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# rocket mass heater uses EIGHT TIMES less wood than a wood stove

paul wheaton
Trailboss
Posts: 21797

David O'Meara
Rancher
Posts: 13459
when did the phrase ONE EIGHTH go out of style?

paul wheaton
Trailboss
Posts: 21797
I call this algebra. As software engineers, our lives are loaded every day with this stuff.

x = y * 8

x / 8 = y

y is 8 times bigger than x.

x is 8 times smaller than y.

It seems simple to me, and yet today I am learning it appears to annoy lots and lots of people.

Here is the biggest rant so far: http://www.reddit.com/r/Frugal/comments/d1a89/this_guy_heats_his_home_with_eight_times_less/

David O'Meara
Rancher
Posts: 13459
Yes it grates but no I didn't participate in that conversation.
Did someone mention heaters?

Henry Wong
author
Marshal
Posts: 21755
85

It bothers me too... ... A multiple of "times" always imply more to me.

Henry

Henry Wong
author
Marshal
Posts: 21755
85

BTW, the other thing that bothers me (that is math related) are the people who "agree 1000%". How do you agree 1000%? Either you completely agree, don't agree, or partially agree, how do you agree 10 times more than possible? Do you have split personalities, and they all agree? ...

Henry

PS... sorry for the hijack.

paul wheaton
Trailboss
Posts: 21797
Henry Wong wrote:
PS... sorry for the hijack.

Is it possible to hijack in MD?

Henry Wong
author
Marshal
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85

How does that stove work? It looks like it is integrated with the sitting area near the windows. So where does the heat come from? And doesn't it get too hot to sit on?

And I really like that it is not only less wood, but the trimming from the trees. From seeing how many of my neighbors throw away bags and bags of leaves every fall, it could probably heat dozens of homes from what they throw out.

Henry

Chris Baron
Ranch Hand
Posts: 1061
Is the usage of percent not common in the US?
We would say it uses 80% less.

paul wheaton
Trailboss
Posts: 21797
More about how on my rocket mass heater page.

Mike Simmons
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Posts: 3090
14
Chris Baron wrote:Is the usage of percent not common in the US?
We would say it uses 80% less.

It's common. But 80% less leaves you with 20%, which is 1/5 of the original amount, not 1/8 (which is the quantity Paul was attempting to communicate).

I would definitely prefer saying "one eighth as much" rather than "eight times less". The n-times-less usage is certainly common and well-understood, for large enough values of n. But justifying it as "algebra" seems problematic to me - better to just fall back on defending it as common usage, instead. Using strict algebraic usage, it can lead to problems.

Consider if x = 40 and y = 50.

y is 25% more than x: true
x is 25% less than y: NO!
x is 20% less than y: true

"n% more" and "n% less" are clearly not inverse relationships. To me, "n times more" and "n times less" should have a similar relationship. That would be logically consistent, but it isn't common usage.

With a construct like x is 8 times smaller than y, a strict interpretation would be that if y is 100, x must be -700. While it would be really cool to have a heater that creates seven times as much wood as is put into it, this obviously isn't what was meant. We can easily dismiss the strict interpretation because (a) it's impossible, and (b) there would have been other, clearer ways to express it, if that was the intent.

However if someone says something like 120% more, I can't really tell if they really mean 120% more, or just 20% more. Probably it's really 120% more. But if they say 120% less, I have no idea what they really mean, because common usage and logically consistent usage are at odds. Similarly if someone says 2 times more - was that really a total that is 3 times the original quantity? Or just 2 times? And if they said 2 times less - what does that really mean?

So for me, "n times less" is clear enough for larger values of n, but can be ambiguous in other cases - especially if percentages are involved.

paul wheaton
Trailboss
Posts: 21797
When I say "rocket mass heater uses EIGHT TIMES less wood than a wood stove", is there even one person that does not understand what the intent is?

In other words, is the problem with being misled, or is the problem with grammar and convention? Can it be written off as dialect? Like "ain't"?

Mike Simmons
Ranch Hand
Posts: 3090
14
I think it's very much like using "ain't", or a double negative. Probably more acceptable than either of those, actually - this usage is widespread even among many well-educated people. I don't think it's even dialect, really, as it's too widespread. And the intent is quite clear. It just doesn't seem technically correct, to some of us. I wouldn't have commented at all if you hadn't gone with the "I call this algebra" response, which seems to be the area where this usage most contradicts my own understanding. Oh well.

