I stumbled over this concept for the first time last week, and it keeps coming to my mind... Its an extraordinary clever idea! I'm an engineer, and quite used to technical combustion solutions, but this is new to me. I didn't believe it at first and did some calculations of the heat riser draw... and well... I'm convinced... I'm quite sure, that this concept does not comply to the building rules, where I live (germany), but I'm maybe willing to try anyway...
But I have some questions left:
1. What about the condensed water in the mass heater piping? There has to be quite a lot of water, depending on the watercontents of the wood and the end-temperature of the exhaust. Is this removed somewhere?
2. How do you get the ashes out? Isn't there any ashes pulled into the mass heater piping? How do you get that out?
3. Does the barrel or the heat riser oxidize on the interior, due to the high heat? Whenever I see oil barrels, they are heavily rusty, and thus made of quite reactive steel.
Good answers to your questions and some very insightful data for an engineer like yourself.
1. I haven't seen anyone mention a need to address the condensed water vapor.
2. The ash clean-out is usually in the heat bench, however, not needed to be cleaned much because there isn't much ash due to complete burning of the wood smoke and ash.
3. I see how oxidation would be a concern, however, I have not seen anyone mention it to be a problem.
Although I'm not really answering your questions, I hope this is better than nothing.
Yes, there can be condensation if you allow the exhaust temperature to drop too low within the thermal mass before venting. The vent tube is sloped away from the heater until it is outside the building so any condensation will exit if any is created. The bottom of the flue on the outside of the building has a hole for clean out and condensation draining if needed.
A cleanout is placed at the juncture of heater and piping.
Heat riser metal can and does oxidize. The heat riser can be made of firebrick.
I just didn't see any "maintenance hatches" (don't know if the word fits here, I'm no english native) on the concept drawings. Even the cleanest combustion needs some cleaning every now and then.
I try to stick with the engineers view of the concept, because thats where I'm good at.
The good material for the heat riser would definitly be firebrick. Especially if you try to improve the burning temps, like described further on. You can buy round chamotte tubing here, so... this is perfect for keeping the turbulences. Not everybody has access to highly heat resistant materials.
at condensed water:
Some part of the RMH-concept is the improved efficiency to regular wood stoves. To push this advantage further on, you need to optimize the process. You want the highest possible burning temps, and the lowest possible exhaust temps. Both goals increase the water condensing problem. This means on the reverse, if you don't have condensing water in your flue, there is room for efficiency improvements...
To get the highest possible burning temps, you need the exact amount of air for the burning wood. Because the fire is sometimes burning more intense than on other times, some kind of air regulation is generally desirable. Theoretically the heat riser should do some of that task on its own, without human interference, and maybe it can be designed to do the job really good. But I guess that would be more complex, than slightly oversize the heat riser and integrating a lambda sensor and a control system, because thats easy. But maybe some of you wouldn't want such technology in a RMH.
Flame temperature of 2000F are definitly reachable. Theoretical Limit is the adiabatic flame temperature of wood and air at ~3600F so there is room for improvement (but be aware of the NOxes)...
If you have managed a perfect combustion (complete combustion and very few excess oxygen in the exhaust gases, no NOxes), even when you have well dried wood at 15 mass% water, if you drop your exhaust below ~60C/140F, water will condense AND stick to the flue piping. At the beginning of the firering, when your thermal mass is cold, I guess exhaust temps will be significantly below that temperature. Thats just theoretical thinking. I've never seen a RMH in reality. Please correct me, if I'm wrong! If thats true you are building up amounts of water within the flue piping, during burning. Thats wanted for efficiency reasons, but needs to be drained, to keep the piping clean. One easy way would be, to lead the last third of the piping in the thermal mass on a downward angle into the exhaust, where it can drip out.
The thoughts need to buzz around my head some more, and then I try to build a RMH on my own... at first in the garden...
Joel Hollingsworth wrote:Sorry if this is a stupid question, but do you suppose water is condensing in the exhaust pipes? Have you provided for removing that water?
Erica Wisner wrote:General comments on rocket heaters:
Water does condense in the pipes on other RMH systems, but not to the point where it would pool and do damage. Earthen masonry is pretty forgiving up to moisture levels around 13-15%, and cob both absorbs and evaporates water readily.
My gut is that any condensed water gets evaporated out again once the fire dies down. Some amount of cooler, drier air flows through the warm pipes as the fuel load diminishes, especially if you aren't tending the fire super-closely and let it burn out completely before shutting the burn door.
The time when RMHs tend to be really drippy is just after building them, especially if you build it in the fall when the weather is cold and wet. Water comes out everywhere - the cob faces, the pipes, the drainage underneath.
We do try to orient the cleanouts so that water can drain down and out.
the problem with showing clean outs is guests and kids like to pry the covers off to see what is happening in the pipes so we stash them behind tiles. yes the pipe does oxidize badly but the covering is cob and wont let smoke and gasses out into the room. the heat riser is made of several materials depending on what you have on hand. this ranges from duct and stabilized perlite to brick and mineral wool. other stuff can be used but i would have to evaluate it with some long term testing.
Hope this helps
I want to build a house in around half a year to a year. Building a house is a complete different philosophy in the states as it is in germany. I realized that in my terms in Miami. Here its more like you try to build it for all time being, and live there to the end of your days, and I'm quite comfortable with that thought. Following this way, I want to design all the installations to last a long time. It just doesn't fit in there, to build a stove with known issues remaining. But I expect those issues to be easily solvable by picking the right materials. Therefore I'd like to ask you about some more details:
*Which flue material did you use. In the videos it looked like standard stainless steel (1.4301 or x5CrNi1810). Did this material corrode? If yes, the problems are bigger than thought, but I dont really expect that. Its common flue material for oil and gas fired stoves.
*Did it corrode uniformly all over the flue, or is it worse in some parts than on others? Beginning? End?
I'll build the heat riser either from a highly heat resistant steel tube or chamotte. Whatever is available cheaper. I buy the stuff needed, and will not depend that much on what I find by chance.
Although cob as a building material has a long tradition, even today in rural germany ( timber framing => german "Fachwerkhäuser"), I'm not sure, if it will be part of my house, especially if not plastered over. I just don't know it from first hand living in such a house. I'm especially suspicious to the cob-bench, because I would expect it, to be easily abrased. Basically its just dried mud. Thus I'd expect having the dust of it everywhere, at the clothing, in the cushions, the floor, etc. pp... Maybe thats just prejudical, but I wouldn't take the risk, until convinced otherwise. Additionally i want to mate the stove with a water heat exchanger, to support the central heating and to use the heat everywhere in the house. Therefore my thermal battery will be a hot water tank.
heat riser i would make from brick with a insulation layer of 1 inch rock wool.
I was trained by Ianto Evens in building so i to look at housing as a long term idea (about 300 years). Cob is durable and yes you need to plaster it (cant figure out why folks would leave it raw). plaster is three layers base coat, brown coat, and finish coat. A bench should get the same treatment with the finish layer being a good hard plaster. rounding corners and edges makes chipping minimal. If you build a RMH from proven plans we have worked the bugs out of the build. We dont leave a stove Raw but it does take time to dry and plaster.
Cob can be very durable and a good cob with a good plaster on it wont dust off. thats one of the characteristics of a good plaster. Me for Germany i would be building a cob bail. R-40 and 8 inches of interior thermal mass, add a nice plaster and a good wide eaved roof and you have a house that would outlast your great grand children. you heating and cooling cost would be very low and you dont need to work very hard to add rooms if needed, maintenance is an easy job of whitewashing it every few years, patching the few chips and keeping the roof in good repair. that would be my druthers, you have your preferences.