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rocket mass floor heater -- finally completed and it works!

                    


Joined: Oct 23, 2011
Posts: 0
We wanted to heat the interior and floor of our 16 ft yurt with this innovative type of stove.  We started it in April and finished it (mostly) about a month ago.  There was a long summer pause in between start and finish.

We used the book Mass Rocket Heaters by Ianto Evans and Leslie Jackson as our starting point. They recommend that first timers DO NOT attempt to build a floor heater, but that's what we did.     We just told ourselves that if we had to build it twice...we would.  We made sure that the proportions of everything followed the book's recommendations.   

We wanted the firebox outside, which made the length of the burn tunnel pretty long (32" to make it under the wall of the yurt.  That made the whole thing kinda big - the interior cross section of the vertical burn chamber is 7"x9" and 67" tall.  We used four and a half bricks per vertical course. 

We paid $500 for custom fabricated barrels, as the whole system needed to be larger than 55 gallon barrels would allow.  The outside one is 3/8" thick, the interior is 10 gauge, which is I guess a little thinner than 1/8".  The outer barrel is 24" in diameter, and about 4 feet of it is exposed inside. 

We used volcanic cinders (locally sourced - ie, gathered from a road cut) for insulation around the vertical riser, around the horizontal burn tube, and under the entire thing. 

There are four 6" exhaust pipes going thru a pea-gravel filled and insulated box under the floor.  They each have their own clean out holes which are covered with a metal disk and made prettier with some rocks set into the cob.  We surrounded the connections of the exhaust pipes with walls of cob. 

We figure the entire cost of the thing, including delivered sand, pea-gravel, foam insulation, and some new fire bricks (some were salvaged) to be around a thousand dollars. 

We are extremely happy to report that it is working really well.  A fire for 4-6 hours keeps the temperature of the yurt above 70 degrees (sometimes WELL above, we've had it over 100 degrees inside - this thing could heat a larger space) for more than 12 hours, with 40 degree temps outside. With temperatures below freezing, the yurt is close to 60 degrees 12+ hours after the fire has gone out. It has never been colder than 50 degrees, even without a fire for several days.  The yurt is insulated with two layers of wool blankets under the tarp. 

We wrapped 60 feet of copper pipe around the barrel inside, and this thermo-syphons to a water tank outside. Even with our low pressure situation we can take a bath right next to the barrel. The water is slow to heat up; we plan to make a volcano of cob that will cover most of the barrel and surround most of the pipes, but that will have to wait til next summer.

Photo gallery here.
Joel Hollingsworth
volunteer

Joined: Jul 01, 2009
Posts: 2103
Location: Oakland, CA
What a beautiful and exciting project. Congratulations!


"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.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
Very cool!
Thanks for sharing.

Nice workmanship, too. 

Keep us posted about how it works for you.


I'm so used to people asking how to do it (who haven't read the book, yet), I feel like I'm supposed to say something here.

So here's some reflections....

I like the outside-the-barrel copper pipe.  Seems much less likely to explode than the inside-the-barrel variety.  (Heating up slowly is a small price to pay for the absence of flash-steam explosions.)

Ever try using your cleanout caps like a Lorena-stove or sideboard, to heat a small pot of water or soup?  Probably won't boil anything there, but it could be handy for cosy evenings or snowed-in situations.

Watch that straw-cob while the system is heating; some of the straw may burn out with hotter fires.  (You may smell it, or see black smoke for a while.)  Check the structural integrity after that happens, it may be just fine, and it looks like you've got chicken wire too as backup.  But if it fails, you may need to reconfigure that area for all-masonry support (corbels and vaults without straw).

After you've lived with it for a few years, will you come back and tell us how you like that outdoor wood feed
A lot of people like the idea on paper, but most don't realize how tall they'll need to make the heat riser.  You've done a great job there.  How much trouble is it to tend the fire outside during inclement weather?

Thanks again for sharing - a very impressive example of rocket stove design (by yet another fisherman's kid!)  I'm beginning to be convinced there's something about sailors in the family that makes people very handy....

-Erica Wisner
http://www.ErnieAndErica.info


Play with nature, make nifty stuff:
www.ErnieAndErica.info
                          


Joined: Nov 20, 2010
Posts: 140
Very impressive!  Well done.
Jami McBride
volunteer

Joined: Aug 29, 2009
Posts: 1779
    
  10
What a wonderful project - I love the gallery of photos.

Well done 
                    


Joined: Oct 23, 2011
Posts: 0
Thanks everyone!   

Was so much work, plus the expense, and since we weren't entirely convinced it would work, in fact were warned by both the book and a couple of other cob-people that it might NOT....sometimes it was hard to keep going.  Glad we did.  I'm hanging out in the warm yurt right now, actually.   

We had a few test fires during the building process to reassure ourselves that the math was right.  The first one was just with the stacked bricks.  Once the air in the vertical stack heated up it sucked the remains of the not-quite-fully-burnt newspaper all the way down the burn tunnel.  That was encouraging and awesome.  We had another test fire after the barrels were in place, and another after the chamber was built, and another after the exhaust tubes were in place.  It just kept flowing in the right direction, so we went ahead and buried the lot in pea gravel. 

Thank you for the feedback and compliments, Erica!  To respond to a few things: 

Heating up slowly is a small price to pay for the absence of flash-steam explosions.)


There's a pressure relief valve on the tank.  Also, the water heats SO slowly that it takes 6 hours to get just the hot water to bath temperature, and by that time the yurt is SO warm you have to lay on the floor to breathe.  Have never had it close to boiling, I think the hottest water so far is 140.  I'm very very interested to see how much faster it will heat the water when it's covered in cob.  We'll lose a big part of the radiant heat from the barrel, which is nice in that it heats up the air of the yurt really quickly, but I think it will hold the heat a lot longer as well.  I'm all for that trade off. 


Ever try using your cleanout caps like a Lorena-stove or sideboard, to heat a small pot of water or soup?  Probably won't boil anything there, but it could be handy for cosy evenings or snowed-in situations.


Those metal disks that cover the top of the metal elbow are about 8 inches below floor level.  The rocks that you see in the finished photos cover up those holes.  The rocks get warm - are the best foot warmers ever - but not warm enough for food, really, and I don't think a pot of soup or anything would fit down in the holes.  The cats like to sleep on the cob surrounding the barrel. 

