. Seed Co. v. Kale Inoculant Co.
Steve Evans


Joined: Nov 26, 2010
Posts: 8
Location: South Gloucestershire, UK
paul wheaton wrote:

To me the answer is obvious:  smoke leaving the house of a 90% efficient stove is really hot.  Usually 300 to 600 degrees.  It has to be to get the smoke out.   Exhaust leaving a well built rocket mass heater is leaving the house at 70 to 100 degrees.   

That's it.   

Am I overlooking anything?


Hi Paul,

pretty new around the forums but I think you got it.

My short answer...
90% stove: good at burning but poor at utilizing heat => burn a lot.
RMH: good at burning and good at utilizing heat => burn less.

My long answer...
I suspect the 90% figure is talking about how efficient the stove is at converting the theoretical max energy in the wood into heat in the combustion chamber. The stove will probably then dump most of that heat outside the target heating space via the exhaust unlike RMHs which recover a large fraction of the heat from the exhaust.

BTW, there's an interesting link to a low-mass rocket heater project at http://www.iwilltry.org/b/build-a-rocket-stove-for-home-heating/ in case you've not seen it before.

Thank you for running a great forum!
                    


Joined: Oct 23, 2011
Posts: 0
according to : http://www.stovesonline.co.uk/stove-efficiency.html

"The efficiency of a stove tells you how much of the energy in a fuel will be given to the room when you use the stove."

This is a listing of stove effciencies as determiend by the EPA:

http://thelograck.com/wood-stove-manufacturers.html

note that the mid 70 percentile is the average max. I skimmed quickly and saw no "80%" or better ratings.

energy in a fuel is rated in BTU's; this is the ammount of energy it takes to raise one lb. of H20 by 1f.

So, an 75% efficient stove, given wood that combusts to 20k BTU's in 1 hour (@35# of alder, which is quite a bit in one hour!) would put 15kbtu into the room while 5k went up the chimney.

what does this mean for temperature in a room?

1) It takes .018btu to raise one cubic foot of air 1 degree Fahrenheit.

1000 cf= 18btu=+1f.

a 1000sf home will have, before furnishings, about 8500cf volume. after counting for simple furniture, maybe 7000cf. So to raise an IDEAL home of this size by 10 degrees- from 60 to 70, one would need 1260btu's.

why does the stove produce 20k or more BTUS an hour? if thats the standard, the room just went from 60 to 220f+ in one hour!
rough math time: 20kbtu/7000cf=2.95btu/cf: .018btu/'Air'cf= +1f; so @160f (.o18btux160f/1lb=@2.9+ per cf in 7000cf of air!

the stove is 75% efficient, so only 15k of those BTU's got in. phew, were down were down to 180f.  we still have a few moments to think this through before we die of heat exhaustion.

we arent just heating air. were heating all the object in the house.  1500cf plus the walls and ceiling. with higher mass and therefor BTU input requirements to raise thier temp 1f per 1#, you can see how furniture can add up. especially metal and glass furniture which is highly conductive. More important, the draftier the space is the more unheated air is sucked in as hot air leaks out, also stealing BTU's.

In fact,  in most homes a 75% stove heats at 6-7% its design model potentials. Even in these fuilly wrapped homes for bubble people, a 12-15 house efficiency is probably the best they can hope for. See, the fewer air circuits a day, the worse air quality gets. sealed homes must fliter and pump HVAC air in and out- and in the winter that means warm air through Out the Pipes.

it takes .22 btus to heat 1#dry clay by 1f. sooo... a 5000# bench (a single bed size, just about) would require 11000 btus to heat from 60f to 70f (10 degrees). so about 18# of wood in an hour. in ideal heating conditions. lets use the 75% effciency again, and say we need to get 24# of wood to ramp that baby up. Thats already just 30% less than the conventional unit. the difference? The heat lasts longer. Its held in the space by the mass. It doesnt leave with the drafts or the HVAC.

while this math is off the cuff stuff, And roughshod, Id wager a RHM with 50-70kBtu's/day under a well insulated roof strawbale walls  could easily run 50%-70% more effcient than a conventional stove rated at 30-40k Btu and hour (100-160kbtus/day) in the same construction.

Dont trust my cafe napkin analysis. do some. Been along time since I did any of that math. I dont reckon I should be abraided if people make it clearer

Len Ovens
pollinator

Joined: Aug 26, 2010
Posts: 1315
Location: Vancouver Island
    
  18
do you mean like gas water heaters? They may be 90% efficient at getting the heat from the gas flame to the water, but 100% of the time 10% of the heat is going out the pipe so the gas has to come on more often to keep the water at temp. The question here (with the stove heater) is what efficiency are we talking about? Under what conditions? the 90% measured is with the wood in the stove burning quickly (no one ever uses it this way because it makes too much heat) But what is that efficiency? It could be 90% of the heat produced makes it to the room, it could be 90% of the heat available in the wood is converted to heat... but not both. Yet, if either one is true, the manufacture can claim 90% efficiency and not be lying.... though still dishonest in my opinion.

If you have a stove that in the lab converts 90% of the available fuel energy to heat, but 50% of that goes up the flue.... you have 45% really in the lab.... and less in the home where you smolder the fire to keep the room comfortable..... maybe a whole lot less depending on how lazy the owner is... the bigger the fuel load is compared to the needed heat at the moment the worse it gets.... 90% heater 20 something% in real use..... blah blah blah.... Anyone who has done tests with a well run masonry heater is happy with a total efficiency over about 70% (this is a mass heater with exhaust temp around 200C)

With the RMH (or a masonry heater in general for that matter) efficiency is about how much of the heat available in the fuel makes it to the room under normal operating conditions. 90% would be incredibly good. With the RMH the "flue" temp can be lower for a horizontal exhaust and so some where in the 80s might be possible.
                          


Joined: Mar 13, 2010
Posts: 94
Location: Colorado
paul wheaton wrote:
When i talk about rocket mass heaters, the most common question is that how can it use five to ten times less wood to heat a home, if the existing wood stove is 90% efficient.

