Questions for Ernie and Erica----Large RMH

Mr. Wisner and his wife Erica are our resident experts on these matters. Please give them a couple days to respond before the wild speculation begins in ernest.

Hi Ernie. I wonder if you have had experience with any very large units? I'm thinking of something that would have a feed door between 12 and 18 inches in diameter and able to accept large diameter wood. I would like to enlarge everything from pipe diameter to length of run and have a riser that is also proportinately larger. The reason for this is simple. I want to heat a large space and to use the RMH to heat water for a hot tub and for other uses. I could use a portion of a 200 gallon propane tank for the riser. A 6 to 8 ft. rise would be possible since it will start out in a sunken pit.

Has a unit of this scale been tried?

Would I be better off building a seperate unit for each purpose and sticking to standard sized components?

Has a unit with multiple feed tubes been built?

Does the riser need to be round? Could something oval be just as good? I'm thinking an old oil tank stood on end.

I intend to build a typical sized unit in a small guesthouse before going bigger.

I intend to purchase the book. Anyone feel free to drop in purchase info for the book by Ianto Evans.

I would like to hire some expertise when doing the big one and would provide accomodation for as many people as required for a workshop. Food , entertainment and a bus tour of the area included. It is a very nice location. I could pick up people at the airport, the ferry and in Victoria. I'm 8 miles from Nanaimo on Vancouver island. I'd also be interested in attending any event in the Vancouver area and I could transport those who live on Southern Vancouver Island.

Pursuing permits under the alternative building code. All of my installations will be tested in various ways and the data made available to all.

Thank you in advance: Dale Hodgins

yes to most and it works as long as your materials can take the heats; to be frank not many can. a 10 inch system is going to burn at around 4500 degrees in the hot spot of the heat riser on a regular fuel load. plenty high to melt aluminum and steel in seconds it will also vitrify clay.

no the heat riser doest have to be round.

On large systems you are looking at masonry for most of the combustion unit and barrel/bell that is the only material I know of that will handle the heat. ducting needs to be pretty hefty as well. at the 10 to 12 inch size you are dealing with very high duct heats. the last 12 inch system had 1000 degrees in the first part of the ducting (its scary when Cob starts glowing) we had 6 inches of cob over the ducting. I would suggest that you test a big system well away from things that can burn and keep it in mind that you are playing with something that in the smaller sizes already generates thousands of degrees.

When you are ready to plan your stoves give us a call and we can help. we also have the book and plans already drawn up.

Dale Erica and I are in Tonasket WA. if you wanted to stop in and look at our stove let us know. the number in my Sig is our cell phone.

a 10 inch system is going to burn at around 4500 degrees in the hot spot of the heat riser on a regular fuel load.

What fuel and what oxidizer? I don't think you use pure oxygen, do you? Because 4500F is roughly, what you could possibly expect from that... hellfire...

Wood and air are strictly limited to ~3600F => adiabatic flame temperature
The value is the upper limit, because you can't reach it in a real setup.

ya folks keep telling me that; then i burn steel in the heat riser and melt pyrometer ends. I believe that what happens is the gasses begin to burn and many of those can go to higher temps. We have seen evidence of smelting temps in the burn tunnel of some designs and have burned stainless in a matter of hours but i cant tell you what the reaction is that is making the rocket hit temps so high. I am assuming that we are seeing a gas breakdown and the systems is scavenging O2, or the bricks are pushing the heats higher and higher. at this point i just dont know; its not like i get to much call for 10 and 12 inch systems. most folks dont want a thousand degree object in the house.

Well, nearly every steel is molten at 2750F (i know no "steel" that isn't) and subject to fast oxidation even significantly below that. One of the best materials sometimes available on the consumer market (2.4605 or "Alloy 59") is resistant to scaling up to ~2200F, and this is a LOT! Wouldn't expect the wall temperatures higher than that, even if the gas is hotter, because the metal looses energy by conduction. Normal stainless steel (1.4301) starts scaling somewhere above 1100F, non-stainless steel much below. Very slow at first, but roughly doubling corrosion speed every 20F. Thats just a rule of thumb, to get the picture... exponential development is not our friend, again...