We offset the spacing of the outer barrel so that there's 2" of space on the room-side, and about 3/4" on the wall side.  I notice that I can boil a kettle on top of the barrel if it's sitting on the room side, but the side close to the wall doesn't get nearly as hot.  That's the food warming/very slow cooking spot, excellent for stocks and things I want to gently simmer a really long time, but don't really have space for on the cook stove in our cabin.  I plan to make a big batch of apple butter on the barrel-top later today. 

Watch that straw-cob while the system is heating; some of the straw may burn out with hotter fires.


I kinda thought that would be a bad thing, so I built the chamber a little bigger on purpose and then - and this was a giant pain in the back but I'm glad I did it - put two substantial layers of plaster on the entirety of the chamber interior, both a scratch coat to smooth out the lumpy straw corbels and a very fine finish plaster to make it strong and very smooth.  There's a photo of the inside of the chamber we took by sticking the camera down in there, you can see that's very smooth and there is no exposed straw whatsoever.  Like you noticed, we used a strip of hardware cloth screwed into the bottom of the barrel to support the ends of the corbels.  Deston Denniston came to visit in June; these were the suggestions he had for the project and I am SO GLAD we got to hear that advice.  Made all the difference. 

How much trouble is it to tend the fire outside during inclement weather?


I was actually pretty worried that the outside firebox would be annoying.  But -

We ended up modifying the firebox to hold a load of horizontal wood (we were getting too much smoke-back with vertical loading), and we can easily fit five or six fairly large pieces of wood in it at a time.  It lasts for one or two hours, depending on the wood species in there, at which point one of us has to pee or get another beer or something, and we stoke the fire at the same time.  It's obvious when the fire's almost all the way out, things get very quiet in the yurt, but the coal bed down in the bottom of the firebox is pretty gnarly, you just put a thinner piece of soft wood on the bottom of the next pile and away she burns.  I love the noise of rocket heaters. 

We built a shed roof over the fire box, so when you're stoking it it's not like you're getting rained on.  It's actually relatively warm and toasty under there, sometimes the chickens come hang out around it, and I've seen one of our pigs on more than one occasion laying in the gravel right next to it.  The firebox is only a few steps from the door of the yurt and from the wood shed.  Relative location is key. 

I'm mostly happy to NOT have the whole firewood mess inside!  It's a real revelation to have a space that STAYS CLEAN.  It's a take-your-shoes-off space, which I've never ever had in my life.  Also, the footprint of the stove takes up enough space inside the yurt as it is, having the firebox outside frees up interior space - and have I mentioned how much I love the fact that we don't have to carry wood in and ashes out?  I LOVE THAT SO MUCH. 

We still cook and sleep in our cabin, where we have a normal wood cookstove, and man, it's just filthy in there all the time.  Dust and wood shards all over the floor, even when I sweep everyday.  And when I sweep everyday I spread the ash dust in a thin layer over every single horizontal surface.     The clutter of the cabin used to send me on cleaning frenzies, now it just sends me over to the yurt to escape!   

We also usually start the fire and stoke it a few times when we're NOT in the yurt, when we're running around outside doing animal things (that have to be done everyday, weather permitting or not).  Then by the time we get around to "retiring" (that's what we like to say - shall we retire to the yurt, darling?) to the yurt we can take our wet shoes off, and let the fire die.  It actually gets more comfortable in here after the fire dies- after several hours of a rager it's HOT. 

I want to make a wool cover that fits on the top of the fire box to completely stop airflow when the fire is all the way out.  I know that will keep heat in the system longer.  We've tried it a couple of times with just a wool blanket and it makes a big difference.  I once put the blanket on too soon and ended up with a seriously singed blanket.     Lesson learned.  Burned wool smells awful. 

I was nervous that a detached space with a wood stove would mean we'd have two simultaneous fires frequently and that we'd use up an obscene amount of wood.  But....generally unless it's seriously cold (which for these parts is like, in the teens), we let the fire in the cabin die after breakfast, and start a fire in the yurt soon after or even later in the day, depending on if we're going to spend daylight hours in there (which is dependent on if we can work outside that day).  Then we let that yurt fire die later in the afternoon/evening, have a quick fire in the cabin to cook dinner and heat things up before we go to bed.  So there's some overlap, but generally the fire is going either one place or the other. 

We are burning more wood, but we are also heating up more interior space.  Having the yurt brings our living space to a grand total of about 350 square feet!  It's our living room, the place where we hang out with company, the yoga spot, the computer/tv area, the winter guest sleeping quarters.  It's wonderful. 

I'm beginning to be convinced there's something about sailors in the family that makes people very handy....


I'm the daughter of a fisherman and the granddaughter of a navy man.  That history has influenced my life very much.  I've never thought to connect seafaring ancestry and handiness....but you may be onto something there.  I've been told my dad liked to tie knots to pass the time out at sea....and make fireworks....perhaps he was a pyro too. 
                          


Joined: Dec 01, 2009
Posts: 211
Location: Northern California
Congratulations, Marina! Sounds great. Those photos are excellent and very helpful for visualizing the process. You could send them to Ianto and Leslie for the next edition of the book.

Those tailpipes really make it look like it will take off!
                            


Joined: Nov 13, 2010
Posts: 29
for heating water, it may help if the coils were moved up near the top of the barrel and wrapped in insulation as only the small area actually touching the barrel is heated when exposed as it is

the water in coil should be the bottom coil and the water out at the top
                    


Joined: Oct 23, 2011
Posts: 0
Thanks, Kerrick!  (and hello after a long summer)  I made sure to document the whole thing.  The authors of the book encourage people to send them pictures of larger systems especially, so I plan to do that.  Maybe I'll do that now! 

Those tailpipes really make it look like it will take off!


Ha, yeah we had to angle the tail end of the exhaust pipes up like that because if the wind blew from that direction it reversed the fire entirely.  Not good.  They do make the floor look like a big weird rocket though, huh? 

for heating water, it may help if the coils were moved up near the top of the barrel and wrapped in insulation as only the small area actually touching the barrel is heated when exposed as it is

the water in coil should be the bottom coil and the water out at the top


Yeah, like I already said we plan to encase the coils in cob, which will also get warm and the whole pipe will be surrounded in warm stuff that stays warm a long time.  Better than simply insulating it, in my opinion.

I don't understand that last sentence?  Cold water falls out the bottom of the tank into the bottom coil and the hot water expands itself out the top coil back up into the top of the tank.  This is the only way a thermo syphon will function. 
                            


