David Blackton

Well, several years later, and I finally have the house to a point where I am getting ready to put in the RMH (I was rebuilding the roofs, backyard, masonry walls, etc.). I just found a whole pallet of new fire bricks for $1/each, and I also bought some burn barrels for $10. I've been having a really hard time finding clay though. I checked Home Depot, and they just stared at me, and finally someone sent me to an art store where it would have cost me thousands of dollars to buy enough clay for a RMH. I went to some masonry suppliers and tried some other construction materials wholesalers, and no one seemed to know what clay was. So I have two questions regarding the clay:

1. I found one place near me that sells "fire clay" (although they didn't know what just plain "clay" was). I got a few bags of this, but is that any different from just regular clay? Can I mix that with dirt or sand to make cob?

2. I did a soil test on my soil, and I can't figure out if it has any clay or not. Water seems to pool in our yard fairly well when it rain, so I had assumed the soil would have some clay, but when I did the glass jar test, it only has two layers, not three. Obviously the bottom 70% is sand/rocks, but the top layer I can't tell if it's silt of clay. On the one hand, everything settled out of the water in much less than an hour, which I've read is a strong indication that there is little or no clay, only silt. On the other hand though, I scooped off a bunch of this top layer and let it try out, then added a tiny bit of water and rolled it around in my hand until I was able to make little "snakes" out of it, which implies that that layer is clay, not silt. And of course the rain pooling in the yard also tells me it's clay. I'm afraid to carry in several tons of dirt into my house and build my whole project, only to find out that the dirt has no clay in it. Any thoughts on this? Also, assuming I can use the dirt from my yard, if it IS actually 30% clay, does that mean I don't need to add any clay at all? Or would I even have to add some sand into it to get the right ratio of sand to clay? Or can I just use it as is?

It would be nice to use the dirt from my yard for a number of reasons, not the least of which is that I have way to much dirt in my yard now that I've put in a 4' deep foundation wall all around my yard. Otherwise I might just be throwing away hundreds of bags of dirt, which I don't really even believe in!

Thanks Allen! I just downloaded the book, so I'll read through it and see if the information there helps! That's strange though; every time I went to the site before this, it was a completely different site with videos and things on it, and the book link took you to an Amazon page which only offered the physical book. At least I have the book this time!

I'll read through the book before coming back with many more questions.

But to quickly respond to what you said about the cob versus cement, I THINK that cement would be cheaper, judging by the price of clay that this site linked to on Amazon, but of course if I can find wild clay, then it's free! I usually get the torn bags of cement and lime for $1 from Lowe's, then I get sand for $30/ton, so I can do a tone of cement for about $30. So from a cost and availability standpoint, cement is cheaper and much more easy to find. But the more important question is the functionality. I guess I should get some clay anyway to make the clay "slip."

And yes, the fire bricks that I found are VERY strong! The diamond blades were supposedly designed for cutting through masonry as well as cast iron and steel, but they burned right up as soon as I hit the fire brick! I do have some of the softer ones inside my wood stove, but the wood stove probably needs those ones! Were you implying using just normal masonry (non fire) bricks? What is the benefit of fire bricks in a stove setting over just normal bricks? I guess the normal bricks would crack over time? And cracking is why you also don't reccomend the thinner fire bricks? I think the kind I have in my wood stove are the thinner type anyway (although not too many of them have cracked yet, but then again it wasn't a rocket stove!).

I don't think pyrolysis should be a problem with my joists, as the brick chimney shields most of the heat. I don't think the outside of the brick even gets hot enough to be more than warm to the touch. And it sounds like the RMH should actually emit COOLER air than our wood stove is! And after over 100 years of wood, coal, oil, and finally gas fire exhaust, none of the joists seem damaged, so I'm not too worried about that (I've looked at a lot of old row homes, and 100 percent of them had rotten joists under the bathrooms, always in the exact same part of the house, but usually everywhere else the joists are almost perfect after all these years!). Are you saying that I could have charcoalized joist ends but not realize it?

OK, well let me look through the book before I ask any more questions!

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OK, well I wasn't clear on how best to seal the stove if I am not using cement in that part of it. And what is clay slip? The basement floor is concrete with dirt underneath that. My plan is to utilize the fire bricks that I salvaged from the old coal furnace to build the heat riser. So what about packing just normal dirt (not topsoil or clay) around stove pipe, then topping that off with wet cement to take the gravity pressure off of the stove pipe? Or what if I just pack sand all around the pipe? A friend of mine buried an insulated swimming pool full of sand underneath his yard to store solar-thermal energy from the summer months for use in the winter months. The sand or dirt directly around the pipe would protect from the extreme heat, while the brick walls and concrete over the top would provide the structural support. Or maybe I just need to figure out how to make cob...

Can I just find a stream or pond and dig up some wild clay to use for this? I know a place where I used to find it...

Thanks for the responses everyone!

To answer your questions Allen, I am located in Philadelphia. This is a row home basement with concrete floor poured over dirt and a high basement ceiling. I own the house next door as well, which is where we currently live. Although it is possible I might build on wood floors in the future, these old houses are build so much stronger than new houses, so the weight is not nearly as much of an issue. I always design my rooms to center any heavy weight near the side (structural) walls rather than near the middle of the rooms or the front or back sides of the rooms (unsupported, as the joists go from side to side, not front to back). Also, I have been pouring concrete radiant slabs over the wood floors and they have no trouble with this weight. At several points we actually had several tons of sand piled in the middle of the living room floor inside a dumpster, and the floor didn't seem to notice (that's not to mention another several tons of cement, furniture, and other supplies stacked around it! Oh yeah, and about 10 more tons of bagged rubble, around 800 bags total!)!