And by "question" I mean that there is usually a long list of my obvious mental deficiency and how I must be selling snake oil. 

To me the answer is obvious:  smoke leaving the house of a 90% efficient stove is really hot.  Usually 300 to 600 degrees.  It has to be to get the smoke out.   Exhaust leaving a well built rocket mass heater is leaving the house at 70 to 100 degrees.   

That's it.   

Am I overlooking anything?



I do not know what kind of wood stove you use, but but my stove pipe going to the chimeny is only about 150 F, or less,

I have the wood stove, (a down draft unit made in 1980), with a stove pipe oven on it and a magic heat, and then about 3 foot to the chmmeny, and it draws fine,  it has been used in two diffnret houses, and has worked well in both of them,



                            


Joined: Jun 12, 2010
Posts: 13
I'd also say that it burns the wood more completely.  When you have a wood stove you have a lot of ash left over and smoke.
                          


Joined: Jan 17, 2010
Posts: 31
a 1000sf home will have, before furnishings, about 8500cf volume. after counting for simple furniture, maybe 7000cf. So to raise an IDEAL home of this size by 10 degrees- from 60 to 70, one would need 1260btu's.

why does the stove produce 20k or more BTUS an hour? if thats the standard, the room just went from 60 to 220f+ in one hour!


This would be the actual case if your home was surrounded by a gigantic thermos bottle which stopped ALL external heat losses.  In the real world, homes lose heat to the outside world like a sieve.  Granted that installing higher R rated wall and ceiling insulation, higher R rated windows and doors etc. can reduce the 'mesh size' of that sieve from large to small.  But in the grand scheme of things, external heat losses for real world homes are still on the order of magnitude of a sieve !

In it's simplest form, the stove heat input versus external heat loss equation is a variant of the ever famous 'filling the bathtub while the stopper is removed from the drain' equation.  Unfortunately, there are lots of additional variables such as the day to day 'height of the tub' ( temperature differential from inside of home to outside ) changing.

In the final analysis, overall wood stove efficiency is a combination of efficient combustion ( i.e. being hot enough to create secondary combustion of still flammable gases emitted by primary combustion ), and efficient transfer of the resulting heat to the home ( as opposed to much of it going up the stack in the form of very high temperature exhaust gases ).

With most stove designs, the two tend to be mutually exclusive.  However, with the rocket stove design, it attempts to achieve the best of both worlds ... relying in huge thermal mass to prevent sustained high temperature combustion of the wood plus primary combustion gas byproducts from driving the surrounding temperature to uncomfortably high levels, plus relying on extensive stack heat transfer to reduce the temperature of the exhaust gases as much as is practical before they are released.

In terms of operational efficiency, every wood stove operates very inefficiently when it is first lit and coming up to temperature, as well as when it is 'throttled back' to the point where secondary combustion can no longer occur ( i.e. reduced stove temperature allowing incompletely combusted creosote vapors up the stack instead of burning them completely before they exit the stove. 

Some Canadian gov't research on various wood stoves tends to show that wood stove efficiency in the absence of secondary combustion tops out around the 55% mark ... which rises to 70% or so when secondary combustion is taking place.  "Trick' wood stoves such as the rocket heater, soapstone stoves, 'high tech' combustion control stoves etc. can allow the efficiency to rise to 80% ... with the caveat that these high efficiencies are only achievable once the stove's combustion chamber has reached optimal operating temperature. 

Thus actually being able to take advantage of 70% to 80% combustion efficiency means that the stove needs to be sized and selected such that the BTU output capacity versus the home's need for BTU's are fairly well matched ... so that once lit the stove can continue to operate at optimal combustion temperature for a sustained period of time.  This is less of a problem with a rocket stove than with commercial product stoves since everything is 'scalable' by the builder. 

Personally, I went another route ... mostly so that I could do some serious cooking as well as heating.



However, one mistake I made was failing to order the ( now available ) air to water heat exchanger option ... which if installed would allow me to capture even more combustion heat for a useful purpose ( hot water supplement ) before having to throttle back or shut off the stove. 
Len Ovens
pollinator

Joined: Aug 26, 2010
Posts: 1315
Location: Vancouver Island
    
  18
Birdman wrote:

I do not know what kind of wood stove you use, but but my stove pipe going to the chimeny is only about 150 F, or less,


Ok, but what is the gas temp? Like in the center of the pipe? The gas moves much slower close to the metal of the pipe and so cools much faster than the fast moving gas in the center.
                          


Joined: Mar 13, 2010
Posts: 94
Location: Colorado
I would think that after going through the Magic heat, and so froth the heat would be reasonable even in the pipe, 
when I lived in MT, the chimney had Icicles hanging from the cover on it most all winter,
I know it has to be reasonably cool,

and I do not know why there seems to difficulty in thinking that a low temperature chimney would draft, as gas appliances have hoods that pull in fresh air, on them that lower the temperature to not much more than 100F, 

even the rocket heater, it draws even tho it has a low temperature flue,

one of the keys to efficiency is to remove the heat into the living area,  and not let it go up the chimney.

for a standard wood stove the magic heat is a good item, in some cases it can extract double the heat that the stove would produce by it self,
                            


Joined: Dec 15, 2010
Posts: 105
paul wheaton wrote:   

Am I overlooking anything?



Yes, the first law of thermodynamics, energy neither created or destroyed.

When is a lie not a lie and when is it actually a lie? Thank the lawyers and our courts and the perpetual redefining of words in our world.

It is a well established fact, the heat to drive the flue on a natural draft system requires energy, to keep the flue above the dew point REQUIRES heat energy in the flue, The established MINIMUM required energy is either 14 or 16% I do not recall, I would have to look it up. So what this means is simply that the maximum possible heat output of the stove is either 84 or 86% of the input. Since that is the established maximum the manufactures just use it as the 100% mark so if a stove has a performance actual efficiency of 77.4% that would indeed be 90% of the established maximum.

