I plan to build a large rocket mass heater before cobbing in any of the exterior walls of my new house. I'm using a post-and beam structure in order to keep everything dry during construction since the roof will go on before cob infill. This will keep the rain and sun off of the building and the workers. Whenever it looks like rain I'll stockpile 5 to 10 tons of premixed cob inside the building so that the process is not weather dependent. Temporary form work attached to the posts will make the process a hybrid cob/rammed earth/cordwood process. I've had good results with test blocks containing woodchips, sawdust and larger chunks of scrap wood. This material can be nailed. Walls will be given an earth plaster finish and will therefore appear to be pure cob but  with greater thermal efficiency, quicker drying , and better resistance to earthquake.

    Since all materials will be mechanically mixed and hauled in with a tractor it makes sense to build the heater before anything inside the house is finished. I plan to use up to 15 tons of cob since the mass heater will also be a Trombe wall for solar heat storage. The plan also calls for a cook top and two ovens, 2  cast iron tubs  embedded in cob, hot-water heating, showers and a hot tub. The heater will also be used as an air conditioner, dehumidifier and exhaust fan in summer.

      Workers will be camping on site during this process so it only makes sense to camp inside the house. And what campground is complete without a campfire?  With a functional RMH we'll be able to cook food and ourselves if necessary. People will be able to sleep on or near the heater depending on how warm they like to be. And most importantly everyone will be able to take a hot shower before roasting in the hot tub while watching a movie on a big flat screen.

    My camping bus which is set up for 24 people complete with tents, cooking facilities and entertainment will also be an important part of this project. I'll haul everyone to the job with the bus and then park it strategically so that the facilities are available in the evening.

    I plan to use this bus to haul people to many other green building events and then hire one of the leaders of these events to do workshops at my place.

    After all cob work is complete the mass heater will be run constantly in order to dry out the material. Windows will be open and south facing glass will allow plenty of solar heat as well. It doesn't matter how much firewood I go through since I have an abundant free supply.

    If you can think of anything I've missed here that would make this even more efficient please post a reply. I like to overthink things and do them once and do them right.

    I'll never consider doing cob work in the traditional hand and foot mixing style so let's limit this to a discussion of how to make mechanical mixing and building using a crane and tractor as efficient as possible. I plan to keep track of all costs on this project including $25 per hour for my own time. I'll post things as I go and then do a cost summary when all is said and done. I believe cob building can become a mainstream process if appropriate technology used.

The cob mix for the giant mass heater is different than for the walls?

      Yes, the mass heater will be made of regular cob and the walls will use a wood chip/cob mix as mortar for cordwood construction. Everything will be parged with a regular cob mix so that the walls don't have the cordwood look or all of the work involved with pointing the log ends.

    I like the thermal efficiency of cordwood but am not a fan of all of the fiddly work involved in treating the log ends and getting everything just so. I'll be building inside a form similar to what is used for rammed Earth. The forms will be boards which are temporarily screwed to my posts. That way I can slam the material together and tamp it without worrying about how the wood looks since it will all be covered up.

    In the end I expect the walls to look more like rammed Earth than like cob because I'm not going with any sort of gingerbread house look.

Are you using soft wood? Maybe two foot lengths?
Are you using all sawdust and chips  or some cord wood construction? like you would cut for firewood?

That mass heater sounds to me to be almost 2 stories tall or is it shorter than i think?

Most of my land was clear-cut 11 years ago and the rest contains mature firs and maples which are worth more standing than cut into cordwood so I won't be harvesting any filler wood from the property. There is a Cedar Mill 1 mile from me and 40 yard containers of slabs with sawdust are available cheaply. The bark strips off this wood  easily and it dries quickly.

    There's also a strong possibility that I will distribute small bins to construction sites within the city and charge contractors for disposal of wood waste. Just about any crap wood can be used with my forming system.  The length of scrap wood can be highly variable since all is being hidden with a cob parging in the end. Everything from sawdust to 4 inch chunks to 5 foot long two by fours can be jammed into the form. Finely ground woodchips will be mixed with the cob mortar.

