rocket mass heater winter projects

We are talking about doing some rocket mass heater projects here december 7-10. Finishing up some cob stuff, including the finishing layers of cob; possibly building a dirt based quickie rmh in the wofati (it has been built with a cold thermal mass, so it needs warming); modifications to several of the pebble style rocket mass heaters; new shippable cores, with new forms and new designs; etc. etc. etc.

We will be doing this stuff for sure. Not yet sure if ernie and erica will be here. Putting this up in case there are people that want to be involved in these projects.

We charged $500 for two days. Plus extra for camping and what-not. Now we are charging $100 for four days which will include a bunk and organic food. And we don't have a kitchen commander, so I'm not quite sure how that's going to happen either. If E&E come, we will have room for two individuals and one couple in the house. Plus two couches in heated areas.

If you are interested, send email to jocelyn at richsoil.com

We are doing two gatherings. the first is december 7th through the 10th. The second is december 12th through the 15th.

We already have several people coming to each and we have a little more room in each.

Erica will join us for the second weekend, arriving on or near Dec. 12. Ernie will not be here this time.

We have nice groups forming for each weekend. I'll e-mail confirmations to folks next.

If Erica is not gonna be here for the first round, then that means we have extra bunks, right?

Yep, that does mean more room the first weekend! We know what needs to be done, so there will be plenty of RMH doings both weekends.

I am going to make some comments about the latest podcast (267) here because it will hopefully keep related information in one place.

Re: pocket rockets: I am wondering if a 12inch section of clay/ceramic flue pipe could be used on the bottom of the feed. I would suggest square section as round seems to come in smaller sizes (6in) The corners could have some clay(cob) stuffed in them to seal it. I have not played with these at all because in my mind the mass is the more important part of my projects.

air flow past the feed(slanted fuel): I have found that putting a 1inch pipe from fresh air to a point beyond the feed (say half way down the feed tunnel or so ... exactness is not big here) of a 6inch system makes a big difference in clean burning. The reason for vertical fuel is to make such a pipe not needed, but the pipe makes fuel that falls into the tunnel not matter so much. I would think 1.5 inch pipe (1.5 inch I.D.) would work well for an 8in system. I like the pipe because it means less time fire tending. In my case the exhaust went from wispy white with no pipe to clear (no visible smoke at all) with the pipe.

Burning pipe: Pipe that is next to cob will be kept cooler than the flue gas in the middle of the gas flow. It is like the whole boiling water in a paper paper over a fire thing. So my point is that once the flue pipe gets very far into the cob at all, it will not be very hot.

Cob with no pipe, one word... bricks. They don't have to be cured, make cob bricks to surround the the flue area. Then use cob around and to seal.

Shorter flue with fewer turns: YES! Bells do this well, almost anything that keeps the flue gas in one place will work... even a T with a capped pipe on it pointing up will extract heat (clean outs in a through the floor system are a great example). however Ts and pipe cost money A barrel on it's side with intake at in the middle and exhaust on the bottom is cheaper and works well. just cover it with cob to collect the heat.... put as much of the cob on top as possible and maybe even insulate the bottom because mine worked well enough to collect water. I started with an 18in barrel and sliced it lengthwise so it was only 10 or so inches high so I could get 4 to 6 inches of mass on top.

Barrel gap... 2 inches in an 8 inch is just constant CSA with the riser so 2 inches is minimum. The problem with wider gap is that the riser has to remain a certain height to work with the J. So widening the gap to bring the torus down the barrel will also mean a higher barrel which means heat will be radiating higher up in the room as well as down more. A bigger barrel means more heat is being harvested directly into the room rather than into the mass. But maybe adding 4inches of gap (to 6inch) will push the torus down 10 inches and the trade-off would be worth while. Personally, I like to add mass to the barrel, but I would use brick rather than cob because brick conducts better. In other words, mass added to the barrel needs to more conductive (soapstone or brick) to keep the barrel surface temperatures reasonable. I have not had any noticeable rocket effect difference doing this and in fact there have been brick "barrels" built for RMHs.