So its not very remarkable to destroy even a stainless steel-heatriser in a matter of hours, if you really push it. This adds energy to the combustion, but it cannot go that high. If it scales, theres oxygen left, which means, its not a perfect combustion for maximum temperatures. If you could reach the melting-temperature of steel, it would be quite awesome with such a design and fuel!

Oh we do get temps that melt steel and glass and aluminum and clay and pretty much anything you care to toss in. the sweet spot in the heat riser is about 2/3 rds the height. this is why we worked on 10 inch systems in the first place. We managed to melt a crucible. but i have not made a steel lined heat riser in the system due in large part to the heat. If i get demand for those big systems i will start working on them again. for most applications an 8 inch system suffices.

Ernie, thanks for the invite. I'll definately do that one day. Hopefully in conjunction with a workshop for myself and a bus load of eager Canadians. The book is something I need to get. I'll drop by your website to see if you have any manuals to sell. (I'm not one of those idiots who thinks that it's OK to steal. Development of this technology has taken much effort and time and I will pay my fair share for educational materials.)

Is the move to Washington permanent? You'll get some good cold winters for testing new designs. And your thermal mass may help moderate in summer as well. I experienced 104F in downtown Kelowna not too far north of your location. There must have been plenty of mild days in Portland when you would long for some real winter conditions to prove your designs.--------------------------------------

Back to oversized rockets----I didn't realize those temperatures were possible or likely or that a larger system automatically equals higher temperature. So it looks like I need to stop giving away all of my firebrick. Increase size in moderation.

Another consideration will be the maximum temperature that the hot end of a Stirling engine can handle. I suspect that they can't take those extremes. I'm shopping for antique models.

Would the inclusion of a copper coil for water heating say half way up the riser be able to prevent the tube from melting while still allowing 2000F+ in the center of flow?

Would the convoluted surface of such coil contribute to desireable turbulence?

I've done some pottery and used cones that melt at various temperatures. Do you use those?

Some plumbers use little cameras on the end of a snake for inspections. Have you used those?

And finally do you ever install mica windows for inspection or asthetics and is mica the best substance for a window?

just thought I'd put in my $0.02 worth as a founder and metalurgist. My guess is that you are creating a very oxidizing atmosphere in your heat riser. you might want to try using a high alumina refractory for your riser. Dale and Erie, I am not sure who is in the seatle area for refractrories put I have used united western supply in portland for refractories and foundry supplies and would recomend them if you don't have someone closer.
kent

Ken Peavy works in refractories and he has also mentioned high alumina refractory for the riser. Seems to be a consensus.

If you are gonna get a bunch of eager Canadians We will come to you better us travel than several folks. Send us an email and we can get you the book.
Yes we moved to wa permanently this winter has been a nice test and has shown that we are on the right track with our basic stoves. its a bit warm today but its been below freezing here for weeks and we have a good snow cover. our neighbors have used 3 cord so far this winter while we are still working on a cord. not bad.
ok i'll get to the rest later.

3 more things:

1) If you get heat riser temperatures of >2500F, maybe it would be be useful to reduce the heat riser insulation a little. Thus, more heat is conducted away, lowering the material stresses, while the temperatures of 1800F still do it to ensure a complete combustion. Has someone ever tried that?

2) I just did some "coaster-calculations" (some fast assessment) on the power of a Rocket stove. A 10''-System would be able to reach 40 kilowatts. With a 1meter heatriser, 2200F Tmax, 77F room temperature. It surely somewhat idealized, but is that halfway realistic? To get it more precise it would take me much longer...
1. Volume flow equation from Stack effect
2. Ideal gas law (gas is still of nitrogen majority) to get the density of air at 2200F
1+2 => mass flow through the stove
3. "heat capacity of air" * "mass flow" * deltaT = ~40 kilowatts... If thats true, who personnally needs more?