    The mass heater will be fairly long and the portion closest to the chimney will rise to ceiling height. This height is both aesthetic and functional. I want to include shelving which rises to the ceiling and I need to have some thermal mass up high so that the mass heater will give proper draft at night in the summer when it will be used as an exhaust fan and passive air conditioner.

     In order to distribute plenty of heat to the embedded bathtubs and hot tub I will incorporate some recycled aluminum extrusion. 6 foot lengths of aluminum will be placed perpendicular to the exhaust and run under the tubs. Aluminum conducts heat many times faster than the cob mix. If the hot tub doesn't get warm enough I'll build it a separate rocket stove. Everything will be built near south facing windows so that they can be solar heated for much of the year and so that the mass heater functions as a Trombe wall as well.

    The tubs and shower of this spa/mass heater are all going within the heated space so that no energy is lost in letting a hot tub cool off. All of the water in a very large hot tub will add to the thermal mass.(I'll get my containers for free or be paid to remove them from demolition projects)

   The mass heater plan has changed since my original post. Well it's more of an order of operations change. I still plan to do the giant mass heater for the house as planned but I'd like something more immediate since my property is not the most comfortable place to be in the wet winter months. I'm currently concentrating on creating giant hugel beds while clearing overgrown road and pathways. During this process I have spent six nights sleeping in my minivan. There is nowhere to wash and the only escape from rain is the van or deep under the cedar trees.   

    So I decided that I need a spa much sooner than originally planned. Located a perfect spot on a south facing slope where I will construct an 8 x 10 post-and beam greenhouse. Inside the greenhouse I will place a salvaged hot tub. These tubs are available for free. This tub will be heated by a rocket mass heater. The clay will be molded around the tub with the top of the heat riser being slightly above the rim. I'm looking to heat the water more than the air so the inner flue system will be closer to the tub than it is to the outer edge of the cob mix. The little building will be constructed to look much like breadbox solar water heater and that will be its function most of the time. All of the cob will be stained dark brown or black so that solar heating is most efficient.

    Operation of the tub.

When not in use the tub will have its own clear plastic cover which will prevent evaporation and increase efficiency. If the water requires more heat than what the sun provides then the rocket will be fired up. During most of the year some wood burning will be necessary. Heat will transfer from the cob mix but will also be available from a container which sits atop the heat riser. Water for a shower will also be drawn from this container. The hot tub is likely to take up half of the available space in the small building. The remaining space will be used to store and dry out firewood and there will be a bed which sits 5 feet off the ground. It will be like a bunk bed with no bottom bunk. That area will be filled with firewood. Suspended from the ceiling above the bed will be a television with DVD player. This will be powered by a cord run from my vehicle. The vehicle always contains everything I need to survive at job sites and in the wilderness so it will be handy to have it just outside of my sleeping area. After the tub is used each night the lid will be tightly affixed and with a small amount of ventilation a comfortable humidity level can be maintained. Although this may seem a little rustic, I'm sure that the presence of a large vat of hot water will maintain comfortable temperatures for sleeping. I've lived outside in Canada at my job sites with no heat for most of the last 12 years. I've managed to be relatively comfortable through proper planning. So this little hovel will be a step up in the world.

   I'll feed the stove using the rocket stove slide which I described in another post. The rocket stove feed will be positioned so it's visible from my bed and a basket full of wood will be suspended immediately beside the slide so that I can feed the stove without ever leaving the comfort of my bed. I've set up several wood-burning situations in the past where I could do this but none of them where as high-tech as the slide and all were considerably more dangerous. The only other addition necessary to make my sleeping area complete is the urinal which will feed out a pipe and over the bank. I'm not kidding about the urinal which will be made from a bleach bottle connected to a garden hose and will be suspended 1 foot above the bed against the wall down by my knees. I'll use it from a kneeling position without having to get down off of the bunk bed. It's funny I'm an automatic working machine but when it comes to things like this I can get quite lazy. I prefer to think of it as an efficiency thing

   All a man really needs after a hard days work is a bath, food and a television. I've spent 12 years in vehicles and unheated houses usually without electricity, believe me you need a television

Sounds like a good plan to build the Spa building to live in while you get the rest done, will be small and easier to get done.  The water will actually be a great thermal mass too, better than stone or cob per volume.

