Building RMH in NH

[rjdudley]

Matt, what a frickin' brilliant idea using the sawmill!! And mine was sitting about 3ft away from me!
Just checked on my core and it's hard as a rock. I really had doubts yesterday, it just didn't seem like cement texture, but, hey it's concrete now. Curing may be a little tough without firing it up because it's 90˚ here with HIGH humidity.
Manny, glad to meet you. I've read your posts about your 6" build. I'll have some questions about your bench in a couple of weeks. I'm struggling with what to put under the piping (just cob? stone surrounded in cob? chunks of rock? surround that in cob?) and I'd like to pick your (and anyone else's) brain about the build.
I'm also getting the feeling that building a RMH is very forgiving. Meaning that I was scared to death to mess up measurements, getting the wrong materials, etc. but it seems that as long as I seal up the duct work in the bench and wherever else it needs sealing, then cover with a could about of cob then I may not mess things up too much. Have I got that right?
Ray

[matt walker]

Ray, that may be my attitude rubbing off on you, but I feel like these things are incredibly forgiving. There a few things that absolutely have to be right or they just plain won't work, but once you are in the ballpark they are kinda hard to screw up. As for the seals, the whole thing, when warm, is running at negative pressure, so any leaks are going to be pulling air in as long as they are not massive. One finicky thing about most of these is cold starts, which should only happen a couple times a year since the bench can coast for days and not be truly cold. When starting cold any tiny leaks will show themselves with little traces of smoke, but once warm they disappear. I've found them most often at the cleanouts. After building many I now just accept them as normal and not an issue. My old wood stove put more smoke in the house in one day than this thing does all season, and my stove wasn't particularly smokey. Anyway, the cob does an amazing job of sealing the whole thing up, and what I'm trying to say is that it's really dang forgiving in general.

As for the base, I propped up my flue pipes with rocks or bricks and then filled underneath with cob, removing the supports as I went. The reason for removing them was two fold, I didn't want to crush the pipes onto the rigid support once I started filling over the top with the heavy cob, and I wanted to decouple the pipe from the floor since the cob transfers heat slowly and evenly as opposed to the faster heat transfer through most rock/brick.

[mannytheseacow]

[mannytheseacow]

[rjdudley]

So, the base layer onto which the pipes will rest CAN be just cob? There's no reason to have to stuff "urbanite" or rocks into that layer?
On top of that you're going to put a layer of "...bend fenceposts I will probably bury in there, large rocks, some broken concrete, and some lime-gravel mix too…". Questions: 1) is one layer of this "stonework" enough? And how thick should it be? 2) You'll be filling in around this stonework with cob/clay, but, is this the level when straw is added?
Thanks Manny for whatever info you can give.
Ray

[mannytheseacow]

Hi Ray. If you haven't checked out Ianto Evans and Leslie Jackson's book on rocket mass heaters you should- it helped me answer a lot of my questions. I had my library order a copy and I got to keep it for 3 weeks, though I would like my own copy to support all their work. Also, Portland Oregon has a building code for RMHs that helped me be a little more confident too. Just google Portland's RMH building code. If I get some time I'll try to find a link to it and post.

Not that I'm an expert or anything- this is still my first build and I'm learning along the way... your first question:

I went with 6" of earth, then my 6" ducts, then planning 6" more of earth for a comfortable seat height of 18". Ianto and Leslie talk about running the duct closer to the floor to store more heat or running the duct closer to the surface if you want to heat the surface of the bench faster. I went for the middle of the road since this is my first build. They also talk about running the duct slightly higher towards the core and running the elevation downslope to the exit flue. I did this- slightly. My hot end is about 6 1/2" and the exit end is about 5 1/2" off the floor.

Your second question: I don't know much about cob. I've never worked with it and I'm hoping to not use it if I can help it. I have access to a bale shredder but straw is super expensive around here due to the drought last year. It's my understanding that you need it for strength, which I've counteracted by using recycled brick for the containment area (I'm going to post some pictures this morning). I have read that you shouldn't use any combustibles (ie. straw) close to the ducts. So, how close exactly you can use it, I'm not sure. It might be good to use near the surface to keep that surface layer from cracking? I might still have to do this. I haven't thought about it much since I have a long way to go before I get there. Good luck!

[matt walker]

Ray, what are you building on? If it's concrete or dirt or the like than you don't absolutely need a rock footing, but one of the main reasons to use urbanite or rock or other fill is that it reduces the amount of cob you have to mix. Adds a bunch of mass for relatively small effort. As for the cob, you want as dense a mix as you can get for the mass, so like Manny, I used straight clay/sand for most of my fill, with random rocks and stuff away from the flue pipes, and then used a typical cob with straw for the last few inches to tie it all together.

[rjdudley]

Matt, I'm building on a main floor which has a full basement underneath it. Actually, to be more truthful, it's also covered in carpet.
I'm thinking I may want to pull the carpet back, place a layer of durarock, then some 4"x8"x16 cinder blocks (the kind with the holes through them) which would give me the 4" air vent spacing Ianto talks about in his book, then another layer of durarock, the 4-6 inches of clay/sand onto which I'll put the pipes.
Sound right or do you think it's overkill?
I'm also going to put some lollicolumns in the basement for extra support.
Today I did my first burn!!! Happy Independence Day!!! Here are some photos. I had to mud a galv. pipe onto one end because I forgot to put one in during the initial pour of the refractory to make a round form for it.




http://i1366.photobucket.com/albums/r77 ... 99aba7.jpg



Some observations:
1) It was sweet! Burned well. It did put out some small ash from time to time, but, given that the stack isn't insulated and the form is still curing, plus some of the wood still had bark on it, I can't complain.
2) Ditto for some smoke. Periodic, but, see #1.
3) The galv pipe did get HOT. You can see from the photos that there is a white heat marking on the stack.
4) I was surprise at how hot the wooden box DIDN'T get. That cement did one heck of job! It was warm, but, never too warm to touch.
Soon I'll start building indoors. Although with the temps around 90˚ here it's hard to get psyched to build a stove
Ray

[matt walker]

Right on!! That's always fun. I think you've got a good plan for the underlayment and base. I'd peel that carpet back myself. Probably a few feet away from the bench and maybe make a brick or tile hearth a foot or two out from the bench, certainly near the feed. Partly for combustibles and clearances, since it's possible to have an occasional ember pop out the feed, but because these things put a whole lot of water on the floor during construction and particularly on dry out.

Great photos Ray, thanks for sharing them.

[rjdudley]

Just a quick update on the burn chamber. Did a second burn today for another couple of hours. Matt, I haven't had any shrinkage with the burn chamber. I'm thinking it's due to using refractory cement.
It does have a LOT of ash in the bottom of the feed tube. Maybe because I'm burning sticks? Not something like pine boards?
Does anyone think it's possibly due to the burn chamber isn't hot enough? Just wondering, but, it's not a total burn.
Ray