Joined: Nov 13, 2010
Posts: 29
you want the cold water to enter the system against the flow of heat so the temp builds to max as it exits

in most any heat transfer system the flows should be against each other

the siphon flow will be determined by the hot water tank

and as i understand it you want the hot air in the barrel to cool so that it falls and forms a draft so insulating too much of the barrel will hinder that

just bunch the coil at the top and insulate just that much so the coil absorbs the heat
                        


Joined: May 26, 2010
Posts: 278
Location: Iowa, border of regions 5 and 6
Kerrick wrote:
Those tailpipes really make it look like it will take off!


Naah.  Not until you put tail fins on it, and a fancy grill in front of the burn unit.  And don't forget to hang a pair of fuzzy dice in there somewhere.
                          


Joined: Dec 01, 2009
Posts: 211
Location: Northern California
Only fuzzy D20s fit in yurts; 6-sided dice are too square.
                    


Joined: Oct 23, 2011
Posts: 0
haha!  Ya'll are funny.  Oh man the image of the yurt with a grill makes me laugh. 

wardd, I can't call myself an expert on rocket stoves, only having made three of them (and one of those was the tin can version), but in my experience and understanding:

The single thing that makes a rocket stove work is the action of the rockety burn chamber - the turbulance and crazy high temps in the insulated vertical stack.  The temps inside there get so hot that the fire gets PULLED sideways down the burn tube, and exhaust is PUSHED up, PUSHED down the sides of the barrel, and PUSHED out the exhaust tubes.  It may help if the barrel is warmer than outside air to get the thing started, but the temperature of the outer barrel really has nothing to do with the draw.  It's just a way to get radiant heat out of a system that otherwise transfers most of the heat into thermal mass.  Our system is a little heavy on radiant heat for the size of our space.  The cob that we cover it with will probably not cool all the way down until about a month after the last spring fire. 

Also, I've found that the tin can aprovecho stove is actually a really handy thing to have around when you're explaining mass rocket heaters to someone who's never seen one before, especially when it's totally finished and the most important part of the thing is hidden.  You can hold up a little model of the internal parts, and then people are like "Ooohhhh!"
                            


Joined: Nov 13, 2010
Posts: 29
marina phillips wrote:
haha!  Ya'll are funny.  Oh man the image of the yurt with a grill makes me laugh. 

wardd, I can't call myself an expert on rocket stoves, only having made three of them (and one of those was the tin can version), but in my experience and understanding:

The single thing that makes a rocket stove work is the action of the rockety burn chamber - the turbulance and crazy high temps in the insulated vertical stack.  The temps inside there get so hot that the fire gets PULLED sideways down the burn tube, and exhaust is PUSHED up, PUSHED down the sides of the barrel, and PUSHED out the exhaust tubes.  It may help if the barrel is warmer than outside air to get the thing started, but the temperature of the outer barrel really has nothing to do with the draw.  It's just a way to get radiant heat out of a system that otherwise transfers most of the heat into thermal mass.  Our system is a little heavy on radiant heat for the size of our space.  The cob that we cover it with will probably not cool all the way down until about a month after the last spring fire. 

Also, I've found that the tin can aprovecho stove is actually a really handy thing to have around when you're explaining mass rocket heaters to someone who's never seen one before, especially when it's totally finished and the most important part of the thing is hidden.  You can hold up a little model of the internal parts, and then people are like "Ooohhhh!"


true enough but the cooling air between the riser and barrel falling also helps

my point being most of the heat is at the top and that is where i'd put the coil slightly seperated and covered with insulation
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
wardd wrote:
for heating water, it may help if the coils were moved up near the top of the barrel and wrapped in insulation as only the small area actually touching the barrel is heated when exposed as it is

the water in coil should be the bottom coil and the water out at the top


Never wrap a rocket mass heater barrel in insulation - it needs to be actively cooling the exhaust gas for the draft to work properly.

Wrapping it in a thin layer of masonry, tile, or cob can work OK, as long as it can still radiate to shed excess heat.

-Erica Wisner
http://www.ErnieAndErica.info
                            


Joined: Jun 12, 2010
Posts: 13
Erica Wisner wrote:
Never wrap a rocket mass heater barrel in insulation - it needs to be actively cooling the exhaust gas for the draft to work properly.

Wrapping it in a thin layer of masonry, tile, or cob can work OK, as long as it can still radiate to shed excess heat.

-Erica Wisner
http://www.ErnieAndErica.info


Well, if you had the barrel wrapped in some insulation and surrounded with a copper tube with cold water flowing through it constantly, then it would be actively cooling it, one might argue even better than the surrounding air.
                            


Joined: Nov 13, 2010
Posts: 29
Brian wrote:
Well, if you had the barrel wrapped in some insulation and surrounded with a copper tube with cold water flowing through it constantly, then it would be actively cooling it, one might argue even better than the surrounding air.


and we're only talking a few inches
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
Brian wrote:
Well, if you had the barrel wrapped in some insulation and surrounded with a copper tube with cold water flowing through it constantly, then it would be actively cooling it, one might argue even better than the surrounding air.


You could argue that.

That's one reason I was glad to see the copper pipe on the outside of the barrel, rather than inside along the heat riser.  (Cooling the fire =  bad.  Steam explosions = bad.  Cooling the downdraft barrel = good.)

We could also argue the risks and merits of heating the water faster, vs. too fast, and whether you'd be able to control this in a hotter location with insulation.

It all depends on execution.  It sounds like most of the people attempting water-heating are aware and pro-active about the risks of steam explosions, so I'm biting my tongue and hoping it all comes out well.

I just wanted to get it out there, because we've seen well-meaning and intelligent people make basic mistakes like insulating the whole barrel (thinking hotter=better draft).

Ernie likes to tell people to try it in their backyard and see; I like to warn people away from time-intensive mistakes that have already been made.
tel jetson
steward

Joined: May 17, 2007
Posts: 3097
Location: woodland, washington
    
  53
marina:
any idea what the temperature of the exhaust is when it leaves the system?


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Joined: Nov 13, 2010
Posts: 29
i just had an idea about steam explosion containment.

coming of the heater put a t or y in the line and off the t attach a length of plastic or rubber hose capped at the other end

the thing is that the rubber or plastic tube will be the weakest link but strong enough for normal use

this works on straight physics , nothing to jam or stick

i would use this as a backup to a pressure relief valve

i would shield it so no one is burned if it lets go
                    


Joined: Oct 23, 2011
Posts: 0
With Robert's nifty laser temperature taker, this is what we found for various temps in the system. 
(only a short fire in it thus far this morning)
Exhaust is around 100 degrees, and mostly water vapor, you can put your face in it no problem.  It's cool to see billowing clouds coming out the pipes but smell no smoke.