/edit:
3) I've somewhat surprisingly found the rocket stove book in a library. It`s the 2006 edition. Are there any major changes since then?

wrong post - didnt find he delete button, yet!

Ernie-The only reason I would do the drive is if it were part of a green building bus tour. There are a few towns in B.C. where cob, earthships and strawbale have taken off. Nelson comes to mind and many homesteads in the Okanagan Valley just north of you.

The idea would be to have one of your workshops as the final event and destination. It could be on either side of the boarder but right now I'm only licensed for B.C.

Dale, I've got a piece of 8' diameter by 15' long by 1/2" wall pipe if you need to upsize your outer barrel. Hahahahahahahaha

we could work with that I think. I didn't know there was a tour set up for the local natural buildings. Well when one is scheduled let us know in advance and we will see what we can do.

C.J. Murray wrote:Dale, I've got a piece of 8' diameter by 15' long by 1/2" wall pipe if you need to upsize your outer barrel. Hahahahahahahaha

Sounds like just what I need for the drum on a large paddle mixer for preparing cob. Or it could be the beginning of a trommel. Can you drop it off? Stood on end with a good lid, you'd have a nice little silo. The kids might like going down hill in this thing. Sliced down the middle it'd make two great elephant barbeques. Or with wooden walls capping the ends you've got a guest house.

I could insert it into the side of my hill, loaded with shrapnel and use it as a propane cannon to control canoe trade on the river 120 ft below.

Okay, boys,
I've completely forgotten Dale's original questions, but at least I'm getting a good laugh out of it.

We would be delighted to drive a few minutes across the border to join your tour in the Canadian Okanogan; spring or summer would be the best times this year, if we set dates soon.

Although I don't think we should make your silo-rocket-project a topic of conversation with either border patrol.

-Erica W

Paul talks with Ernie and Erica about rocket mass heaters in this podcast: rocket mass heater podcast

They answer this question.

Hi there I'm just new to these ideas and concepts but am very excited by it all!! I just had a question about whether or not you need an outside air source to feed the stove. It seems to me that with all the draw of the stove it would suck the oxygen out of the area which could be dangerous. Any info would be greatly appreciated!! Thanks for al you do guys this site is amazing!

Shane if you can breathe in your house the stove can to. you dont want any house to be the current norm of a ziplock baggy.

Shane Williams wrote:Hi there I'm just new to these ideas and concepts but am very excited by it all!! I just had a question about whether or not you need an outside air source to feed the stove. It seems to me that with all the draw of the stove it would suck the oxygen out of the area which could be dangerous. Any info would be greatly appreciated!! Thanks for al you do guys this site is amazing!

Hi Shane, and welcome!
We've been getting this question a lot lately; seems the 'outside air feed' is popular again. Gary tried to improvise one onto his project, and I'm hoping it doesn't wreck his rocket mass heater by cracking the firebox or providing the smoke with an unintended path to freedom.

Thanks for posting it, though, and I'm just going to give you the whole lecture piecemeal. You may want to read the book or keep scoping out these forums for the basic principles.

As I write, I have our heater running, and the door ajar, as we just broiler-grilled a massive steak. I am warm and the air smells good. I will get up and shut the door eventually. When you consider the length of this post, and the fact that I am now re-writing this paragraph for the third time, you can see how worried I am about drafts. If my toes get cold, I can stick them under the cushions on my heated bench, and enjoy some 'me' time. I want to build a RMH in our yard here, with a little trombe-walled gazebo, so we can sit on it in our 0-degree weather with 3 feet of snow and show how very little 'warm air' you really need to stay cozy.