In a bit simpler terms, since the flue losses are so to speak equal across the same "type" of design (natural draft) they are indeed allowed not to include flue losses as they would indeed be the same across all within the same type.


Professor of Thermal and Electrical Engineering, Welding/metallurgy: Licenses: PE license, Mechanical license Variety of other "certifications" from industry groups such as Refrigeration Service Engineers Society http://www.rses.org/, ASHRE http://www.ashrae.org/ Ect.
Joel Hollingsworth
volunteer

Joined: Jul 01, 2009
Posts: 2103
Location: Oakland, CA
NedReck wrote:
Yes, the first law of thermodynamics, energy neither created or destroyed.

When is a lie not a lie and when is it actually a lie? Thank the lawyers and our courts and the perpetual redefining of words in our world.

It is a well established fact, the heat to drive the flue on a natural draft system requires energy, to keep the flue above the dew point REQUIRES heat energy in the flue, The established MINIMUM required energy is either 14 or 16% I do not recall, I would have to look it up. So what this means is simply that the maximum possible heat output of the stove is either 84 or 86% of the input. Since that is the established maximum the manufactures just use it as the 100% mark so if a stove has a performance actual efficiency of 77.4% that would indeed be 90% of the established maximum.

In a bit simpler terms, since the flue losses are so to speak equal across the same "type" of design (natural draft) they are indeed allowed not to include flue losses as they would indeed be the same across all within the same type.


So good to know!

So it seems that two major differences in the design of an RMH vs. a wood stove are that:

1) the (convective) heat engine driving air through the system does not use exterior air to reject heat, but, rather, a fixed thermal mass that remains above room temperature, but significantly below the temperature of the flame, and

2) (per Erica's comments on the under-floor yurt heater) RMH exhaust gasses are allowed to cool below the dew point

The comment about heat driving the natural draft system seemed to snap everything into focus.

Another important point I forgot to mention earlier: An RMH remains warm between firings. Efficiency is measured as the stove is running well, not integrated over the smoky start-up time plus the main burn time. Smoke not only is a form of wasted fuel, it also encourages people to open up airflow a lot more than they otherwise would, and represents a hassle that might encourage people to keep things running when they don't need the heat. In that case, even if a stove is efficient under ideal conditions, it might drive actual patterns of use that consume more fuel than one that stays warm and easy to start over most of a season of use.
                            


Joined: Dec 01, 2010
Posts: 158
Location: Abilene, KS
Sorry for the off topic question here, but Melonie, what kind of stove do you have pictured there?


Be kinder than necessary, for everyone you meet is fighting some kind of battle.
                      


Joined: Nov 30, 2010
Posts: 53
well 90% effeciant at WHAT? burning the fuel which is what stoves are rated or harvesting the heat as rockets are referenced . given the volatiles are burnt off at 1000-1300 F if the stove achieves that (or 90% of that) its effeciant even if all the heat made is shot up a chimney?  on the other hand if a rocket achieves 1300 burn temp and the exit temp is ~130 not only did we burn it efficiently we harvested the energy at 90% which is the true test of efficancy.
not only burning it 90% well but burning it 100% well extracting 90% the resulting energy
                            


Joined: Dec 15, 2010
Posts: 105
sticky_burr wrote:
well 90% effeciant at WHAT? burning the fuel which is what stoves are rated or harvesting the heat as rockets are referenced . given the volatiles are burnt off at 1000-1300 F if the stove achieves that (or 90% of that) its effeciant even if all the heat made is shot up a chimney?  on the other hand if a rocket achieves 1300 burn temp and the exit temp is ~130 not only did we burn it efficiently we harvested the energy at 90% which is the true test of efficancy.
not only burning it 90% well but burning it 100% well extracting 90% the resulting energy


There is a LOT of misinformation surrounding the performance of RMH units. The desire to have such a grand success has blinded many to the hazards of achieving that goal.

There is two really cool processes taking place inside a RMH that give it a great deal of potential to perform to an outstanding level. I have spend some time studying the design and can almost without any doubt confirm those two processes and am highly suspicious that a third event may be taking place. I can test the first two and gather a lot of data to confirm them and am planning out some testing trials this week. The third one I will have to consult with another qualified person in another discipline of the sciences.

Too many distortions and claims which are without any doubt are false exist about RMH units, however, they may very well be one of the better uses of wood burning energies. My motivation is to identify the operational parameters and clearly define what can be expected so ideas can expand in a safe manner and improve these heaters to a point in which fairly reliable and safe units can be field built in a variety of sizes by persons of moderate skill.

There is a great deal of potential with these units and I hope to see it come to fruition for one and all.
Len Ovens
pollinator

Joined: Aug 26, 2010
Posts: 1315
Location: Vancouver Island
    
  18
NedReck wrote:
There is a LOT of misinformation surrounding the performance of RMH units. The desire to have such a grand success has blinded many to the hazards of achieving that goal.



Who are you and what makes you authoritative in this field? The tone of your post was very innuendo-ish with no substance at all. Not the kind of post I would expect from someone with training in this field. (or any other field) The normal course is experiment and measure first (do your research first) and then bring forth the data gained. Instead, you have poisoned your data and research by saying... there's something wrong here and I'm gonna find it.

At least thats the way it hits me.

If there is " a LOT of misinformation surrounding the performance of RMH units." Start by at least listing one with some backup.
Burra Maluca
Mother Tree

Joined: Apr 03, 2010
Posts: 5253
Location: Portugal Zone 9 Mediterranean Climate
    
208
NedReck is doing the testing at the moment - he may sound a bit authoritative, but he is actually trying to figure stuff out and keeps coming back here to try to help. 

