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Rocket Mass Heaters on wooden floors  RSS feed

 
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I'm looking to build a rocket mass heater above a suspended wooden floor and would like to know how to go about insulating the floor from the extreme heat that the stove will generate.  I have Ianto's book with a brief description of one technique but would like a little more guidance on this.  In particular how to mix and set a slab of insulating mass that is also capable of supporting the weight of the stove.

This stove is going to be used to demonstrate the benefits of rocket mass heaters at the Eden Project, Corwall, England.

Cheers!
 
gardener
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Just wondering.... Do you have a full basement under this suspended wood floor, just a crawl space, or what?
 
Cornish Nathan
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The wooden floor is actually a large outdoor-stage under the cover of a huge permanent tent structure.  The stage is supported via massive metal posts and metal scaffolding onto which is a sub-floor of laminated ply-wood, topped off with plywood.  Underneath all this is a lake!

The owners of the stage (my bosses!) definitely do not want to lift the floor so everything must sit on top of the ply wood, must protect the floor from the heat generated and be able to bear the heavy load of brick and cob!

The rest of the stage is being transformed into an ice-rink for the winter season and the whole thing will only last until late February.  So what I'm really trying to do is to build a rocket mass heater with possibly up to 8 metre cob bench to demonstrate to our visitors (and my bosses) what these fabulous contraptions can do.  Hopefully after this winter trial I will then be able to build a permanent rocket mass heater in another location, and NOT on a wooden stage.

We're a major environmental/educational visitor attraction and will expect between 150-200 thousand people to come to see, play and learn stuff with us this winter.  My intention is for a large percentage of these people to 'warm their butt's' on the bench whilst I explain to them the benefits of Rocket Stoves, Mass Heaters, etc.  I'm also keen to be warming soup or maybe even roast chestnuts on top of the barrel.

I've been demonstrating the Winiarski Institutional size rocket stove for the last 2 years here at Eden with great results, and this would be a fantastic development of all things Rockety!

If you'd like to see more about Eden go to www.edenproject.com.  It's a truly inspirational place.
 
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Any chance of some pictures and more info on the strength of the structure? 

I think the primary concern would be the weight of it all. 

I'll send erica an email and ask her to take a peek at this thread.

 
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the largest full system i would put on a wood floor is 8" and figure the load accordingly. cob is lighter than water by the cubic foot so you can calculate you load requirements by water and build in some safety margin.
I think the best small demo would be 6" system with thirty foot of exhaust ducting in three passes through a bench. build a frame for the whole thing and fill with the perlite and clay mix. the critical area is under the burn tunnel but to be safe as you can put the foundation under the whole thing.


Perlite and clay mixed and packed into a 2"X4"  or brick frame  then build the stove on it.  this should give you a perlit/clay layer 1"3/4" (2"X4" to 2"1/2" (Brick) thick that supports the stove fine. if you want more insurance  build the frame  for the whole stove with brick on edge then layer in; 1 inch thick sand bed then fill with dry perlite , cap with a trimmed bit of 3/4" mason board  build the stove right on top of it.

the only part of the stove that gets really hot us right around the burn tunnel this section is about 1 meter long by 2/3 of a meter wide.

I will have Erica look at this later and she will i am sure make it far more clear then I. 
 
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You're building a big heated cob bench... beside an ice rink... on a wooden floor... suspended above a lake?

Wow, that's either the coolest thing I've ever heard, or a recipe for a giant mud-pie.
I'd love to help with it.  (Reminds me of a blacksmithing class I once took, that was housed in a temporary, plastic-lined snow cave beside the arts building at my college campus.)

Here are my responses to the issues you raised, and to the whole project... please let us know if any other questions arise.

- Weight is a factor.  If the floor was built for the ice rink, it may already be engineered for excess weight.  Cob weighs about 95 lbs/square foot, maybe 80 lbs/sf when you subtract the ducting voids.  Brick and concrete (used in foundations and burn tunnel) are 100-150 lbs/sf.  An 18" tall cob bench might weigh 2-5 tons, but it's spread out at about 140 lbs/sf.