I learned from an energy auditor that a healthy house needs to 'cycle' - meaning ventilate and replace - air at about 1/3 of its volume per hour. That means that if you rely on warm air to heat your house, you are effectively re-heating your house every 3 hours. It is very difficult to prevent some 'drafts,' by which most people mean the cold air flows around windows (actually adiabatic cooling), or the actual cold air leaks around doors and sagging house timbers. In fact, the healthiest way to prevent these cold drafts is to deliberately create some warm ones: you could channel outside air along the back of the rocket mass heater, out into the room, so you had a warm 'draft' and the house didn't pull in as much cold air. Or channel some air in along the walls and ceiling, with the outside vent low and the inside vent high, so it can't draft warm air out of your house like the roof vents do. All houses must breathe, to prevent molding and rot in the house materials as well as for the occupant's ongoing supply of vital oxygen.

Assuming your house has adequate ventilation through smaller cracks in walls and floors that you don't notice (proven by you not being dead yet), you can also put a sand-stuffed legwarmer snake in front of the door.

It is true that if your house is too well sealed, or if it has good roof vents but no air inlets lower down, you can get a situation called 'negative pressure'. That just means the pressure inside the room is lower than outside, so when you put in a new hole (like a chimney), air wants to come in it rather than flow out. Any combustion device may not be able to draw properly with negative pressures (the lab tests check for building pressures up to about -5 something-or-others, units anyone? Pascals?).
If you burn stuff and it doesn't go out fast enough, you are more likely to get smoke or carbon monoxide poisoning in your house from stove or chimney leaks. A well-ventilated house will draft more air out the same leaks and keep you safe. The problem is that your house is too well sealed; adding outside air for your stove just mitigates the new problem of the woodstove suffocating. It does nothing about your old problem of YOU suffocating: breathing unventilated air that probably contains mold spores, off-gassing VOCs from your sealant materials, cooking smoke, etc.

We can talk about ziplock baggies and other unsuitable human habitat until we are hoarse. This was one of the lectures that Paul was lucky enough to get from 7 different instructors at once, and he was brave enough to keep asking until he got enough answers to really let it sink in.
But you won't really 'get' it until you experience radiant, masonry heat, in a setting where warm air would be useless. Paul describes this in one of his podcasts.

I can't stack bricks for you over the Internet.

But I can tell you that for the first year or two I lived with a masonry heater, I sometimes woke up thinking "Oh, cool, spring came early! It's going to be a warm day today." And on several of those occasions, I looked outside and it was actually snowing. Our frugal friends who keep their house at 60 degrees, used to come over for dinner, and routinely fall asleep on our warm couch.

We just aren't that concerned about drafts, because the thermal inertia keeps the ambient temperature much more even.
We do ensure adequate ventilation for our own comfort, and the fire does fine too.

If you did want to add external air, without scotching your project:
The J-tube downdraft heaters and stoves need to suck air in alongside the fuel to avoid smoke-back in the house; adding an air feed anywhere else in your system is like putting a hole in your siphon to relieve the 'pressure' on the water intake filter.
The fuel feed is low enough that it mostly tends to use cooler air from the floor, and you can set them below grade so the feed is at floor level. Ianto's is right next to his door which has about 1/2" gap on all sides for moisture expansion; I'm sure that the 'draft' just rolls right down into the stove and I've never felt drafts in that building even when there was a cat-door in the temporary plastic window. It's passive solar, lots of thermal mass, and good temperature stability. There's probably an updraft on the outside south-facing walls that means the windows aren't cold, either.

You can bring outside air close to the mouth, if you really want to, and it will work OK with the draft. It is best if the RMH's air is not pre-heated, as you want the air to cool the top of the wood and keep the fire down at the bottom of the sticks. You also want the sticks below the lip of the fuel feed- many pictures show extra-long sticks so it's clear where they go, but this is like showing a woodstove with a stick poking out its door. Not how they're designed to work.

Another option for the air-feed obsessed would be to build an almost-rocket-mass-heater with a horizontal fuel box instead of the downdraft feed, and plumb outside air in. But it will not burn as cleanly, or efficiently, though possibly still a vast improvement on a woodstove.