Please forgive his tone - here's a link to one of his threads on another forum where, hopefully, he'll keep us all informed about both his experiments and results.

http://donkey32.proboards.com/index.cgi?action=display&board=discuss&thread=282&page=1


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Joined: Nov 30, 2010
Posts: 53
i was wondering what was claimed and i got 93% for the 5000$ wood stoves cataylic has to be checked 3 times a cold season so red is the expert as stated what does the 93% represent the thermal extraction or the burn? if its the burn only and of the ~1300 degree heat we are shooting 400-600 ( i seem to recall you saying it needs to be 400 degrees for draft or something to the point) degree heat up out the flue i dont think thats at all 93% seems more like ~30-50% thats why i am more inclined to think that a rocket mass maybe around 90% if the 1300 degree flame pputs out ~130 degree water vapor etc thats 90%

so i ask what does both sides define as 90% ?
Len Ovens
pollinator

Joined: Aug 26, 2010
Posts: 1315
Location: Vancouver Island
    
  18
Burra Maluca wrote:
NedReck is doing the testing at the moment - he may sound a bit authoritative, but he is actually trying to figure stuff out and keeps coming back here to try to help. 

Please forgive his tone - here's a link to one of his threads on another forum where, hopefully, he'll keep us all informed about both his experiments and results.

http://donkey32.proboards.com/index.cgi?action=display&board=discuss&thread=282&page=1


Ok, I accept that. I understand the part of his post about coming across as being course (actually I don't think that is the word he used, just my paraphrase). I've been there.... try relating to some other cultures enough and it is pretty hard to avoid.... I tend to be blunt, my wife seems to dance around the bushes and never say what she means, but expect the hearer to understand anyway.
                            


Joined: Dec 15, 2010
Posts: 105
Len wrote:
Who are you and what makes you authoritative in this field?


A man with more than 30 years experience with a vast variety of heat transfer equipment from very experimental to the common wood furnace. A man who is well aware that more than co2 and water vapor comes out of the flue, a man who knows there is a lot of stuff contained in wood that 1300-1500 degrees will indeed not destroy. A man who knows that some of the design limitations of this stove can be overcome, the parameters of operation just need to be found. A man who seeks to share information with a bunch of good folks looking to do things their own way, a way that has as little impact on others as possible. A man with a bunch of licenses and other pieces of paper hanging on the wall who just kind of thinks this whole idea is a huge experiment that is fantastic for his students to explore and learn from in contrast to only mainstream technologies.

Who am I? just a man whom despite the first rule of these forums is far far from perfect.

You are quite welcome to like or dislike me of your own free will, I do not seek to influence that either way and if you can take that totally to heart, then you understand that I mean no offense nor care beyond this post if you chose to become offended.

Ned
                            


Joined: Dec 15, 2010
Posts: 105
sticky_burr wrote:
i was wondering what was claimed and i got 93% for the 5000$ wood stoves cataylic has to be checked 3 times a cold season so red is the expert as stated what does the 93% represent the thermal extraction or the burn?


Cat systems are very effective at destroying certain components of combustion gases. I have little doubt that they just as the non-cat stoves do subtract the heat energies required for lift at the flue to develop the 100% mark since they all have to do it.

It is also quite possible the 93% reflects 93% of the CO is burned off through the cat system.

I would have to research it a bit to see what the EPA and DOE regs are on rating them, but one thing I can assure you of, that 93% will be a huge stretch of the truth compared to an input-output true ratio.

Never forget it is all marketing hype.
Max Kennedy


Joined: Feb 16, 2010
Posts: 462
Location: Kirkland Lake, Ontario, Canada
This has been touched on but I don't see it explicitly stated so here goes.  There are 2 common measures of efficiency when one sees literature relating to wood stoves.  The 1st is burn efficiency.  This is defined as the percentage of the fuel completely combusted to CO2 and H2O.  Smoke, soot, creosote, tars, CO etc are incomplete combustion products and represent losses in burn efficiency.  This of course is a useless number except it is a good indicator how clean the system burns.  The other efficiency, and the one we need to focus on is delivered heat efficiency.  This is the % of heat energy in the wood that stays in the home.  The best rough measure of this is the temperature of the exhaust as it exits the home.  The lower the temperature the better the delivered efficiency.  Don't mistake this as the temperature the exhaust exits the chimney!  For example there can be quite a temperature difference between the bottom and top of an external chimney.  The temperature of concern for delivered efficiency is the one at the bottom of an external chimney as any heat losses above that don't heat your home.  Thus 2 stoves having the same burn efficiency may burn a lot different amounts of wood depending on how well heat is retained in the house vs lost to the outside.  This assumes the 2 houses in question have the same heating needs and the same losses due to thermal breaks, windows, etc..  Hope this helps the discussion.

Max


It can be done!
paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15631
Location: missoula, montana (zone 4)
    ∞
Well,

If nobody objects, I'm gonna leave the posts as is.  Mostly because I am lazy and it seems that things have been smoothed out later.

Back on topic:  It seems we have somebody that is hooking up a few dozen guages, dials and measuring gear to give us some rather exact figures on efficiency on rocket mass heaters.  Plus, some exact info on how they compare to more conventional woodstoves. 

As far as lots of misinformation, I would like to better understand this.  And at first pass, it would seem that there is a great big world of numbers that I would first need to get my head wrapped around.  Since I am lazy, I am going to make a feeble attempt at dodging all that my taking a different tack:

Suppose there is a 20x20 cabin occupied for the last 25 years by "Bob".  Bob installed a really nice wood stove, brand new 25 years ago.  Five years ago, Bob put in a RMH.  Bob claims that he used ten cords of wood every year for the first 20 years and a little more than a cord every year for the last five years.  (granted, I'm making this up - based on many other stories I've been told)

I kinda think that this is the core of the question.  When Bob felt cold, he burned wood to feel warm again.

Is this part of the "misinformation"?  I suspect that this is not part of the misinformation.  I suspect that "misinformation" is within my feeble attempt to explain how it happened.

True?
                          