- Insulating to protect the wooden floor is good - Ernie said 2" or so of perlite, but I'd be tempted to go with 4" especially under the combustion area. 
  You may also need to insulate all the way around the stove.  Air gaps outside this insulation are a very good idea, to let convection move any excess heat away from the supports and make it available to your guests.

Structural insulation is pretty do-able, you can even look up the commercial standard for Masonry Heaters or woodstoves and follow their parameters for heat shielding and weight support.

- Cold Ice, hot earth:
The other priority is to make sure you don't have thermal transmission between the ice rink and the rocket bench.  Ice rink that needs to stay frozen, earthen mass needs to stay dry.  Working against you are conductive metal support members, water vapor from the lake, spilled drinks... Wet cob and wet insulation transmit heat better, which could result in a runaway conflict between your melting ice rink and your melting hot mud bench.
  Earthen masonry can take about 13 percent core moisture before it becomes structurally vulnerable, according to Mike Wye from Devon - since you're in the UK, you might want to check his website.  Good tips on cob generally, and might have some insight into using local earth etc.

  Even with 2-4" of DRY insulation, the plywood may get warm.  Ordinary, the floor or walls reaching 70 degrees to 85 degrees (20-30C?) would be a benefit indoors.  But if you're trying to maintain an ambient temperature below freezing, for the ice rink, then more separation is desirable. 
  Raised floor, with air space in between, could help.  Both vertically, and horizontally.  I imagine you'd have floor space between the ice and the cob anyway, so people can enjoy the warm spot without getting bowled over.  There might be a place for a foam-filled barrier somewhere between the two, to keep the hot side hot and the cold side cold.

- Dry:  If there may ever be liquid water leaking from the ice rink (when it gets installed, removed, or Zamboni'd for example) you need a good barrier between it and the cob, and good drainage for any water that does get in there.  That's where the sand bed underneath the perlite comes in; if a little water leaks through here, it may not impact the bench.

- Other tips...
For proportions, stick closely to the book.  Don't go stretching the burn tunnel or shortening the heat riser or anything, 'cause it would suck to have your demonstrating stove smoking at your thousands of visitors.

-Exhaust: how you gonna get it out of the tent? 
  You may want a combination vertical/horizontal option, to allow you to choose what works best for maximum draft on any given day's temperature and wind conditions. 
  The horizontal outlet can be a capped T "cleanout" that you can open if needed.  This can be used to show off the quality of the exhaust, to let the exhaust excape downward if it needs to, or to reach in and prime the stove by heating the chimney with a candle or burning twist of newspaper for secondary draft.

- Draft: Are there any corners of the tent that are open to the air?  It looks like an airplane hanger. 
  If it's got a really high ceiling, and openings to the outside at the top, you may have a conflict if the stove is trying to draft out down low, and the tent is trying to draft IN down low and out way up top.  This problem occurs in houses too, and is one reason why interior, vertical chimneys that extend above the roof ridge are the normal best practice.  This makes sure that the "out" end of the thermosiphon is pointed in the right direction whenever the building/fireplace is cooler than the air.
  If there's no way for air to escape out the top (or plenty of ways for air to come in the sides to compensate), then you have the option of exhausting the stove out the side or down onto the lake.  (rocket mass heaters are fun that way: the cooler exhaust can go down instead of up.) I'd go for a side location in preference to a central one, for exhaust dissipation.
 
The temperature dynamics of the space may be tricky - if it's cold indoors, draft can be an issue; and the thermal mass will also lose heat faster. 
  Extra thermal mass (6-8" thick around the pipes) will help the interior of the stove stay warm without overheating the surface.  A masonry heat riser will stand up better and hold heat, especially if you're using it constantly.  A big barrel will dissipate the extra heat, and taller is better on the heat riser if you can possibly arrange it.  Shorter feed tube helps too.
  If you want it to perceptibly heat a larger space (like one corner of tbe big tent, or a 2-story barn) then 8" is a better size.  If you want it to mind its own business in a corner and just be a warm little kitten, then 6" is better.