That is way more information than you asked for, and I admit I'm just gathering it all up into one place.
Maybe we should combine all these into a book ... there's some useful technical stuff on these forums that wasn't included in the original RMH book, and sometimes builders want to skip ahead to the technical stuff without wading through the principles that make it all work.

Ok, it's been 40 minutes or so. Instead of going back and adding something about stick-frame houses being a gussied-up tent for self-deluded nomads who think they have permanent housing, yet move every 5 years, I'm going to shut the door now. One of my toes is finally getting cold.

-Erica

Erica Wisner wrote: Instead of going back and adding something about stick-frame houses being a gussied-up tent for self-deluded nomads who think they have permanent housing, yet move every 5 years,..........

lol, sums it up well. The construction industry dresses it up as "high quality, sustainable, efficient, modern housing" and extracts loads-a-money for what is really a shed with drywall walls and some cladding.

Any combustion device may not be able to draw properly with negative pressures (the lab tests check for building pressures up to about -5 something-or-others, units anyone? Pascals?).

-5 psig or Pascals would be way negative.

I work at an old nuke plant (to kept food on the table and a roof over our heads) and we want the plant to be negative to keep the bad guys in the filters. -2 psig is almost too negative. More than that and we can pull water out of the loop seals in the floor drains.

So I think -5 is -5 water column, which would be -.18 psig which is doable without fans.

Or I am complete wrong and full of shit...

no we see some odd stuff in stove placement when the negative pressure can stick an entire sheet of 3/4 inch ply to a vent its pretty strong. the stove cant handle some of these situations.

we once built a stove in a building shaped like a wing; we went with what the client insisted and the stove failed. had we followed our instinct and experience we would have either done it our way or walked.

I also remember building 'punchlist' walkthroughs where we were trying to get the pressures below 5 lbs - on the doors or handle - to meet ADA.
If you had to push the door open or shut with any force, the air-handler needed more calibration so people could get in and out of the rooms as 'reasonable accommodation.'

There were a couple of doors I could barely get open, and I sure wasn't going to let them shut with me inside. I can't pull as hard as I can push.

This is one of the reasons I think the HVAC engineers don't have it all quite figured out yet. (The building had been engineered, and these adjustments were common before turning large buildings over to the owners. My boss said their equations are derived from water, which is non-compressiible.) Modeling the rocket mass heater interior draft seems likely to be in the delicate realm of what we don't know yet.

Playing it safe for a given building means adding more fans and louvers for more control.

Playing it safe for the planet might involve getting a better intuition for the passive ventilation systems like solar chimneys (and stairways) and how to operate them with less outside power.

Otherwise a lot more of us smart people are going to need to take on your type of job, Daniel.
And however smart we may be about wearing bunny suits, collectively I don't think we're wise enough to handle waste disposal with a 10,000-year half-life. Maybe Hanford's glass turds are sufficient, I don't know, but I suspect we already have messes out there we can't clean up. Every tsunami is a bit more in the biosphere. History and statistics are against us, but until we quit cold-turkey, somebody has got to mind the monsters. It sure won't get better if we leave it to people who see nothing wrong with the idea, any more than it is a good idea to leave jury duty to dumb folks who can't get a note from their boss.

Hmmm. Grumpy erica. Must be bedtime.

I sometimes hear people talk about wood burning devices "sucking all the oxygen out of the room" as if the stove can somehow refuse the nitrogen and minor gasses that make up aprox. 79% of the air. You tell them that stoves take in air and they'll insist that it isn't so because you can't burn nitrogen. It's actually a good qualifying tool as idiots self identify. :

Dale Hodgins wrote:I sometimes hear people talk about wood burning devices "sucking all the oxygen out of the room" as if the stove can somehow refuse the nitrogen and minor gasses that make up aprox. 79% of the air. You tell them that stoves take in air and they'll insist that it isn't so because you can't burn nitrogen. It's actually a good qualifying tool as idiots self identify. :

I think this kind of mistake is pretty common among people who are book-educated. (Smart maybe, but inexperienced / unwise.)
Similar thinking as far as "if white light is all the colors, why do I get a muddy dark mess when I mix all my paints together?"
(http://erica-k-wisner.hubpages.com/hub/Why-are-primary-colors-different-for-paint-and-screens)
If the only context you have for something is words on a page, not experience, it's easy to mistake the relationships between the things people are discussing.