Joined: Mar 13, 2010
Posts: 94
Location: Colorado
IMO

that is the only way one can justify the claim you first started with,

you would have to "TEST" the two different stoves in the same given set of parameters,

with out the parameters changing,  and the basic fuel (wood) the same in both.

one can do a base calculation on estimated heat loss by measuring the building and it components and the R values,  and one could weigh the wood and estimate the BTU it has, and come up with some type a comparison as to efficiency that way as well,

the only efficiency rating that is important is how much heat are you getting in the house, from the wood burnt, 


paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15631
Location: missoula, montana (zone 4)
    ∞
I present "exibit A":



So, we have the reports of the people using less wood to heat the same structure. 

I think it is possible for the room to be colder and the people inside feel just as warm.  This could be due to more radiant or conductive heat instead of convective heat.



tel jetson
steward

Joined: May 17, 2007
Posts: 3112
Location: woodland, washington
    
  58
first problem I see is ambiguous language.  what exactly does "eight times less" mean?  sort of like "eight times as cold, " which is also fairly confusing, if not meaningless.  "eight times as much" is pretty easy to understand.  probably, folks mean one-eighth when they're making this "eight times less" claim.  may sound like picking nits to some, but it seems important to me.

the different things being described by "efficiency" is also a pretty big problem that others have delved into.  that word seems to be very prone to abuse.  or maybe our culture is very prone to abuse it.

at the heart of it, these claims are marketing.  most marketing is intended to sell products and amass riches.  this particular marketing has a rather worthier goal, but that doesn't change the fact that there have been exaggerations and hyperbole thrown around in the interest of attaining that worthy goal.  and, just to be clear, the goal that I believe is inspiring these marketing tactics is: reduce pollution, reduce consumption, and increase comfort in the context of heating with wood.


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Joined: Dec 15, 2010
Posts: 105
Paul,

Thank you for the flexibility to the hazards of dealing with me and understanding that I mean no one any disrespect. I will try and explain this as best I can and certainly mean no one any offense at all, not even a little. To best prove that point, people whom I do not know have no impact on me at all so I certainly have no reason to insult or offend one I do not know. My circle of influence is small and those whom know me look past my shortcomings and I look past theirs, everything is real mechanical in my world and none of it makes me "feel" in any manner.

Efficiency is a relative term and we are talking apples and oranges 100% when it is being discussed. A stove manufacture must prove his claim to a degree, however nothing says that claim must be represented for what it is in the marketing of his product. A cat furnace, well I could say it is 93% efficient, but at WHAT? Perhaps 93% of the VOC's are destroyed, 93% efficient at delivering usable heat to the space, burning 10 lbs of wood down to .7 lbs of ash etc.

Now add the human factor! I can adjust the dampers on this cat stove so that I maintain the operation of the stove where the flue gases leaving are right at the minimum standards and if I do so then the performance of the stove would be X. So if the flue standard is 15% (I know it is 14 or 16 I just split it) then if I work and tune the stove to that point I could say it is 100% efficient as the 15% does not count it is used for vertical lift for exhaust.

Doing this would require me to use some fairly expensive instrumentation and make several adjustments to balance the burn rate and the draft, but once I achieved that goal, wall ah! I CAN PROVE MY CLAIM of 100% efficient. So in the lab I have done this, some what of an expert trained professional with a lot of expensive toys, I can indeed make that rascal hum, IN A LAB.

Now your friend buys one of my stoves, loads it up with wood and fires it off. Does he have the ability, training, and expensive toys to tune it? Even if he does, the temperature changes in the room affect the performance of the stove and each degree of change alter the air density and retuning the stove would be required to maintain the same performance level because less oxygen would be delivered by the same CFM of air due to the reduced density. So in other words, to certify the stove at X efficiency I have to be able to prove in a controlled lab the performance claimed. That means very little if not nothing to the actual real world performance Joe Blow, toss the wood in, set the damper half way on intake and 3/4 on outlet is going to get!

Now bring the RMH into play. I had about an hour long conversation with Donkey (I have no permissions beyond that, but I think you may be familiar with the person, if not PM and I will confirm) and through that conversation I am at a 99.999% sure handle on two of the principles these bad boys run on. I can not effectively explain it here due to my own issues and how I know I will respond to questions or criticisms by persons whom do not state their point technically correct enough. I accept full responsibility for that and can only try and help those whom do not understand get a better picture by pointing out one of those issues has been classified and labeled as Asperger's Syndrome, if you seek a better view than I can give, http://en.wikipedia.org/wiki/Asperger_syndrome

Folks say X about a RMH system, if X is not 100% accurate I have a great deal of hardship in not saying that it is not true, I can not get past that even when X and what they said from a practical view is fine, I just am obsessive about such things. As an Aspie there are advantages and disadvantages, its fine in the sciences but it fails miserably in the grand social event a vast majority of folks live in and while I fully grasp your thoughts on maintaining continuity and peace on these boards, my ability not to offend is drastically hindered by the simple fact it is nearly impossible to offend me if I do not know you and you have developed great meaning in my life.

With all that said back on topic here, RMH. When other stoves are given these lets call them "fudge factors" where they can eliminate the 15%, the RMH does not need that so to speak. Please give me some leeway on this until I can confirm through experiments certain things about the operation. A LOT about thermodynamics is counter intuitive and I do not have the time to explain it all here, but to a large degree flue temp and efficiency even on the input-output scale has little to do with performance.

Operation of these units have some rules surrounding them and I can without any doubt say that most if not all folks involved with them are not in tune with what is happening or at least I should say I have not found any information that indicates anyone talking openly on the internet about these has a sound understanding of the concept AND the implications that would dictate the standards by which one could prove safe operation by prescribed regulations of the Uniform Mechanical Code, the International Mechanical code, or the Fuel Gas Code.

It is very hard for me to express the "differentness" of the RMH from the systems that the above codes were developed for safe operation. I simply will have to get a greater understanding of the "why" behind them. Even with that knowledge, the folks trying to get one approved, Eric and Erica if I recall correctly would get an automatic disapproval from myself. Now this does not mean by any shape or form the unit is not effective or safe, but it does mean that even once the operations are fully understood and explainable, there are deficiencies that would not be code compliant.