Have you thought about including a big food dryer in your main-house RMH?  Possibly a smoker, too, although smoked food probably isn't really good for you, even if it does taste good!

Kathleen

   Regarding the thermal mass of the hot tub in the small picnic shelter/spa sleeping shelter.   

       Yes, it will have an enormous amount of thermal mass for such a small structure since it's being built in the style of a breadbox solar water heater. The thermal mass  to volume ratio is somewhere around 3 to1 as compared to cob. And of course with water there is little lag time in moving heat to the edges of the container since convective flow will do that whether you want it to or not. Then there's the thermal mass of all the firewood.

     Regarding a dehydrating room and smoker in the giant household unit.

   Yes, I plan to build a good-sized room which can be used for dehydrating vegetables and fruit but that won't be it's primary function. This will be an approximately 8 x 8 vault type room made from cob. It will be set up as a dry sauna since this is all part of a heating system/spa area. But there will also be foldable racks which allow it to become a dehydrating room and a clothes dryer. Any hot space such as this will dry clothing quite well if airflow is induced. I will configure the exhaust fan function of the mass heater so that air is drawn from the top of the dome shaped roof. Since this will be almost like a separate little building I'll probably style it to look like a Roman vault Temple. This room will not have its own heat source but will instead be molded into the mass heater and it will form the tallest portion of the Trombe wall. The doorway will face south toward the indoor swimming pond and aquaponics area. There's a big structure in Washington DC containing a statue of Lincoln. It looks over a water feature. I'm not going to try to copy it but I'm sure that visitors sitting inside the dome will look just as regal . I may give the entire interior of this structure a half-inch coating of ferro cement. This would allow it to be blasted with a hose for cleaning purposes. If it starts to smell like swimmers or fish I'll mix mint or some other herb with the wash water. Minty fresh. The only temperature control will be the door and a few adjustable vents. I envision it's everyday usage will be as a spot for swimmers to take a rest and to warm up. There will be times when it simply functions as part of the Trombe wall.

    I don't like smoked meats or rather I refused to develop a taste for them because of the carcinogenic qualities so this will not be a smoker.

    What will this all cost?

   I expect to spend less than $1000 on parts for this elaborate contraption. It's more likely that I will make money in the process of procuring supplies since most will come from demolition projects. I've already accumulated six cast-iron clean out doors, 10 sections of clay flue, and at any time I can choose to save tons of firebrick which commonly goes as fill since few of my customers want it.

      Undoubtedly it will consume plenty of labor. But since I'm building a house far larger than what is required for my own needs, I'll recruit helpers who wish to stay in the house for a few months. People pay good money to attend workshops for this sort of thing so it shouldn't be too hard to locate a novice cob builder who wants experience and a roof over their head. This structure will be something that would feature strongly in their resume should they decide to turn pro. I can imagine the whole thing consuming $10,000 worth of labor. That's about what I would expect to pay for a decent oil furnace and the accompanying ductwork. So although this may be the world's most expensive rocket mass heater it will actually be quite inexpensive when you consider the multitude of functions.     

     When I added 11,000 ft.² to my ex-wife's home 14 years ago it cost me nine dollars per square foot including labor. That was for a house which was sitting on pilings and simply needed extra bedrooms, bathroom, living room and rec room built beneath an existing home, so there was no roof or foundation cost. This time around I expect to spend between $30 and $50 per square foot for a far superior building. This will be a massive savings over the $200 per square foot general cost of building in my area so I think I'll be okay to splurge on the heating system/spa.      I'll post cost figures as I proceed and will include $30 per hour for my own time and $12 per hour for unskilled laborers so that everyone can have a realistic picture of the costs involved. I hate it when people understate the cost of things by pretending that labor has no value. So any time that I post the cost of anything done to the property it will include an estimate of hours spent and the approximate value of that work. I'll also include an approximate value of any resources harvested from the land.

en, good suggestions, thanks.

dale i read with interest your posts, do you have any drawings of this rocketstove/heater I cant visualise what its going to look like

    Currently the design for this wonder of engineering only resides between my ears and on this forum. I'll draw something or have my niece draw something and then I guess I need to learn how to send that sort of thing. Computer novice.

They have some kind of google drafting app...but i just take a digital pic of the drawing and upload to my desktop then download to a post.