It's scary, because the same kind of thinking can be found (and encouraged) in political and marketing circles, and can be very misleading, and influences a lot of major public decisions.

Like 'high-quality protein' being a characteristic defined by which diets cause farm animals to pack on weight the fastest, for agricultural production. And then the meat industry taking this up as a commendation of their product, which often is quite tasty food. And then people trying to include lean, 'high quality protein' in diets to lose weight. And getting ketosis on the Atkins diet.
And well-meaning NGOs and missionaries, and school cafeterias, trying to increase the 'high quality protein' in the diets of malnourished children everywhere, who do indeed need more food, and a 'high quality diet'... which for a vulnerable human young person is very different than for a porker or beef cow with a 2-year life expectancy.
http://www.ted.com/talks/ann_cooper_talks_school_lunches.html (<-- Ann Cooper offers a longer rant on this subject.)

As a person whose metabolism gets really messed up by eating refined sugars, I find it sadly laughable that 'lite' salad dressings substitute HFCorn syrup for the more expensive salad oils.

There are a lot of very, very subtle variations on this: biochar, for example. CFL's. Anything labeled 'good' without the relationship being defined: "good for...." Good when.... Look who's advocating it; and the most consistent relationship will often be, "Good for my interests."

Ernie and I are no exception - yet we are, sorta, because we keep getting deeper into stoves and heating technology when Ernie's major dream in life is to get back out on boats. He could be doing very moving presentations on the reality and importance of global warming, based on his Arctic experiences... but maybe the smokeless, fuel-miser heating is the most efficient thing we can do, and the thing people are most likely to appreciate, to directly impact this issue. Reduce our American carbon footprint and save the Arctic and the oceans we know and love.
Distant connection, at first glance: aren't fire and water opposites? but starkly real, when you look at where our household energy (from fossil fuels) goes. We have a much better shot at helping with this than with transportation or agriculture, because we have more to offer. By encouraging a lifestyle where people on farms get to feel luxuriously well-off while staying cozy at home... and people who flit back and forth between vacation homes are ineligible for this superior technology... maybe we are addressing all three.

The big picture has near-infinite resolution, and it can be hard to spot the little errors of definition.
Mostly I look for glossy spots that may indicate air-brushing, or dark shadows that may have been added to make us fear the unknown (and self-sufficient) so we purchase the glossied-up stuff instead.

-Erica

We are considering replacing our wood stove with a rocket stove. The wood stove is fairly small and has an oven but at 16 years it's showing it's age and parts aren't available anymore (at least in US). It sure is nice though to sit and watch the fire.




Will a rocket stove heat a very tight, efficient (R38 on 6 sides) 2 1/2 story house which is 2400 sq ft? We currently burn 2 - 2 1/2 cords per year depending on the weather and the wood. People are stunned that we use so little wood.

We have not settled on a design, I haven't done enough research yet.

Very interested in a glass door (like matt walker's), an oven, and warming plates.

Cj, did you ever get an answer on heating your 2400 sq ft home?

No answer.

I'm sure it will work, it's just that there isn't much low hanging fruit (energy efficiency wise) in the house so it's a bit hard to get sooo excited. To go from 2 1/2 cords to 1 1/2 doesn't seem huge, since it's all wood from our property.

I'm hoping to set up an outdoor rocket stove this summer for bread and whatnot, though I can't get sooo excited about that either because I'm on a low carb diet. If the rest of the family wants bread, they'll work on it.