So that said, my goal for a lack of better terms is to prove out the operational parameters, bring them to the RMH community in plain speak, possibly prove out a third principle which would possibly change the way the above codes are applied to this specific device as long as it is constructed within proven parameters. I do not want to openly discuss this third parameter because 1 I am not qualified, 2 it could be wrong but some might agree and spread it as gospel, 3 if correct might have interesting commercial applications at which point it would lose its "open source" and become someone's patented discovery which I nether seek nor seek to be restrained by at all.

Within limits the first unit went into production today and may be completed for some testing next week.

I hope I remain welcome to post here despite my poor social interactions, however if I become more than you seek to deal with, please speak only the words and I will indeed discontinue beyond alerting that new information has been forwarded to Donkey. I truly seek to be of assistance to this community while having no real intent of becoming "part" of the community. I like the way folks here think, I find it very appealing whether or not I am capable of it has nothing to do with the value I place upon it.
                            


Joined: Dec 15, 2010
Posts: 105
Just in case it was not clear, I have some issues with the 10 cord to .5 cord discussion, however, I do understand the point and agree that one may very well be able to heat a space with significantly less wood with a rmh and be very comfortable with the unit.

This in no way what so ever indicates efficiency differences of 10-.5 or any where near, but it does mean a RMH can make folks happy. I hope to be able to pen all of it soon including "creature comfort" application improvements the RMH has to offer.

I have no doubt of the potential and agree with Donkey, the low capitol gains from these boys works in the favor of the people, there just simply is no real profitable angle to drive development of the RMH, but that sure does not mean it can't be huge effective.

Ned
                          


Joined: Mar 13, 2010
Posts: 94
Location: Colorado
wood stoves vary greatly, on the amount of wood that they can use for the heat that is put out,

IN my own experience, and my SIL, he will go through 4 or 5 times amount of wood, than I do go though,

I had my folks wood stove, before I moved mine in to the house here, it would burn about twice the wood amount for same amount of heat that was put out, and this was a "air tight" stove.

and if you have a old non air tight, it may take a double the poorer air tight,

depending on the stove one is comparing against, I could see the possibility of a 8 to one savings on wood usage, but my guess is that would be the extreme, not the normal,  to a good quality manufactured wood burner,

another factor is/was the fuel the same, was it the same species, the same moisture, the same degrade do to age, or pounds per cubit feet,

 
paul wheaton
steward

Joined: Apr 01, 2005
Posts: 15631
Location: missoula, montana (zone 4)
    ∞
Donkey:  I have heard the name many times, but I don't think I have yet met him or talked to him.

Eric and Erica:  Do you mean Ernie and Erica?

Disrespect:  I run a tight ship.  I like the idea of folks talking about "stuff" instead of clubbing each other.  The key things to keep in mind are "never suggest that anyone on permies is anything less than perfect" and that qualifying one's statements as opinion really helps the general smoothitude.

cords:  the example I gave was 10 vs. a little more than 1.  In the video it is 4 cords vs. 1/2.  Reports are usually five to ten times less wood (1/5 to 1/10 of the wood previously used). 

One other thing that might be of help:  I have heard reports that people have been able to cut their wood consumption in half (or better) by stacking lots of rocks around their conventional wood stove.  So I think that the mass plays a big role.
                            


Joined: Dec 15, 2010
Posts: 105
paul wheaton wrote:
Donkey:  I have heard the name many times, but I don't think I have yet met him or talked to him.

Eric and Erica:  Do you mean Ernie and Erica?

cords:  the example I gave was 10 vs. a little more than 1.  In the video it is 4 cords vs. 1/2.  Reports are usually five to ten times less wood (1/5 to 1/10 of the wood previously used). 

One other thing that might be of help:  I have heard reports that people have been able to cut their wood consumption in half (or better) by stacking lots of rocks around their conventional wood stove.  So I think that the mass plays a big role.


Ianto mentions him in the book.

I do mean Ernie, I am also really bad with names.

The mass in some ways plays very little role and in others it is the reason folks find them appealing and working within parameters they like.

Folks around here seem smarter than average so my simplification here may seem condescending, its not, just trying to broaden folks scope of vision on how these things work and why things are sort of different.

Temperature has little to nothing to do with heat content, it is a measure of heat intensity.

BTU on the other hand has to uses, a unit of energy and it is also an expression of a rate as well and this gets a little twisted up at times.

I am going to use some bogus numbers to drive the point and make math easy.

Q=U*A*TD is the rate of heat transfer equation.
Q BTU, U conductivity of heat factor for a material or the inverse of R factor, A is area, and TD is temperature difference.

If we assume that the same building is used then U and A do not change at all so we can insert 1's in their place.

So if we have a 100 square foot roof with a U factor of 1 and it is 40 outside and 70 inside we would have 100*1*30 (70-40) or 3,000 BTU's of heat lost through that roof, if we warm the room to 80 we would increase the TD to 40 and have 4,000 btus of heat lost etc.

Now if we consider the differences in a RMH and a standard high performance cat stove we have some issues that are drastically different. The cat stove must fire at a fairly high rate in order to work properly. In doing so, it is going to raise the temp in the room very fast and consume the wood fast too, we will come back to this in a second. Since natural convection dictates that the hot air is going to rise, you would find the ceiling above the stove to be very hot in contrast to the other side of the room, well that higher TD is going to result in higher heat losses.

Now with the RMH, you are not firing it at a high rate and your distributing the heat over a larger area lowering the TD and not putting all the heat up and out so to speak.

Now lets look at that firing rate. Again numbers for easy math, the cat stove firing at 4 lbs per hour and the rocket at 2. again a bogus number for ease, if each pound is 5k BTU then the cat is giving us 20k btu into the space (both assumed for this discussion 100% efficient) and the rocket is giving us 10k the question is why are folks still comfy?