I guess DOM, Henry and I will just have to give a 110% effort to stomp out this sort of language misuse.

fred rosenberger
lowercase baba
Bartender
Posts: 12234
36
saying 'times' does not necessarily mean more. "1/8 times at much wood" makes perfect sense, and is obviously less. But I have always found phrases like "8 times less" confusing. I do understand what you mean, but I have to think about it a little.

It's similar do writing code like

if !(isFalse())

it is a perfectly valid way to write code, but it may not be the clearest way to express things.

paul wheaton
Trailboss
Posts: 21797
How about "reduced wood use by a factor of eight"?

Frank Silbermann
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Posts: 1408
He never said what a rocket mass heater was or how it worked. Also, he lost credibility with me for not bothering to finish getting dressed (buttoning his shirt and tucking it in).

paul wheaton
Trailboss
Posts: 21797
It's a youtube video. They are supposed to be short. If you go to my rocket mass heater page you will get more on how.

In the rocket mass heater world, ernie is a rock star. Rock stars get to dress like that.

Ryan McGuire
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Posts: 1085
4
paul wheaton wrote:How about "reduced wood use by a factor of eight"?

The meaning of that idiom is still not clear for all factors. Sure, when number is greater than 1 (or 100%), it's more-or-less obvious what you mean. But let's say instead of being 8 times more efficient than a regular wood stove, a rocket mass heater is only 1.5 times as efficient. In that case, would it be correct to say that its uses "reduced wood used by a factor of 0.5"? I would say no. A reduction of .5 of the wood used means that rocket mass heater was 2 times as efficient, instead of 1.5 times.

Earlier, paul wheaton wrote:
x = y * 8
x / 8 = y
y is 8 times bigger than x.
x is 8 times smaller than y.

I'll add myself as a data point against that wording for either statement.

if we go with your original setup...
x = 8 * y
y = x/8

I would say...
y is eight times x
y is seven times larger than x
y is 800% of x
y is 700% larger than x
x is 1/8 of y
x is 7/8 smaller than y
x is 12.5% of y
x is 87.5% smaller than y

Paul Clapham
Sheriff
Posts: 21443
33
paul wheaton wrote:When I say "rocket mass heater uses EIGHT TIMES less wood than a wood stove", is there even one person that does not understand what the intent is?

Sure, I understand what the intent is. The intent is to say it uses a WHOLE HECKUVA LOT less wood. And most people would understand it that way. If the sentence had instead said it uses NINE TIMES less wood, people wouldn't understand that any differently then EIGHT TIMES less wood.

However I seem to come from a dialect community where "Eight times more wood" and "Nine times as much wood" are synonymous. (In general "N times more" and "N+1 times as much" are synonymous.) So that makes it very hard for me to figure out what "N times less" means in numerical terms, even when I know what those other people mean when they say "N times more".

Frank Silbermann
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Posts: 1408
paul wheaton wrote:It's a youtube video. They are supposed to be short. If you go to my rocket mass heater page you will get more on how.

In the rocket mass heater world, ernie is a rock star. Rock stars get to dress like that.

I went to the web page. It discussed details of its construction, but maybe I just don't know enough about rockets to understand it. What is the essential principle behind its efficiency. How is so much heat wasted from conventional stoves, and how does this design better capture that heat. Saying "It works like a rocket engine" doesn't tell me anything.

Ryan McGuire
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Posts: 1085
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Paul Clapham wrote:Would understand it that way. If the sentence had instead said it uses NINE TIMES less wood, people wouldn't understand that any differently then EIGHT TIMES less wood.

However I seem to come from a dialect community where "Eight times more wood" and "Nine times as much wood" are synonymous. (In general "N times more" and "N+1 times as much" are synonymous.) So that makes it very hard for me to figure out what "N times less" means in numerical terms, even when I know what those other people mean when they say "N times more".

I fall in this camp as well.

As just one example...
I heard an ad ont he radio for a speed reading course. It said it could get you to read "1000% percent faster." Ok, I can accept that. It then went on, "Imagine reading 10 books in the time it takes others to read 1." Well... no. I'd be able to read 11 books while everyone else reads 1. A 1000% increase is a multiplication by 11.

Sure, "1000% faster" sounds like I'd be going 10 times as fast as everyone else. That seems reasonable until you start using smaller numbers. "10% faster" would imply a factor of 1.10, not 0.10. "100% faster" would imply a factor of 2, not 1. In general "X% faster" implies a factor of 1+X/100. Also, "10% faster" is not synonymous with "10% as fast."

There, now I think I've driven my viewpoint into the ground.

Ryan McGuire
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In one of the messages in the "rocket stove and butt warmer" thread in the permaculture forums...
Jocelyn Campbell wrote:
It is amazing how efficient these are and how anyone can build them. The people at Cob Cottage were warm and welcoming and quite interesting.