Would you consider being a "sideshow?"
  Building the stove on solid ground near the entrance/exit of the ice skating tent, or off a side exit, would allow you to create a special warm place for people to enjoy the stove, hot soup (spiced cider! mulled wine!) and chestnuts.  (Doesn't it sound iconic?)
  Building on grade beside the main tent would allow you to skip the added complexities of engineering the supports, insulation, and exhaust.  You could spend that creative energy on decorating and refreshments instead ... I'm seeing the bench covered with a giant patterned wool afghan like old-fashioned ski sweaters.  The side chamber could be a red and white tipi- tent like a Santa hat, maybe with a felt or fabric interior liner... or choose a motif that suits the Eden Project, maybe earthen murals depicting Adam, Eve, and the apple...
  If you've never built a rocket mass heater before, this smaller chamber would also give you (and your guests) a better sense of their normal performance.

Building the bench on the same floor as the ice rink is an engineering adventure with great creative potential.  Building it as a sideshow would be easier, but may not fit your original vision or the space.
 
  Either way, it's an awesome, fun project. Let us know if your budget extends to flying us out to help!

Yours,
Erica Wisner
 
Cornish Nathan
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Firstly, thanks so much for your really informative replies, you've really given me lots to think about and consider.  Unfortunately I can't send any photos today as I have no camera but the issues you have raised I will try to cover so you get a better picture of what I am dealing with over here.

The stage is very strong structurally so the weight of both stove and bench will not be a problem.  Your description of it looking like an aircraft hanger is very much the case, it's approximately 80 feet tall and 200 feet square, completely covered but with clear open space between roof and walls allowing the breeze to blow through but keep the rain out.  Due to this amount of permanent ventilation I think it virtually impossible to heat the air space so I was thinking that the bench's primary function in this would be as a 'butt warming little kitten'.

The amount of ventilation in the tent should hopefully help with getting the exhaust out of the tent as there is plenty of room above to let it dissapate.  I was hoping to have the chimney visable and open vertically so the visitors could see the lack of smoke.  Would a 10 feet vertical stack be sufficient?

I like Ernie's idea of a smaller wider bench with 3 passes and a 6'' system built within a frame and filled with perlite and clay.  Is there a reason for the 3 passes?  I was thinking of doing maybe 2 passes, like a big U-shape and have the chimney return close to the stove.  Also what ratio of clay to perlite would you use?

Another question, could I use the instructions in Ianto's book for an 8'' system but with a 6'' tube or would this overload it? 

Also what temperatures could i expect within both the stove and also the bench?  My Health and Safety people will be very interested in this?

Unfortunately my budget for this does not quite stretch to flying you guys over here to help with this, but you would be made very welcome guests should you ever find yourselves this way.  I love the Santa hat tipi idea, I'll suggest it for next year.

Once again thank you so much for your speedy and knowledgable replies, this is a great site.

Cheers all, Nathan











There is a space of just under 30 feet between my proposed bench and the ice rink so heat transfer should not be a problem, there is also drainage in between.



 
Ernie Wisner
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it depends on the size barrel for your system. a 55 gal drum will be much cooler than a 120 lb grease can.

theres more surface area to dissipate heat on the 55gal drum.

This is what i have found with temps on the drum surface.

55 usually about 250 to 300 surface temp in lab conditions.