This brings up another question, are RMHs usually built along the outside wall of a room like conventional heating systems usually are? I could probably make an L shape from the front wall of the house to the side wall where the chimney is, but the thermal bench would only have a single run of stove pipe running through it then, which is why I was thinking of having the bench on the side wall only with a the pipe going out and back again for a smaller but warmer bench. Is the reasoning for putting a heater on the exterior wall of a house mainly for uniformity of heat distribution, or are there other factors as well? I often think you would lose more heat that way, because locating the hottest part of the house next to the coldest part would create the biggest heat differential, which would cause the greatest heat loss over time. I like and agree with the concept around here that you should focus on heating the person, not the house, so why not locate the heat bench AWAY from the exterior walls, and then just agree to locate your body on the heat bench rather than over by the cold window?

Regarding insurance, I am much more concerned with it actually being safe than keeping the insurance company happy. If they really have a problem with it, I can either find another insurance company or change the design to appease them. The state of the house right now (and any of the other 100+ year-old houses around here) is so bad that there would be a dozen other much bigger insurance concerns, and whatever I do is going to be a MAJOR improvement on how things are now. I've also seen the extremely shoddy work of most contractors, and although I don't consider myself an expert, I know I am doing MUCH higher quality work than most professionals! In fact, I hired two contractors to do my electrical and plumbing on my first house, and when the electrician came to inspect it, he made me totally tear out and redo everything they had done. I did it myself the second time, and he was happy with my work. As I'm living in my own house, I know the work I've done is good quality, and I'd much rather have it ACTUALLY work than to have someone who does "certified" work that is actually a mess of leaking pipes and shorting wires. I realize that there are legal risks with anything though, which is why I am moving forward with caution.

The existing chimney is brick, starting in the basement, going through the first, second, and third floors, then out through the roof. The roof is a slightly sloped flat roof with torch-down rubber on top of wood decking. There is an existing 6" round metal chimney sleeve in the chimney now. It's the same as the chimney on the house that we are living in, which we sweep out ourselves every few months. We are mostly burning old wood that we collect around our block for our wood stove, so there is very little build-up anyway, but always better to be safe than sorry!

One thing I haven't been able to figure out is why people here keep saying that concrete is forever or that it is hard to work with. Am I missing something? I mean if you don't want it in the future, why not just take it out again? I understand the comments on using cob to withstand the temperatures though, so I'll have to look into how to build cob.

Here is a picture that I found of what my fire bricks look like: http://www.hotelrestaurantsupply.com/TWN-225044.html?utm_source=froogle&utm_medium=shopping&utm_campaign=products&gclid=CIvWzvX2r7oCFYqi4AodajsAsw
Mine are probably bigger than the ones in the picture, as this furnace was maybe three feet across, but they look exactly the same! I actually burned out several diamond metal-cutting blades cutting into the fire brick before I realized it was there! I thought I had run into some super-strong metal or something! I was really excited because I'd been wanting to build a RMH for a long time, but needed to find an affordable source of fire bricks, and there it was! They are shaped a little funny though to fit a large round chamber, but I was thinking of maybe breaking them.

So I still wasn't quite clear on the fire-brick portion of the design. Would I just build the feed tube, burn tunnel, and heat riser out of fire-brick without using any mortar, then coat just the outside with cob? And which parts can be normal brick, just the outside wall of the bench?

I did look at that page again for the book download, but there doesn't seem to be any download link, only a link to buy a physical copy of the book. Is the e-version no longer available?

Wow, that's great information, and great news! That design will probably work perfectly here then! I actually have a pile of fire bricks that I pulled out of a giant old coal furnace that I cut up from the basement when I got this house, so I can probably use those bricks for the portions that will be touching the fire. If I build the heat riser out of fire bricks, would that do away with my need for perlite insulation and cob completely? Or will I still need those for mortar on the heat riser?

Also, I want to get a good book to help with some of the technicals of the design. Any suggestions? The comments on "Rocket Mass Heaters" says that book is getting outdated...

Another question about doing a RMH in the city. I prefer to use locally available materials, which around here are masonry bricks and rubble. Can the thermal mass around the exhaust ducting be infilled with cement and/or brick rubble (crushed)? And can the outside be a brick/cement wall? I know I can go out of the city to find clay somewhere, but why not use local materials if they are available? And what about around the base of the reburn chamber? Can that be cement (I'm assuming not because it could explode or at least crack in the heat, but I thought I would ask!)?

I've been thinking of switching from wood stove to a RMH. I noticed that the designs usually have a horizontal exhaust pipe, which is usually embedded in the thermal mass bench or floor or whatever. Does anyone know if that is merely just for spacial/layout reasons or does it have to do with the physics of the rocket effect and/or exhaust effectiveness? The reason I am asking this is because in the city it might have to be vented upwards through the house chimney. My thought was to run the horizontal line through a heated bench, then end it into the existing chimney, where it would go up another 30 feet to the exit through the house chimney. Would this be OK?

Was this list ever made? I've been searching all day for a list like this, and this seems to be the closest I've found to something that would actually be useful for city-dwellers who want to produce food indoors without any sunlight (north facing windows).