From the EPA's website, normal wood stoves are 40-60% efficient, and the newer certified stoves are 60-80% efficient. Well, in case this wasn't clear before, when rocket stoves work correctly, they are over 90% efficient.

While 90% efficiency out of pretty much ANYthing is nice, that's only a factor of 2.25 (or a 125% increase) over a normal 40% efficient wood stove. Is that measuring something different than the "factor of eight" calculation we've been batting around?

Even if you say you go from losing 60% of the possible heat up the chimney to losing only 10%, that's still only a factor of 6.

Paul Clapham
Sheriff
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Ryan McGuire wrote:Even if you say you go from losing 60% of the possible heat up the chimney to losing only 10%, that's still only a factor of 6.

Okay. And "eight times less" means the same as "one-seventh as much"... do I have that right? So with a little bit more precision, say you go from losing 63% of the heat to losing only 9%, there's your factor of 7.

Frank Silbermann
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Posts: 1408
Paul Clapham wrote:
Ryan McGuire wrote:Even if you say you go from losing 60% of the possible heat up the chimney to losing only 10%, that's still only a factor of 6.

Okay. And "eight times less" means the same as "one-seventh as much"... do I have that right? So with a little bit more precision, say you go from losing 63% of the heat to losing only 9%, there's your factor of 7.
In that case, I would say that the old-style stove uses only one-eight as much wood to waste the same amount of heat.

However, most people are more interested in the amount of useful, non-wasted heat that is produced.

And I'm still interested in learning the reason this kind of heater is better at burning scrap wood, how it avoids waste, and whether it can be adapted to burn oil or coal.

Ryan McGuire
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Posts: 1085
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Frank Silbermann wrote:
And I'm still interested in learning the reason this kind of heater is better at burning scrap wood, how it avoids waste, and whether it can be adapted to burn oil or coal.

IF I UNDERSTAND CORRECTLY...
The increased efficiency comes from two differences:

1. Routing the heated air back around the combustion chamber makes the combustion chamber hotter, thus increasing the amount of flammable material in the wood that actually gets burned. In a regular fireplace fire, some amount of what goes up the chimney is flammable but just didn't get a chance to burn. It's this material that causes chimney fires. More combustion mean s more heat from a given amount of wood.

2. The hot air is routed through a earthen (or concrete or whatever) bench that absorbs the heat and then radiates the heat over an extended period. Contrast this to regular wood stoves or fireplaces where much of the heated air goes straight up and out the chimney before it transfers any heat to the house.

I'm certainly no expert in either the science of combustion nor rocket mass heaters. This is just what I've gleaned from a couple web sites, a couple decades of owning/operating fireplaces and wood stoves and my engineering degree.

I do notice one problem with the rocket mass heater that regular fireplaces and wood stoves suffer from as well. The air that gets sucked into the heater to burn the wood comes from inside the house. This air is replaced by cold air from outdoors. That's why many fireplaces actually make a house colder in winter. The small amount of radiant heat they do generate is not quite enough to heat the cold air that gets sucked into the house to replace what goes up the chimney. To fix this, many fireplaces are now sealed off from the house and use air that is drawn in directly from outdoors (sometimes via a remote blower; sometimes directly through the back of the fireplace).

I wonder if the rocket mass heater could be modified to operate like this? Could you construct the heater so that after you put the wood in and light the fire, you then close off the wood-burning area from the rest of the house? e.g. This ugly drawing. Thoughts?

Paul Clapham
Sheriff
Posts: 21443
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Here's a recent link about the application of "rocket stoves" in Africa... Rocket Stoves in Mwamgongo

paul wheaton
Trailboss
Posts: 21797
In order for a conventional stove to take smoke out of the house, it must keep the chimney hot. So once the fire gets rolling, there is 400 degree (F) smoke leaving the house really fast.

With a rocket mass heater, the chimney is shortened to about three feet tall, and then capped and the exhaust is rerouted into something that will capture as much of that heat as possible: the thermal mass. Afterward, the exhaust dribbles out at typically less than 100 degrees (F).

Frank Silbermann
Ranch Hand
Posts: 1408
paul wheaton wrote:In order for a conventional stove to take smoke out of the house, it must keep the chimney hot. So once the fire gets rolling, there is 400 degree (F) smoke leaving the house really fast.

With a rocket mass heater, the chimney is shortened to about three feet tall, and then capped and the exhaust is rerouted into something that will capture as much of that heat as possible: the thermal mass. Afterward, the exhaust dribbles out at typically less than 100 degrees (F).
So if we didn't keep the chimney hot, in a conventional stove, the smoke would not leave fast enough.