Because less heat left the house with the rmh due to the lower rate of heat transfer out and (trust me this is a big and) the RMH burn rate of 50% means it drew 50% less of the cold outside air in to operate so it has 50% less air to reheat.

So if we were to say that both stoves were equal at burning wood fuel say 90% efficient that does not mean they would be equal at heating the space at all, it just means that when we stuck 10k btu's of wood in them we would indeed get 9k btus of heat out of both of them, by getting that 9k out of the cat stove at a much faster rate, we increase our losses within the structure while also bringing in more cold air to feed the faster stove. Not only is it colder air, it is drier air typically in the winter and moving he air around the house at twice the speed so to speak might make one cooler and tend to not feel as warm even when the temp was the same because of the cool dry breeze.

The true reality is, if one was feeling cooler, they would indeed increase the stove firing rate which would increase the problems proportionately and while they felt warm finally they would indeed be consuming far more wood that is required to heat the space by increasing the losses.

I hope this makes it a little easier for folks to understand why it is very possibly accurate that he used 2 instead of 4 or whatever the rate differences were and with proper tending may well have kept the space very near the same temp with both, the 90% efficient stove was not inefficient, it however did increase the losses of his space dramatically through its operation.

This is why radiant heat in the floors is so efficient, no outside air, no dramatic temperature differences in certain spots increasing losses etc.

I hope this helped and if folks have questions regarding it I have no issue trying to explain it further. My post regarding these units are scattered about, but this is the same reason I stated clearly there is no reason to try and rob the heat from the flue gases beyond the dew point, the energy that is lost staying above it is quite minimal and the damage caused by the condensation can happen so fast there is just no reason to try and grab that last little bit and it completely changes the rules when you are looking to see it certified or approved.
Len Ovens
pollinator

Joined: Aug 26, 2010
Posts: 1315
Location: Vancouver Island
    
  18
NedReck wrote:

I hope this makes it a little easier for folks to understand why it is very possibly accurate that he used 2 instead of 4 or whatever the rate differences were and with proper tending may well have kept the space very near the same temp with both, the 90% efficient stove was not inefficient, it however did increase the losses of his space dramatically through its operation.



And I think it is a mistake using the word efficient in this discussion.... or at least in trying to compare it between stoves. I had not really thought of it in the way you mentioned... or at least not so clearly.

What I had realized though, is that running a steel or iron stove all the time is going to use more wood than running a mass heater (RMH or masonry) for 2 to 4 hours out of 12 to 24 hours. This is not about how efficient the heater is, but how the heat generated is used. First, as you said, how the heat is released (low down and not to the roof) means less of the heat is use to heat those in the room, this means there is more heat available to store for later. Second, the fire goes out, but the stored heat keeps the room comfortable so no new fire is kindled and no cold air is brought into the house for draft and to provide oxygen for burning... so the residents feel comfortable for that reason as well.

This certainly doesn't tell the whole story, but my point is a stove that isn't burning uses less fuel than one that is.

As you can tell, I don't have the background in this field that a lot of others do. I am playing around with some bricks in my back yard trying to find out more.

BTW, I understand things a lot better when I found out you have A.S. My son has some of the same problems with high functioning Autism. (though not any of the math skills) I know it is not the same at all and even people with the same diagnosis are vastly different... so a lot better might mean 3% rather than .5%.

I think what I was originally asking was if you were in the industry, or just an armchair reader (I may as well say, like me). I realize I wasn't very delicate in the way I asked... Text tends to make me look even more blunt than I am in person.
Kirk Mobert


Joined: Jan 07, 2011
Posts: 136
Location: Point Arena, Ca
    
    4
One thing I see rarely mentioned..
Houses don't need to be heated, people do.
If the house is 40 deg. F (uncomfortably cold) but the occupant is sitting on a 100 deg. F cob bench, that person can be quite comfy.

In a lot of ways the discussion of efficiency can get SO far away from what really matters SO fast.. Don't get me wrong, it's an important subject and something that I'm personally quite interested in.
On the other hand, what really matters is personal comfort.. How do you quantify that with numbers??  How many BTU's are required to heat a human and how can THAT be delivered most efficiently?


Build it yourself, make it small, occupy it.
tel jetson
steward

Joined: May 17, 2007
Posts: 3112
Location: woodland, washington
    
  58
the shape of the radiating body could play into this as well.  I would think that a typical stove acts roughly like a point source, where a thermal mass could act more like a line or a plane depending on the system.

as I understand it, the intensity of the energy decreases with the square of the distance from a point source radiator.  I believe that intensity decreases proportionally to the distance from an infinite line source, and doesn't decrease at all with distance from an infinite plane source.

physical reality doesn't exactly conform to any of those theoretical models, but they might be useful for understanding another reason why a large mass can be more effective than a small stove for making folks comfortable.

anyone with more knowledge in this department want to weigh in on impact of the radiator's shape?
Burra Maluca
Mother Tree

Joined: Apr 03, 2010
Posts: 5253
Location: Portugal Zone 9 Mediterranean Climate
    
208
Donkey wrote:
One thing I see rarely mentioned..
Houses don't need to be heated, people do.
If the house is 40 deg. F (uncomfortably cold) but the occupant is sitting on a 100 deg. F cob bench, that person can be quite comfy.


I'm going to disagree with that, to a certain extent.  With no heating at all, if the weather is wet the whole house can get damp and mouldy, which isn't healthy for the occupants or the house contents.  When wet weather sets in I like to up the heating a little, which usually means lighting a fire *before* the damp starts moving in through the walls. 

Also, if you have elderly or sick family members, it's not always possible to keep them comfortable just using things like electric blankets.  I find that the old man, who is totally bedridden, will sweat too much in some places (with associated hygiene problems when fungal infections set in) but still feel cold around the head and shoulders unless I keep the room itself reasonably well heated.  I even admit to having an electric radiator with a thermostat in the room with the od boy so he's never exposed to low temperatures no matter what the weather is doing. 