Top of the barrel plate in the middle is 330.  Edges of the top of the barrel are 230.

Bottom of the barrel is 160.

The water flowing through the copper isn't constantly cold.  Only the first two coils are cold to the touch, then it gets warmer as it gets towards the top. 

Spacing the copper pipes evenly along the barrel provides more room for more cob between and around each individual course of pipe. 

If we bunched them all together the pipes would be touching each other on two sides, and I don't think that this would be as effective to heat the water because less surface area of the pipe would be surrounded by warmer-than-the-water mass.

The top of the barrel is warmer, but the bottom of the barrel is still warmer than the water.  For the above reasons, I don't think it's worth it to bunch them at the top of the barrel. 

I am genuinely not that concerned about steam explosions.  The nature of our stove makes it resistant to constant stoking, so creating a super hot barrel that would perhaps be an explosion problem is just not really that much of a possibility. 

We don't NEED to get the system as hot as possible to make the room extremely comfortable.  Neither are we interested in feeding it oil or adding fans or doing other things that might make the fire super hot.  It's plenty hot with semi-regular fire feedings, and we let the thing go out after a few hours because frankly, it's sort of oversized for the space and tends to make it uncomfortably warm in there. 

"You can debate procedure all day long, in the end you need to proceed." - one of my partner's axioms.

There are a million ways to do things.  We accomplished all of our goals of a warm room, a warm floor, and bath temperature water.  We are happy with that. 
                            


Joined: Nov 13, 2010
Posts: 29
marina phillips wrote:
With Robert's nifty laser temperature taker, this is what we found for various temps in the system. 
(only a short fire in it thus far this morning)
Exhaust is around 100 degrees, and mostly water vapor, you can put your face in it no problem.  It's cool to see billowing clouds coming out the pipes but smell no smoke.

Top of the barrel plate in the middle is 330.  Edges of the top of the barrel are 230.

Bottom of the barrel is 160.

The water flowing through the copper isn't constantly cold.  Only the first two coils are cold to the touch, then it gets warmer as it gets towards the top. 

Spacing the copper pipes evenly along the barrel provides more room for more cob between and around each individual course of pipe. 

If we bunched them all together the pipes would be touching each other on two sides, and I don't think that this would be as effective to heat the water because less surface area of the pipe would be surrounded by warmer-than-the-water mass.

The top of the barrel is warmer, but the bottom of the barrel is still warmer than the water.  For the above reasons, I don't think it's worth it to bunch them at the top of the barrel. 

I am genuinely not that concerned about steam explosions.  The nature of our stove makes it resistant to constant stoking, so creating a super hot barrel that would perhaps be an explosion problem is just not really that much of a possibility.   

We don't NEED to get the system as hot as possible to make the room extremely comfortable.  Neither are we interested in feeding it oil or adding fans or doing other things that might make the fire super hot.  It's plenty hot with semi-regular fire feedings, and we let the thing go out after a few hours because frankly, it's sort of oversized for the space and tends to make it uncomfortably warm in there. 

"You can debate procedure all day long, in the end you need to proceed." - one of my partner's axioms.

There are a million ways to do things.  We accomplished all of our goals of a warm room, a warm floor, and bath temperature water.  We are happy with that. 


if it ain't broken don't fix it
                        


Joined: May 26, 2010
Posts: 278
Location: Iowa, border of regions 5 and 6
Exhaust is around 100 degrees, and mostly water vapor, you can put your face in it no problem.  It's cool to see billowing clouds coming out the pipes but smell no smoke.


So in addition to taking a bath, you can give yourself a facial.  Cool.
                    


Joined: Oct 23, 2011
Posts: 0
So in addition to taking a bath, you can give yourself a facial.
     I might just start calling the yurt "la spa". 

I should mention that the tub is our only bathing option here in the winter.  The way it goes, we're waiting around til it gets just hot enough to clean ourselves, at which point we fill the tank back up with very cold water. 

I would like it to get to a point where it takes a lot less wood to get the water hot enough.  It's way beyond what we need to just hang out inside.  Hard to make cob with a foot of snow on the ground, I think our clay is frozen, and I don't want to mix it in here.  This winter we will just deal, cause it is working to a good enough extent. 

I'm kinda surprised no one's really asked about the firebox modification.  That's the thing we changed that we weren't planning on.  We were doing vertical feeding at first, but a lot of our wood just isn't perfectly straight, and sometimes the fire would climb up the wood if it weren't touching the bottom of the box, and we weren't able fix it when it happened because we were sitting inside....We needed to do it differently. 

The fire mostly draws from a hole in the front of an elongated fire brick box (around which we will also add more cob for stability), its dimensions are 22"L x 10"W x 9".  You can lay odd shaped pieces of wood in there (this is handy - we have a lot of curvy twisted oak), in a neat but jumbled pile, and the fire burns down.  We place two metal plates on top of the box to limit the draw there.  I still start the fire with vertical kindling up against the burn tunnel entrance, then lay it over when it gets going and add the rest of the wood. 

I would only recommend this for outside feed systems.  Especially when things have cooled off for a day or more in between firings, it can be smokey when you're starting a fire or filling it back up with wood - until the metal plates are replaced and the fire shoots sideways again.  It doesn't smoke much at all after the third stoking in a row. 

That's another advantage of the outside firebox - there's never ever any smoke inside. 

A couple hours later, the top edge of the barrel is 230 and the bottom is 190.  I just added wood for the third time, only one piece since I don't want to over heat. 

It's 83 in here, and the copper pipe coming off the top of the barrel and back to the tank is 98. 
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
marina phillips wrote:
     I might just start calling the yurt "la spa". 

I should mention that the tub is our only bathing option here in the winter.  The way it goes, we're waiting around til it gets just hot enough to clean ourselves, at which point we fill the tank back up with very cold water. 
...
It's 83 in here, and the copper pipe coming off the top of the barrel and back to the tank is 98. 


Don't let the nit-picking bother you.  We are a bunch of wood heat geeks who love working out possibilities when it's too cold to build outside.  Everybody's compliments are completely genuine- this is a very impressive system you've created, especially for a first try.  (Some of us are frankly jealous.)