How does a rocket stove keep the smoke moving? Fans?

paul wheaton
Trailboss
Posts: 21797
The chimney in a rocket mass heater is about three feet tall. It gets way hotter on the inside because it is insulated. This serves multiple functions, including the smoke reburn and getting a bigger push to the exhaust.

paul wheaton
Trailboss
Posts: 21797
I just overhauled my rocket mass heater article: http://www.richsoil.com/rocket-stove-mass-heater.jsp

Ryan McGuire
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paul wheaton wrote:I just overhauled my rocket mass heater article: http://www.richsoil.com/rocket-stove-mass-heater.jsp

Whew! When I saw this thread back at the top of the list, I thought we were going to rehash the "8 times less" idiom. :-)

Paul, cool page. I want to give this a try now.

I'm still a bit skeptical about the numbers that are batted around. If someone is going to say traditional wood stoves are 75% efficient, I really want to know how that's measured and calculated. ...ESPECIALLY when Paul wonders about the math himself.

Directly measured metrics, regardless of how rigorously the conditions are maintained, are better than precise but hokey numbers. e.g. Someone heats their house with only a half cord of wood a winter as opposed to 5 cords? I'd rather read that than the "75% efficiency" and "10 times better" assertions. Sure the two winters for the .5 vs 5 cord comparison might not have been exactly as cold. There may also have been different house usage patterns and temperature requirements. Nonetheless, admittedly non-rigorous science is better than unexplained numbers we're expected to take at face value.

Speaking of science (as in measuring and/or controlling as many parameters as possible), I'd love to see some more detailed info about the differences between rocket mass heaters and regular wood stoves. What is the diameter of the exhaust pipe in each case? How fast is the exhaust really leaving it? What is the temperature of the exhaust? What is the exhaust composed of?

On a different subject...
It seems to me that if you want the heater to continue to draw air, the end of the exhaust has to be higher than where the fire is burning. In the real-life video it looks like the fire is at shin level while the end of the chimney is about at eye level. That makes sense to me. The diagrams on the newly overhauled page at richsoil.com show the exhaust at the same level as the fire. I'm not going to go as far as to say that's impossible, but I will say that it's non-intuitive to me.

Also, once the seat (or other thermal mass) is well heated it's going to extract less heat from the heat exchange pipe going through it. You probably get the most efficient heat extraction when the temperature difference between the exhaust and the bench is greatest (i.e. a cold bench). This is also wrapped in a big "it seems to me".

And lastly, I'm still concerned about the amount of air that the heater is pulling from the house. Any house air that's used for the heater has to be replaced by cold outside air being pulled in through poorly sealed doors, windows, electrical outlets, etc. I'm going to mention again my idea of having a sealed system from the outdoors, to the combustion chamber, through the bench and back outdoors. This pipeline could be opened just long enough to add more wood and then sealed again. (See my message in this thread from Aug 18, 2010 for a link to a diagram.) EDIT: Aha... I see that the image of the detailed plans, about halfway down the page, is linked to a page page that shows heaters with what appears to be this type of setup.

paul wheaton
Trailboss
Posts: 21797
And now I have a kickstarter!

kickstarter page

paul wheaton
Trailboss
Posts: 21797
The kickstarter was fully funded and now I have DVDs: streaming or physical DVDs

paul wheaton
Trailboss
Posts: 21797
Here it is, a couple of years later and I have a NEW kickstarter for rocket mass heaters:

https://www.kickstarter.com/projects/paulwheaton/rocket-mass-heaters-4-dvd-set

paul wheaton
Trailboss
Posts: 21797
We have finished the editing, so the digital version is now available for sale.  The physical dvds are at the printer.

To buy the digital stuff, or to pre-order the physical dvds:  http://richsoil.com/wood-heat.jsp

Will Myers
Ranch Hand
Posts: 390
if rocket mass heaters are so great why is everyone in that video wearing a thick coat while inside?

paul wheaton
Trailboss
Posts: 21797
Will Myers wrote:if rocket mass heaters are so great why is everyone in that video wearing a thick coat while inside?

Because they just finished building it in the middle of winter and they are now trying to drive materials and keep the windows open.

paul wheaton
Trailboss
Posts: 21797
Will Myers wrote:if rocket mass heaters are so great why is everyone in that video wearing a thick coat while inside?

Because they just finished building it in the middle of winter and they are now trying to drive materials and keep the windows open.

Paul Clapham
Sheriff
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33