Second pump gas speed: The only second pump I would use would be based on falling cooler gas. On an 8inch system a 6inch exhaust is as low as I would go, 4inch assumes gas temperatures as low as room air or lower. In other words, the flue size depends on the gas temperature going through it as compared to that going through the riser. Personally, I think constant flue speed is over rated. I would prefer the gas to slow down so it can give off more heat in a shorter length and so the hottest gases spend more time directly against the flue enclosure surface (why I like bells) rather than through the centre of the flue insulated by a layer of flue gas that is cooler but can't move out of the way because of friction with the flue walls. The masonry heater guys have done rather a lot of testing on these things and it is good to research what they have done so as not to repeat the same set of tests. (besides they seem to have better access to the tools to measure these things ) I will watch you experiments with a second pump, but lets just say I am skeptical

On the same note: Down hill exhaust sounds good... if it can be controlled. I would think an exhaust that goes down hill to a gravel pit (to collect the water) and only releases CO2 to the plants around it would be a good idea. The weight of the cool gas could suck flue gas through the system. (the gas is not only cooling and shrinking, but changing composition as well, the water would be condensing and so the flue would have to drain properly too) The mass to achieve this would have to be large and may need to conduct heat better than just earth (water maybe?) And of course the site would have to work with it, having CO2 pool could be hazardous.

Not mentioned on the podcast: other types of mass. I really want to try tin on top of the barrel in a steel case. Think phase change. I think it could get hot enough here to melt the tin. That case once fully heated (after the burn) could be put into an insulated container and used to heat another room... or maybe cook on. I just don't know for sure the surface would get hot enough.

Have fun guys!

Len Ovens wrote:...
On the same note: Down hill exhaust sounds good... if it can be controlled. I would think an exhaust that goes down hill to a gravel pit (to collect the water) and only releases CO2 to the plants around it would be a good idea. The weight of the cool gas could suck flue gas through the system. (the gas is not only cooling and shrinking, but changing composition as well, the water would be condensing and so the flue would have to drain properly too) The mass to achieve this would have to be large and may need to conduct heat better than just earth (water maybe?) And of course the site would have to work with it, having CO2 pool could be hazardous.

Len, the downhill exhaust sounded good to me at first too, but after a little thought I realized it cannot work. What I remembered is that it is the difference between the flue gas temperature and the outside temperature (where the exhaust is being vented to) which generates the convective force. No matter how much mass you have, nor how conductive it is, you'll never get the flue gasses as cold as the outside temperature on any day when you actually need the RMH for warmth.

This seems like a good place to post my own response to the latest podcasts about RMHeaters. I am in no way an expert on rocket anything, but I know a thing or two about ceramics. If you are having issues with cracking during either drying or firing, the solution is to dry as slowly and evenly as possible. When ceramics dry, they shrink--the more clay you have, the more obvious this shrinkage will be. Pure porcelain can shrink more than 10%, which means that the big coffee cup you thought you made turns out to be a delicate tea cup!

Cracks happen when one part of your item shrinks faster than another part, thus pulling away--CRACK!

So, to avoid cracks, you want to have things with pretty even thickness (little tiny bits are prone to falling off). If you can't avoid having some parts thicker than others, then dry it as slowly and evenly as possible. Setting an item on a warm bench is far superior to setting it near a hot thing (or, worse, to actually try to burn a fire in a wet item). Even on the warm bench, I would rotate the item to keep things as even as possible as it dries.

With pottery, we mix in "grog" to minimize shrinkage and thus cracking. Grog is fired clay that's been broken up into little bits. I remember one ceramics professor who liked super groggy clay--it's what drove me into porcelain, because the stuff ripped up my hands. Anyway, you run bisque fired clay through a hammer mill and sift what you get from that to remove unreasonably large (and sharp!!) shards, and mix that into your clay. Since it's already bisque fired, it won't shrink as much. It will still shrink when going from bisque to cone 10 (or whatever) but along with the rest of the piece, I think

The fibers you are using should help, but please think about long slow drying for your cast cores. You can go by the weight to figure out when it is safe to expose the new core to fire. I think this will save you some of your cracked cores and thus save time in the long run. Give it a try.

podcast 268 makes mention of problems with too tall of a feed for the riser. I was thinking a small pipe like so may help. This would lower the effective feed height, but have the air feed after the feed should keep it from making a flue of the feed. An ordinary ball valve would allow adjusting air amount from none to full. I have used a pipe like this which worked great for me, but my whole feed arrangement is different too.