So we want to built a rocket mass heater in public space, as a public art installation: It will provide a warm space to hang out during the cold winter in the city. It will be built in November in Germany, and it would be already rather cold outside, perhaps even snowing.
Do you think the cold will affect a lot the effect of the hot air/gas circuit? We thought to insulate extra the bottom of the whole object with vermiculite, as well as the chimney, but we want to be sure that the stove will perform as it normally does, however, we haven’t seen any other stove completely outdoors. (they are usually indoors or in greenhouses)
Apart from that, and since it will be in public space (meaning that it has to be extra safe for regulations), we will not use a barrel for the bell because people might burn, if they don’t know how hot it gets and touch it. And we will not do it with cob, as it is not water resistant. So, we decided to built a complete structure out of bricks, for both the bell and the bench. I can’t find much information about building a bell and a bench out of bricks, so I have a couple of questions.
First of all, the hot air is directed to a shaft which makes a loop and comes back to a chimney which is next to the heat riser. The whole way is about 6 m long. Do you have any clue how hot the air will come out at the end?
I am not sure whether we need to put a metal pipe for the hot air, or simply to built two hollow shafts with the bricks (like in drawing no 2). We thought that maybe with the pipe will be better, because we can insulate it with sand all around and reinforce the thermal mass in that way, but not sure if its necessary.
Another topic is the bench. We have heard that if the top layer is more than 5cm thick, it takes 2 hours to heat up. We wanted to do a layer of bricks and then tiles, and we re wondering if that would be too thick. Any other solutions?
Can someone check the proportions of the wood entrance, the burn tunnel and the heat riser in our drawing and let us know if it seems correct?
Also, do you have any details of how to built the outlet to clean the ashes?
The first thing I notice is that the height of the heat riser is barely more than twice the height of your feed tube. It is recommended that this be 3 or 4 times the feed tube height for best function. As shown, you may have trouble getting the riser to draft stronger than the feed tube, and have problems with the flames creeping up the wood and out of the feed. The burn tunnel looks good - it is as short as it can be. The combustion core does not appear to have any insulation, unless you are using insulating firebrick which would be excellent for all except the feed tube (it is too soft to stand abrasion of wood loading).
The brick bell looks like it would work well. It would be wonderful to lean against.
I would suggest for the bench to eliminate the duct and make it an open cavity inside, continuous with the space of the bell, with the exit to the chimney as low as you can make it. This will give minimum friction for easier draw, and warm the surfaces quicker than round ducts would allow. You could put a row of brick piers down toe middle of the space to hold up capping slabs. Rather than brick, I would use concrete pavers or the like for the top, which you could then tile over.
So, for the 'guts' of the RMH we are using insulated heat-resistant bricks (vermiculite based), while for the top of the bell (pyramid) we are using heat-resistant chamotte-bricks. the rest of the structure is normal baked bricks. Under the bench we will use vermiculate plates as insulation. And the top of the bench is concrete plates.
We think that the proportions of the feed tube and the heat riser should work now, what do you think? The cross-sectional area of the feed tube and heat riser is 360cm2, and it grows to 510cm2 or more at the bench, until it leaves from the chimney where it is only 180cm2. Is this a problem for the air exhaust?
We also designed it now in a way, as we understood from your reference, without stove-pipes: the bench is a hollow space where the air can fill it up fully. As you see on the drawing 'first layer of bricks-plan' we have put some bricks in the middle to support the bench's top. However, we are not sure if we need to fully close this line of bricks, for the air circulation. What do you think?
The feed tube:burn tunnel:heat riser proportions are better, but still not quite up to even the older 1:2:3 ratio, never mind the currently recommended 1:2:4. It might work, but might not, or might be unreliable in certain weather. I think for a public demonstration, you want the most robust system you can get, to give a good impression to the public. You also want a significantly taller chimney to improve the draft. For a J-tube system, the chimney should be about the same size as the core. Half the cross section is a very big reduction and may choke the draft, not letting the fire burn strongly.
The bench looks fine. The gaps will let heat circulate relatively freely. I would even make more gaps in the center row. You want the chimney opening from the bench to be at the lower half of the bench cavity (as you show), so the hottest air stays inside until it cools and falls to the bottom.