120 about 550 to 700 in lab conditions.

this can vary widely because it depends on several factors. moisture content of the wood being a big one.

bench heat is about 100 to 110 degrees after several hours of burning. it is possible to make the bench too hot to sit on. usually takes a week of burning every day all day.

yes you can make a 6 inch system with most of the same dimensions as the book. HOWEVER; DO NOT TRY AND HAVE AN 8" BURN TUNNEL AND 6 " DUCTING. Cross sectional area should remain the same throughout the stove. the gap between the top of the heat riser and the barrel is still going to be approx 2"1/2" to 3" inches.

the reason i suggested a three pass bench was because i assumed you had a demo in mind and needed to be able to heat and construct the bench quickly.  you have a much larger project in mind and i would suggest you contact some of the Cob builders in your area who have rocket stove experiance.

I am going to be kinda blunt here; the instructions in the book for the 8 inch system give you a working stove. build a test bed stove fire it up and let the safety folks take a look.  making changes to a system you dont know and have little experiance with, is not the brightest way to showcase a tech.

this is about the last thing i can say about this; i find it sort of dismaying that you want to show 200 thousand people this cool thing and you have not even built one.
This has been the trouble with getting cob approved in the US; lots of excitement by folks with little experiance and lots of badly built buildings. please take your time, build a test bed, construct the a stove and play with it in a safe place, make sure it is right before inflicting any thing on the public.

Sigh***** i probably put a bunch of folks wanting to play with stove off now; but damnit folks take some bloody caution and dont assume cause a book says something its harmless. it is not acceptable to put others at risk unless you are at the same or a greater level of risk.


its before coffee.
 
Cornish Nathan
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Firstly, thanks for taking the time to respond with your extremely valuable advice, it's very much appreciated.  At the moment I'm just finding out what might be feasible with the space, time, budget, skills, etc, that are available to me.  This is just the research phase.

I fully understand your dismay about excited yet inexperienced people doing more harm than good to the bigger picture, despite the best of intentions.  Please rest assured I will be working with others that do have experience of both cob and rocket mass heaters on the delivery of this project.

If, for whatever reason, I cannot show rocket mass heaters in the best possible light then the project will not happen.

thanks again, Nathan



 
Ernie Wisner
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no problem;when it comes to stoves I am bitchier in person. just ask Paul 

it does me no good to tell you my concerns if i don't attempt to give you as good of data as i can. Folks are just gonna do it any ways so i might as well give them the way to do it as close to right as possible.

I sent you an email Nathen if there are any more specifics please don't be shy about using it.

 
paul wheaton
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Ernie wrote:
no problem;when it comes to stoves I am bitchier in person. just ask Paul 



Oh!  I thought that was just my habit of annoying people in general! 
 
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Ernie Wisner wrote:
the largest full system i would put on a wood floor is 8" and figure the load accordingly. cob is lighter than water by the cubic foot so you can calculate you load requirements by water and build in some safety margin.
I think the best small demo would be 6" system with thirty foot of exhaust ducting in three passes through a bench. build a frame for the whole thing and fill with the perlite and clay mix. the critical area is under the burn tunnel but to be safe as you can put the foundation under the whole thing.


Perlite and clay mixed and packed into a 2"X4"  or brick frame  then build the stove on it.  this should give you a perlit/clay layer 1"3/4" (2"X4" to 2"1/2" (Brick) thick that supports the stove fine. if you want more insurance  build the frame  for the whole stove with brick on edge then layer in; 1 inch thick sand bed then fill with dry perlite , cap with a trimmed bit of 3/4" mason board  build the stove right on top of it.

the only part of the stove that gets really hot us right around the burn tunnel this section is about 1 meter long by 2/3 of a meter wide.

I will have Erica look at this later and she will i am sure make it far more clear then I. 



Ernie, am I misunderstanding when you say perlite and clay mixed and packed into a 2x4 frame? So basically build a timer frame how you normally would on top of the plywood, but pack it with the perlite and clay in between the wooden floor frame? How do you protect the wooden floor frame from the combustion chamber if you build directly on top of this?
 
Whose rules are you playing by? This tiny ad doesn't respect those rules:
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