Re: Only option for bathing:
My grandmother's family spent about 20 years bathing from a bucket that was heated on top of the kitchen range (after their indoor plumbing failed during the Dust Bowl, and their new well was too far downhill from the house and their plumbing was full of mud anyway.).  I can email you her directions for 2 girls to 'freshen up' using 1 pail of water, if you like.
So don't forget that hot running water is not the only kind of hot water.  Sticking a pot on top of the stove, and adding it to the lukewarm bath, might get you there a lot faster than trying to get the pipes and room heat to reach the perfect equilibrium.

(Now I'm seeing skinny galvanized pails, like milk jugs or flower vases, stuck one in each hole over your cleanouts, to get a lot of water up to bathing temperature quickly... sort of a cornucopia-octopus-of-steamy-delights ... funny mental image.)

Re: Moving and cobbing pipes
I do think that your idea of cobbing over the pipes and barrel is a good one.  That's going to increase the heat into your water, and decrease the heat to the room, and might just balance out the comfort levels.  It would get you a lot closer, anyway.  I'd leave some exposed metal on the top for heating pots of water etc.

There may be some benefit to moving the pipes upward, to catch more of the heat.  Not clustering them, like you said, but spaced out a little bit.  The difference in temperature does make a difference to the speed of heat transfer, and I think you'd still be pretty safe as far as steam goes.  Assuming you are using cob and not insulation, and have your relief valves close by and oriented safely.

Re: Firebox
Running a clean fire without tending it is a major problem with the outdoor wood feeds.  I'm glad you've improvised something that works for you.  I'm going to look at your slideshow again and see what I missed.
Erica Wisner
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Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
marina phillips wrote:
'm kinda surprised no one's really asked about the firebox modification.  That's the thing we changed that we weren't planning on.  We were doing vertical feeding at first, but a lot of our wood just isn't perfectly straight, and sometimes the fire would climb up the wood if it weren't touching the bottom of the box, and we weren't able fix it when it happened because we were sitting inside....We needed to do it differently. 

The fire mostly draws from a hole in the front of an elongated fire brick box (around which we will also add more cob for stability), its dimensions are 22"L x 10"W x 9".  You can lay odd shaped pieces of wood in there (this is handy - we have a lot of curvy twisted oak), in a neat but jumbled pile, and the fire burns down.  We place two metal plates on top of the box to limit the draw there.  I still start the fire with vertical kindling up against the burn tunnel entrance, then lay it over when it gets going and add the rest of the wood. 

I would only recommend this for outside feed systems.  Especially when things have cooled off for a day or more in between firings, it can be smokey when you're starting a fire or filling it back up with wood - until the metal plates are replaced and the fire shoots sideways again.  It doesn't smoke much at all after the third stoking in a row. 

That's another advantage of the outside firebox - there's never ever any smoke inside. 


I'd love it if you could post another picture or two of your burn process.

I think you're talking about a batch-burn that's more like a masonry heater here, still very efficient, but not quite as clean as a well-tended rocket heater can be.  But I can see where it's much more convenient given that you need the fire to burn without supervision for several hours.

I think you're right, this would only work for outdoor systems. 
The "roof" of the box is going to let a lot of smoke out when you open it. 

You might be able to avoid this by configuring it for a horizontal door, with different drawbacks:
-  can't fit as much wood into the box easily with a horizontal door feed;
- hard to get the air to draw through the wood for proper mixing at all stages of a batch-burn;
- self-regulation of the air is lost with a big box, regardless of horizontal or vertical opening, compared to a hand-fed or downdraft rocket;
- and you have to reach in pretty far to clean the ashes out if you go horizontal.

It sounds like you've created something that works well for you.  I'm not suggesting any modifications; thanks for telling us more details about how it works, and I'd love to see more pictures when you have time.

-Erica Wisner
http://www.ErnieAndErica.info
Joel Hollingsworth
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marina phillips wrote:Exhaust is around 100 degrees, and mostly water vapor, you can put your face in it no problem.  It's cool to see billowing clouds coming out the pipes but smell no smoke.


Sorry if this is a stupid question, but do you suppose water is condensing in the exhaust pipes? Have you provided for removing that water?
Joel Hollingsworth
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Joined: Jul 01, 2009
Posts: 2103
Location: Oakland, CA
Any worries about galvanic corrosion of the steel, relative to the copper? It might be worth taking some sort of measure against this before you cob, such as painting the copper with barbecue paint.
Erica Wisner
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Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
Joel Hollingsworth wrote:
Sorry if this is a stupid question, but do you suppose water is condensing in the exhaust pipes? Have you provided for removing that water?


General comments on rocket heaters:

Water does condense in the pipes on other RMH systems, but not to the point where it would pool and do damage.  Earthen masonry is pretty forgiving up to moisture levels around 13-15%, and cob both absorbs and evaporates water readily. 

My gut is that any condensed water gets evaporated out again once the fire dies down.  Some amount of cooler, drier air flows through the warm pipes as the fuel load diminishes, especially if you aren't tending the fire super-closely and let it burn out completely before shutting the burn door. 

The time when RMH's tend to be really drippy is just after building them, especially if you build it in the fall when the weather is cold and wet.  Water comes out everywhere - the cob faces, the pipes, the drainage underneath.
We do try to orient the cleanouts so that water can drain down and out.
Joel Hollingsworth
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Thanks, that makes sense. I figured it had been addressed, but it's nice to know details.
                    


Joined: Oct 23, 2011
Posts: 0
Don't let the nit-picking bother you.


Thank you for saying that.    I understand that it's part of the process of talking about it, like you said, and it's productive, even if we don't follow up on suggestions.  It's partly why I waited til it was done to even reveal it to the board. 


Re: Only option for bathing:
My grandmother's family spent about 20 years bathing from a bucket that was heated on top of the kitchen range (after their indoor plumbing failed during the Dust Bowl, and their new well was too far downhill from the house and their plumbing was full of mud anyway.).  I can email you her directions for 2 girls to 'freshen up' using 1 pail of water, if you like.

So don't forget that hot running water is not the only kind of hot water.  Sticking a pot on top of the stove, and adding it to the lukewarm bath, might get you there a lot faster than trying to get the pipes and room heat to reach the perfect equilibrium.

(Now I'm seeing skinny galvanized pails, like milk jugs or flower vases, stuck one in each hole over your cleanouts, to get a lot of water up to bathing temperature quickly... sort of a cornucopia-octopus-of-steamy-delights ... funny mental image.)