Hello again! So, I had a chat with a local oven-builder. He doesn't have experience with Rocket Mass Heater, but with typical german mass heaters. He thinks that we will have problems starting the hot gases circulating in the system, because it is very cold outdoors in the german winter and we will not have any difference in temperature between the feed tube and the chimney. Normally, if it was built for indoors, we would have a difference in temperature because the feed tube would be indoors and the chimney would be outdoors. So, he believes that a) it would be very difficult to start directing the hot gases in the way they have to go, and that it would smoke from the feed tube. So, he suggested to make an by integrating a stove-door in front of the feed tube, and a removable schamotte-slab to close, when the front toor is open, the feed tube from the top. In this way, we could start the fire from there, by feeding the wood horizontally, and once the system is hot, to close that door, and continue feeding the fire from the top. He also said, that it would take 5-6 hours until the bench gets warm, because the bricks will be really cold if it's very cold outside.. I would be interested to know your thoughts on this matter. FYI, we will make the heat riser two bricks higher and the bell will also rise accordingly.
Also, how do you clean the ashes from the feed tube? We need a system that it will be easy for the people in the neighbourhood to understand how they can use it and maintain it.
What is usually done to help in cold starting a RMH fire is to put a bypass so the flue gases do not need to go much down before going out the chimney. As you have the chimney right next to the bell I would suggest putting a bypass damper (a slide damper would probably be simplest) between the upper side of the bell and the chimney. You will need to decide on the best, most reliable and damage-proof way to do this.
I think a front door will just encourage people to use the RMH the wrong way. It is fairly easy to reach to the back of the burn tunnel floor with a small scoop, like a sardine can with a short bent handle. There is not much ash from a properly-fired RMH in any case, unlike an ordinary woodstove, and it may be a good thing to have a thin layer of ash on the floor to insulate it from the flames.
It's not complicated at all. The basic idea is to make a channel or duct that goes from the upper bell to the chimney next to it, and build a tight damper into that channel so it can be completely shut off. For starting, the damper is opened, and once the fire is burning well, it is closed.
The simplest damper (but maybe not what you want in a publicly accessible place) would be a sheet of metal in a slot that can slide in and out to close or open the channel.
Once I built a Finnish style contraflow heater in a house of a friend. Beforehand, the chimney needed to be renovated and he choose to have it done by a professional. Professional one and two who were invited to make an estimate of the cost asked what the purpose of the refurbished stack would be. When they were told what it was for and how it worked they commented that it never would work, leading smoke down for over 1.5 meters (5') would stall the thing and it would never burn properly. Even so, they said all the smoke would come into the inhabited space and having such an idiot thing built was an utterly waste of money.
The third professional only said that he wasn't familiar with the concept so he couldn't possibly comment on the subject. That was the man who did the renovation and the heater was built in the following weeks. The same professional also did chimney sweeps, was called in a year later and wanted to see the thing running after he cleaned the chimney for the first time. He was surprised but also commented that he aquired a new view on how wood heating could be done.
The bottom line: don't trust people who are experts in building boats out of steel when you want to have a boat out of wood. The only thing I would say is that I still feel the chimney is on the short side. And when you want to be entirely sure, do a chimney damper like Glenn said or make a small door at the foot of the chimney so you would be able to prime the chimney.
But, please, pretty please, don't make a door in front of the J-tube. You are running the risk that the turbulence behind the proposed door when it's closed would kill the clean burning of a proper J-tube rocket heater. It would be a bad example of such a heater, when a door is unavoidable you could think it over to switch to a batch box instead.
we have made it! I attach here a couple of pictures. We managed to skip the opening in the front, and while we had engineered a bypass, we didn't even needed it. The smoke started to come out nicely from the chimney. What we want to understand still better, is how the J-tube works, and why the opening of the feed tube is on the top and not in the front. We want to be have better arguments, when people ask. I could not find a clear explanation online about the J-tube.
One little problem is that the fire though still goes vertically- the flames are coming out on the top and not much sideways. Is this a problem of how we feed the wood? Please see the picture.
As I m writing we re in the process of heating it, the bench is already warm one hour later!!! so this is big success. You can't imagine how many passer-bys showed interest and positive feedback to it!
The chimney seemed to be working already in its lower form, but then we added some bricks because the oven-builder who came from the city to supervise us suggested it, but we re not even sure we needed them. In the beginning it seemed to work well with the lower version as well.
Very glad to hear that it's working! If the smoke is reliably coming out the chimney and not back up the feed you have draft. However if the fire tends to creep up the sticks, you do not have strong draft. I think you will see an improvement in this if you make the chimney taller still - another half meter would not be too much, even a meter if you can do it.