Healthy adults are different, and kids are just bouncing bundles of biological heat generator, but the old folk have slightly different needs. 

Great to have you here by the way, Donkey.  Love the username! 
Kirk Mobert


Joined: Jan 07, 2011
Posts: 136
Location: Point Arena, Ca
    
    4
Burra Maluca wrote:
I'm going to disagree with that, to a certain extent.  With no heating at all, if the weather is wet the whole house can get damp and mouldy, which isn't healthy for the occupants or the house contents.  When wet weather sets in I like to up the heating a little, which usually means lighting a fire *before* the damp starts moving in through the walls. 


To the health of the HOUSE, dry and cold tend to be the best. Heat will actually speed up the process of deterioration, accelerating oxidation providing yummy habitat for wood eating critters. Though in damp climates it CAN help to dry the place out, it is PRIMARILY the way in which we build our houses and the conditions in which they are set that determine dryness.

Also, if you have elderly or sick family members, it's not always possible to keep them comfortable just using things like electric blankets.  I find that the old man, who is totally bedridden, will sweat too much in some places (with associated hygiene problems when fungal infections set in) but still feel cold around the head and shoulders unless I keep the room itself reasonably well heated.  I even admit to having an electric radiator with a thermostat in the room with the old boy so he's never exposed to low temperatures no matter what the weather is doing. 

Healthy adults are different, and kids are just bouncing bundles of biological heat generator, but the old folk have slightly different needs.


Human comfort is a different subject.. My mom keeps her house QUITE cold, claiming that she likes it that way. She would almost never have a fire and be just fine and though I've provided her with ample wood supplies since before my pop died, she won't use it unless her grand-kids come to visit. Even then, we've got to bundle 'em up to send 'em there.
I like my space warm, though not too warm.. Different strokes I guess.

Great to have you here by the way, Donkey.  Love the username! 


Thanks mate. I've had the nickname for about 20 years now, since I worked in the comic book industry, though those days are long gone.
                            


Joined: Dec 15, 2010
Posts: 105
Donkey wrote:
On the other hand, what really matters is personal comfort.. How do you quantify that with numbers??  How many BTU's are required to heat a human and how can THAT be delivered most efficiently?


We are "valued" at 85 BTU per hour meaning we give up that much heat from our bodies. While we are 98 on the inside we are about 85 on the outside. Because we are cooled by evaporation we tend to desire cooler areas than our skin temp and obviously when it is warmer than 85 we will typically sweat without regard for exercising. Again this does vary a lot among folks.

Very few persons are comfortable at cold temps like I am, it just does not bother me and I do not wear a jacket until it is about 20 outside, most folks think I am nuts which dependent on view I might qualify lol. I am however not stupid and know exposure has hazards so I have warm clothing near by should I need it and can not get to warmth.

Here is a chart that indicates the creature comfort zone taking into account temps and humidity/

http://hed.arizona.edu/gfx/charts/PsychrometricChart.jpg
Max Kennedy


Joined: Feb 16, 2010
Posts: 462
Location: Kirkland Lake, Ontario, Canada
Donkey wrote:
To the health of the HOUSE, dry and cold tend to be the best. Heat will actually speed up the process of deterioration, accelerating oxidation providing yummy habitat for wood eating critters. Though in damp climates it CAN help to dry the place out, it is PRIMARILY the way in which we build our houses and the conditions in which they are set that determine dryness.


True within limits.  Even when the envelope of a house isn't compromised, ie lets water in somehow, the "house" is compromised in very cold climates.  Floors buckle, plumbing bursts, frost heaving destroys foundations etc.  Having moved across Canada I have seen this 1st hand many times.  Cold, by itself can be very damaging because of the differential rates of contraction of different materials including woods of different species.  Cool and dry preserves but the extremes of temperature and moisture are destructive.  The caveate for this is that a house is meant to be lived in at moderate temperatures and moisture levels and so acclimates to those conditions.  It is destroyed by the introduction of extremes.  If the house is built at very cold temperatures and kept there or built in hot dry conditions and kept constant then degradation will be minimal but these would generally be considered unlivable and thus it would not be a house.

                            


Joined: Dec 15, 2010
Posts: 105
Totally mess some stuff up and deleted my post by mistake.

Here are some psychrometric charts that indicate comfort zones of most folks with the skin temp being 86 degrees f.

Donkey if you are the same one, you may find some of these particularly interesting in other aspects of what you do.

Unfortunatly testing will not begin next week I have been subpoenaed as an SME and have to attend hearings next week at court. I will have my lab manager continue production off the one you pointed out as well, then will have to made to test off of day one.

Now for the links:

http://itknowledgeexchange.techtarget.com/data-center-facilities/2009/01/

http://www.flickr.com/photos/mitopencourseware/3038898862/

http://www.architecture.uwaterloo.ca/faculty_projects/terri/carbon-aia/teaching/laroche/laroche3.html

http://hed.arizona.edu/gfx/charts/PsychrometricChart.jpg

http://www.ecocooling.co.uk/psychr.html

Good explain here:

http://www.acr-news.com/news/news.asp?id=633

For Donkey: http://www.architecture.uwaterloo.ca/faculty_projects/terri/carbon-aia/teaching/laroche/laroche3.html

http://www.squidoo.com/passivecoolingcalc

Folks I teach courses on Psychrometrics, I can not really effectively communicate them here, a vast majority of my student population would say its boring stuff old professor ned likes too much because of the math lol. If anyone has specific questions I will assist as I can, however if someone seeks a deeper understanding I will see what I can do about providing more information upon request.

I personally from observing discussions here and other locations regarding cob building practices think a basic understanding could improve some of the building practices you folks employ and if you agree and want more info, shoot a PM.

Ned
 
I agree. Here's the link: http://stoves2.com
 
subject: how a RMH uses 1/8 of the wood of a 75% efficient wood stove
 
cast iron skillet 49er

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