I never turn down the opportunity to hear about old time wisdom.  I'd love to hear your grandmother's bathing routine.  And I too love that mental image of the various water containers down the clean outs!  ha! 

We actually call our method of bathing "taking a bathe" cause we generally don't fill up the tub, we have a couple of yogurt cups that fill up and we do more of a pour shower.  It only takes a few gallons for both of us, and then after we're clean, if there's still enough water and we feel like it, we take turns soaking in a full tub.  Kinda sorta japanese style.  We were recently discussing how heating some water on top of the stove to add to luke bathwater would definitely help the situation. 

The last two winters we had nothing more than a kettle on the stove or the public showers at the local elementary school.  Just having an actual bathtub is a vaaaast improvement.  The other largest water container we had, Robert wouldn't really fit into. 


There may be some benefit to moving the pipes upward, to catch more of the heat.  Not clustering them, like you said, but spaced out a little bit.  The difference in temperature does make a difference to the speed of heat transfer, and I think you'd still be pretty safe as far as steam goes.  Assuming you are using cob and not insulation, and have your relief valves close by and oriented safely.


Thing is, the pipe coming out the bottom of the water tank goes under the tarp of the yurt (to minimize holes we put in the wall), and to get the pipes further up the barrel we'd have to make the pex peice coming from the tank longer and/or get more copper tubing.  This is do-able, but I have a feeling that just the addition of cob is going to make a huge difference in the speed of the water heating.  If it turns out it doesn't, we can always bust the cob off, do whatever it takes to move the pipes, and re-cob.  We're willing to do things like that, but only after our original plan proves itself to not work.  If were talking about a difference of 15-30 minutes, I'd say it's not worth the effort of changing the pipe configuration. 


I'd love it if you could post another picture or two of your burn process.

I think you're talking about a batch-burn that's more like a masonry heater here, still very efficient, but not quite as clean as a well-tended rocket heater can be.  But I can see where it's much more convenient given that you need the fire to burn without supervision for several hours.

I think you're right, this would only work for outdoor systems.
The "roof" of the box is going to let a lot of smoke out when you open it. 


I almost did a photo shoot of starting the fire today.  But then I didn't.    I will though! 

When there is a fire every day, the amount of smoke coming out of the uncovered box is actually very minimal.  Today for instance there wasn't really any...but I've also spent the last four weeks becoming friends with this fiery beast.  Without the roof in place, the fire still burns sideways, but there is still a bit of smoke that wafts upward.  And when the roof is replaced you can hear the rocket noise get more pronounced as the draft consolidates. 

I would say "batch-burn" is pretty accurate.  We talked about it and it's really only more like an hour between stokings.  After several hours (say, three or four stokings) we let it die and it stays very warm in here for a long time. 

Around noon today, the yurt was warmer than our cabin.  We hadn't had a fire in the yurt in almost 16 hours, in the cabin the morning fire had just died.  And the cabin has better insulation!  I am SO sold on these stoves, heating up a large mass is the way to heat with wood.  If we weren't planning on the cabin being a fairly temporary living situation, we'd build one in there too.  We are definitely incorporating a (potentially even larger?) rocket mass heater into an interior cob wall in our future house design. 


Sorry if this is a stupid question, but do you suppose water is condensing in the exhaust pipes? Have you provided for removing that water?


When have you ever asked a stupid question? 

I read about the possibility of condensed water running back down the exhaust pipes to the exhaust chamber while we were building it.  We haven't seen evidence of that so far, but we haven't had a cap off the exhaust chamber in a few weeks either.  It's due for a look-see. 

I seem to remember that what I read suggested slanting the pipes slightly downward from the exhaust chamber.  I think we wanted that to happen, but then the chamber just sort of evolved as I built it, and the metal collars we used to direct the pipes thru the far wooden wall were a certain size and we just went with it.  This is the part that still has me a little worried.  I think the exhaust pipes are pretty level.  I'll definitely let ya'll know if this causes problems.  I guess at the very least eventually it would rust out our exhaust pipes? 

Any worries about galvanic corrosion of the steel, relative to the copper? It might be worth taking some sort of measure against this before you cob, such as painting the copper with barbecue paint.


Dang it, Joel, another thing we hadn't considered.    Thanks for pointing it out.  I don't actually know how copper corrodes steel or vice versa.  If you can fill me in using non-super-scientist lingo I'd appreciate it. 

Would putting a 1/2" of cob in between the barrel and the pipes solve the problem?  I can only imagine that barbecue paint would mean high temp stove paint, and that stuff off gasses something fierce in my experience.  Or is barbecue paint something different?  I saw in another thread you mentioned grease....what kind of grease wouldn't be flammable in that situation?  And would that prevent cob from adhering to the whole thing?
Erica Wisner
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Joined: Feb 10, 2009
Posts: 747
Location: Okanogan Highlands, Washington
    
  89
Joel Hollingsworth wrote:
Any worries about galvanic corrosion of the steel, relative to the copper? It might be worth taking some sort of measure against this before you cob, such as painting the copper with barbecue paint.


Once the cob is dry (within a few days) there shouldn't be much electrical contact between the two metals.  I'd avoid encasing in cob any joints that might leak, for other reasons as well.

You might expect corrosion also from the hygroscopic nature of clay.  Tools left with clay on them develop rust problems very quickly in our climate, because the clay attracts water and holds it against the surface.  But we see very little corrosion of the unpainted barrels after they're indoors, even encased in cob.  We do tend to oil the surface, but not the area where the cob seals to the barrel. 

Most of the corrosion we see happens to the barrels while sitting out in the rain, either as scrap metal or as prototypes.  We clean off surface rust as part of the building process; we avoid using parts that have rusted through.  Once installed indoors, we have not observed any degradation of these barrels over a decade or more, about 20 years is the oldest one. 

The custom barrel on this particular stove is probably thicker than any of the reclaimed barrels we've observed.

Interesting comment, though.  I was thinking of brazing the surface of our barrel as a decorative touch, and now I wonder if I should use a protective layer.  Or maybe I just need a fancy bronze or copper 'barrel' to go with my brazing. 
Joel Hollingsworth
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Posts: 2103
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Galvanic corrosion is what powers a traditional battery. One side becomes more metallic, the other side, more of a salt or an oxide.

Without getting too technical, metals share electrons. A given electron that holds together a hunk of metal is shared about equally by all the atoms in that hunk of metal.

Some elements really grab on to electrons: gold, platinum, carbon (as graphite) and, to a lesser extent, copper are good examples.

Others don't hold so tightly: iron and (more so) zinc atoms let go of the electrons that hold them in a metallic state fairly easily, to the point where atoms are ready to become free-floating ions.

Putting two different metals together tends to set up a tug-of-war situation, with predictable winners and losers. For a given liquid (seawater is a common example), you can determine a whole "galvanic series" where higher-up metals are safe in contact with lower-down metals, and the farther apart on the series you go, the faster you can expect corrosion to happen.

Put three metals together, and typically, only the lowest one on the series sees significant corrosion.

If everything's dry, this is no problem: a tiny charge builds up on either side, which stops the flow of electrons. But water is a tremendously good solvent in this circumstance, an ions tend to dissolve away into the water, allowing corrosion to continue. Rust isn't particularly soluble, but it is formed by a chemical reaction between iron ions and water (or oxygen plus water, etc.) and it is not waterproof the way aluminum oxide or titanium oxide or chromium oxide would be.

Clay is actually a troublemaker for more reasons than just its affinity for water: like graphite, clay can intercalate ions, so that they work their way between the layers of its atomic structure. That intercalation is what makes clay so good at holding nutrients until plants need them, but it can also absorb the ions that a corroding piece of metal sloughs off. Absorbing new ions can mean releasing old ones; some of the ions that might typically be in clay (nitrate, anyone?) can be corrosive in their own right.

As Erica implied, if you break either the electrical contact, or the path that allows metal ions to flow, galvanic corrosion will stop. Good paint stops both of these, holding out the water and preventing an electrical contact between metals.

Thermal grease Is made of silicone and boron nitride; it's used in appliances, from computers to coffee makers, and is fairly fire-retardant. It might not be trustworthy as a way of preventing corrosion: it's too expensive to cover the whole tube with, only a thin layer between the copper and steel that might wear through and allow contact. I don't imagine it will play nice with clay, either: oily clay doesn't take a shape the way it should, and silicone is oilier than most other sorts of oil.

If temperature-resistant paint puts out too-nasty of fumes, maybe fiberglass mesh, of the sort drywall hangers use to reinforce joints, would be a good way of preventing the copper and steel from making electrical contact? It's also possible to use zinc as a sacrificial anode. Putting scraps of galvanized anything in electrical contact with the steel (not, hopefully, with the copper) and in the cob should help protect it.

Brazing is a mixed bag. Brass includes some zinc, and unless you use cartridge brass, there will be zinc-rich regions at the surface, which act as a sacrificial anode. These don't last forever (I bet Ernie can tell you about zincs), and often a brass object gets burnished as it is used, to the point that the surface becomes more and more copper-rich. There are other brazing compounds than just brass, and I think some are designed to match carbon steel pretty well on the galvanic series.
tel jetson
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Joined: May 17, 2007
Posts: 3097
Location: woodland, washington
    
  53
seems like all the galvanized parts would be getting lower temperatures and high temp paint might not be necessary.  why not just coat the outside of them with linseed oil and either let it cure at ambient temperature or fire the stove before the mass is put in place to cure it faster?  wouldn't that be enough to break electrical contact?
Joel Hollingsworth
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Location: Oakland, CA
tel jetson wrote:
seems like all the galvanized parts would be getting lower temperatures and high temp paint might not be necessary.  why not just coat the outside of them with linseed oil and either let it cure at ambient temperature or fire the stove before the mass is put in place to cure it faster?  wouldn't that be enough to break electrical contact?


A good coat of linseed oil would be a great help. If you're sure it would survive, I think some magnetite, silt, chalk, etc. mixed into the linseed oil would help it in a couple of respects.

I believe it's more important to coat the copper than the steel: there's a much smaller area of copper, and it will be at a much more stable temperature, both of which make it less likely that a gap would open up.
tel jetson
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  53
Joel Hollingsworth wrote:
A good coat of linseed oil would be a great help. If you're sure it would survive, I think some magnetite, silt, chalk, etc. mixed into the linseed oil would help it in a couple of respects.

I believe it's more important to coat the copper than the steel: there's a much smaller area of copper, and it will be at a much more stable temperature, both of which make it less likely that a gap would open up.


I wasn't thinking everything through.  I was thinking the galvanized steel duct was going to be reacting with the steel in the stove.  that, of course, doesn't make any sense at all.  last time I had to think about galvanic issues it involved galvanized plumbing, so my mind was stuck in a rut.

of course it makes more sense to coat the copper.  though it sounds like protecting the duct from the clay might be worthwhile, too.

as far as the oil surviving: well, I hope it does as I'll be trying it out soon.
Erica Wisner
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  89
I don't think there will be significant electrical contact in this case.

Indoor appliances of many kinds use mixed metals (copper/steel, steel/zinced bolts, aluminum/steel screws on pots and pans).

Galvanic corrosion occurs when there is either submersion, or a constant trickle of electric current.

I like explaining things, so I'll give it a try too:
Some metals rust easily, others don't. 
(By 'rust' I mean corrosion, similar to the natural state of the metal's ore: white for zinc and aluminum, russet for iron and steel, green for copper or brass, black for silver, seldom seen on gold, etc.)
The galvanic series organizes metals by how much they 'want' to rust.  (Regardless of the shape or color of the rust - aluminum actually rusts really easily, but forms a skin that stops the rust at a very shallow layer, where iron rusts and flakes off and keeps rusting deeper and deeper.  That's why it's such a surprise to many people when their aluminum boats corrode quickly, because in everyday life we think of aluminum as rust-proof, when in the galvanic series it's right down there at the bottom.)

if you have two metals in contact, of different kinds, one of them is going to rust away entirely before the other one starts. It basically sacrifices itself and in so doing, preserving the other metal.
Or you could say the other metal steals all its anti-corrosive power (electrons) and forces it to sacrifice itself.  Metals which steal electrons in this way are called "noble," reflecting deep-seated social prejudices. 

If conditions are dry, or the metals are immersed together in something non-conductive like rubber or paint, not much happens.  But in a conductive solution like salty water, the reaction can happen before your eyes.

We used to demonstrate this for schoolkids with a home-grown version of the Tarn-X commercial. 
Prep: Polish some silver spoons until they gleam, then shake-n-bake them lightly in a bag of powdered sulfur until you see a thin, but dark, layer of surface corrosion. 
With students:  Warm up some baking-soda water, drop a piece of aluminum foil into it.  Dip the 'tarnished' spoon in.  Nothing happens.  Press it down against the foil, and wait about 20 seconds.  Ta-DAAA!  The Silver is restored!  (There's a faint surface scum of powdery material, but the silver shines through).  After a few spoons are shined this way, the aluminum starts to look like ashes or Swiss cheese.  The foil is the sacrificial metal, giving up its electrons that the silver may shine.
(Since I learned this, I have sometimes used aluminum foil with wet baking soda on it to brighten my few battered pieces of silverware - not as soft as Wrights and flannel, but satisfying nonetheless, and probably better for them than simply rubbing off the tarnished layer.)

Zinc is an even better pushover on the corrosion market, and it's cheap.  It can be welded or bolted onto almost any other metal as a sacrificial anode: the zincs rust, and as lont as they're there, the other metal stays intact until all the zincs near enough to affect it have rotted away.  Zincs are used to protect ships, submarines, and sailboats, and they nowadays come printed with permanent instructions, "O NOT PAINT."  Because of course, if you paint over the zinc, it won't corrode, and it won't protect the steel, and then the steel WILL corrode.  And ships with rust holes are a whole lot uglier, and leakier, than ships with plain, serviceable grey zincs.

This preferential-rusting (galvanic corrosion) happens incessantly wherever dissimilar metals are underwater together, or connected by any electrically-conductive fluid.  It can also be created when just one metal has an electrical current running through it and into the water, where the metal is losing electrons and they are replaced by corrosion ions.  So understanding the galvanic series is critically important for boats, and boilers, electrical devices, and industrial chemistry.

But I can leave silver and cheap steel forks in a drawer together, or even in a dishwasher together, and the worst I see is a little tarnish.  Copper-coated steel pots, and shiny copper-coated steel wires, rust VERY quickly outdoors, but not in my kitchen drawers.  Galvanized and steel fittings are sometimes used on plumbing fixtures to attach joints from the outside (like a wall plate), just not inside the tanks.  Brass is screwed down with steel screws, and is still holding decades later. 

Indoors is usually dry enough that galvanic corrosion is not a problem.

I would be willing to bet that the physical contact between copper and steel won't create any significant problems, even if it does encourage some surface rust.  Oil the barrel, and you shouldn't see even that.  As long as the pipes aren't leaking (and that's WHY the pipes are copper, so they will be least susceptible to corrosion), there shouldn't be enough water for the electrical exchange to occur.

Now, smoke is acidic, and ash is alkaline, and rocket barrels contain ions inside which might freakishly charge the barrel, and yurts drip, and bathtubs steam, and I could be wrong.

But rather than separate the pipe from the barrel with cob and reduce your heating, maybe it would be clever to use galvanic protection? 
You could use zinc ribbon, or thin galvanized metal, to make a slim bridge between the copper and the steel.  Then you can check for the presence of galvanic corrosion by sacrificing the zinc, without sacrificing the heat conductivity.

That would be the fancy geek solution.

The practical solution would be to proceed as planned, note the place most likely to get wet, and inspect from time to time for signs of rust.

I really don't think it's a concern for an indoor situation, though.
Jami McBride
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Joined: Aug 29, 2009
Posts: 1779
    
  10
I am really enjoying all this information on the science behind corrosion - good stuff to know along with the details of building rocket mass stoves & water heating systems.

I'm teaching my daughter about the science of atoms just now (homeschooled you know) and I love your experiment for kids Erica.  Science was never my thing in school, but my daughter is loving it!  So maybe we'll give it a try.  You sound like a fun educator    
                        


Joined: May 26, 2010
Posts: 278
Location: Iowa, border of regions 5 and 6
This may be a stupid question: After reading all this stuff about bending the copper around the barrel, is it worth trying to flatten one side of the copper for better contact with the steel, etc., I was struck with a thought.  If your big concern is heating water, why not build a separate system for heating the water?

By that I mean that it is simple (a LOT simpler that the RMS you built for the yurt) to build a rocket stove  with an inset at the top for a pot.  By that I mean at the top of the chimney, you build a space that will hold a pot with about 1/4 inch space all around the side of the pot.  The gasses will heat not only the bottom of the pot but the sides, and so heat the water MUCH faster.  Bring the water to a full boil, dump it into your water storage that you're using to try to heat the water, fill the pot again and repeat.  You'd cut the time needed to heat your water in half, I'd imagine.
Erica Wisner
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Joined: Feb 10, 2009
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Location: Okanogan Highlands, Washington
    
  89
Muzhik wrote:
This may be a stupid question: After reading all this stuff about bending the copper around the barrel, is it worth trying to flatten one side of the copper for better contact with the steel, etc., I was struck with a thought.  If your big concern is heating water, why not build a separate system for heating the water?

By that I mean that it is simple (a LOT simpler that the RMS you built for the yurt) to build a rocket stove  with an inset at the top for a pot.  By that I mean at the top of the chimney, you build a space that will hold a pot with about 1/4 inch space all around the side of the pot.  The gasses will heat not only the bottom of the pot but the sides, and so heat the water MUCH faster.  Bring the water to a full boil, dump it into your water storage that you're using to try to heat the water, fill the pot again and repeat.  You'd cut the time needed to heat your water in half, I'd imagine.


Not a stupid question.
Wise suggestion, I think.

The "heat a pot" is my best recommendation to anyone trying to create a DIY warm bath for the first time.
- Cheaper by far (the copper tube costs something like $5/foot).
- Safer (provided you don't dump the scalding pot on yourself at too high a temperature).
- Easier to achieve success without advanced degrees or experience of fluid mechanics.
- And likely quicker, as you say, both to build and to reach temperature.

If you want to get fancy, you can build a permanent pot or tank (open at the top, or with a substantial relief valve like a large version of a pressure-cooker, say a spout with a 3lb weight). 

A spigot out the side, and you could draw off whatever amount of hot/warm water you want to use.  This would avoid needing to lift a heavy pot of hot water.

You can either create a custom enclosure that forces exhaust past the pot as you suggest (Aprovecho method, works great as long as you only use one size of pot), or place the pot atop a barrel that serves as a heat-transfer surface (works for any size pot, and makes it easier to remove for refilling or cleaning, but some heat is lost in transfer).

If the pot is not surrounded by an exhaust-channeling sleeve, then an insulative sleeve (and lid) might be useful.
 
 
subject: rocket mass floor heater -- finally completed and it works!
 
cast iron skillet 49er

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