Denny Nebgen

I thought this might have been discussed before but, I couldn't find it in my search results.

Okay, I love the idea of heating with a rocket mass heater. However, I am having trouble wrapping my head around using such a small firebox and burning the wood it holds up in 15-20 minutes, creating enough heat to heat the mass for any period of time. Wouldn't you need to fire the thing several times in order to get the mass up to temperature? If that is the case, couldn't the fire box be bigger? Just wondering how something like this could work say in a green house and not need to be fired several times a day to keep the green house warm? Thanks.

Rob Torcellini wrote:I've posted it elsewhere on this forum, but I have a pellet feeder design on my RMH that burns without having to do anything- just clean out the ash when the pan gets full. The hopper holds 6-7 hours (80 pounds) of pellets. I just fire it up and walk away.

Latest Pellet feeder design video

I don't suppose you would be willing to share any dimensions with the rest of us? The hopper sounds like the way to go. I hate the thought of buying wood pellets but, the firing a fire every 45 minutes will probably nullify the rocket stove for me. I could see walking up and down the hill to the green house every hour through the day but, having to do that through the night would just be something I'm not willing to do. Maybe propane isn't such a bad idea after all. Is there anyway to increase the size of the firebox to the point much more wood could be used to allow a much longer burn time? Thanks

Mathew, you want to keep reading this site and searching. I can't recall where but, I believe someone said 4 inch pipe didn't allow enough draw. The whole idea with the RMH is to heat enough "mass" to release heat over time. Your question is good though and something I am curious about. Just how big is big enough? Perhaps with determining the number of BTU s needed for the space and the number of BTU s a RMH gives off, the answer will be achievable? Then you can determine if it will do what you need. Does your high tunnel have two layers of plastic over it or just one? Some folks out our way are using two layers and a small fan to help create a layer of air space to help insulate.

Hilary thesane wrote:Whatever your greenhouse shape, design and material used, the cheapest way that I can see to add raw thermal mass, is to insulate the perimeter to two times the depth of your frostline. If your frost line is 18 inches, then insulate to 36 inches. This turns the ground under your greenhouse into thermal mass.

Now, the idea is to HEAT that thermal mass, and I believe a rocket Mass stove can do that. (You may add additional thermal mass as you wish. Water as part of an enclosed system requires a very small wattage pump to circulate it. And it can be used to transfer heat from your heat source to your ground.) The problem is where to put the RMH?

The two main designs are 1) Full wall glazing. All the walls allow sunlight in. These work great if you get mostly skydome light. 2) South facing glazing. The northern wall is insulated (and maybe some or all of the east and/or western walls. These work the best for areas that have direct sun light, presumably from a low in the sky sun to the south. In the latter design, the heating system would go against the north facing wall.

The issue that I see with using a RMH in a greenhouse is that people don't live in greenhouses. There isn't going to be someone there to feed the standard RMH little sticks of wood all day long. So either you need to design some continuous fuel feeding system or come up with a way to have your fuel burn by itself for several hours at a time. MY idea is to do the latter, and to do it with a batch charcoal making stove.

The retort design for making batches of charcoal seems like the answer to me. You fill it with wood that you want to turn into charcoal, you surround that with some kindling to get the process started, and you stand back and let the thing go. The gasses that are released by the wood heated without oxygen exit the inner chamber and burn at a very high temperature when they hit air. They continue to off-gas for several hours. And when there is nothing left but carbon, the process stops. There are several videos of these stoves on youtube. I will not bother posting any in this thread. But check them out on your own time.

Now the trick is incorporating a charcoal making retort into a RMH design. This may not be straightforward. The batch method presents some logistical issues. you need to open the thing up and remove your inner drum, barrel or whatever, and allow it to cool before opening. One other issue, a charge of wood turning into charcoal puts off a LOT of heat. Seriously. You would need a fast heat exchanging system to cool all that exhaust fast enough for the gravity draw system of a RMH to work properly. AND you don't want to cook your plants in the process. Charcoal retort stoves tend to have a minimum size or else they will not off-gas long enough to be self sustaining. I think a 25-30 gallon inner drum is about as small as you should go. Smaller than that, and they will not put out enough off-gasses.

Hillary, are you using a system like what you describe at this time? My only experience with the retort furnace is in using one to make bio char and over all it was a big disappointment for me. I used the system where you put the retort inside a concrete block structure and build a fire under the retort furnace in order to get the wood in the retort up to temp to burn off the gasses. I felt it took so way much more wood to make the char and the char we got out of the system just didn't make the whole procedure worth it to us. Perhaps in a much more inclosed area like a RMH the gases would help keep the fire going much better then they did in the bio char setup we had. The gases did help once the whole system got hot enough but they did not keep the fire going by themselves to finish the process and more wood was needed under the retort to keep the process going to the end. That is my only experience using a retort and , as I said, it left a bad taste in my mouth . Do you have any photos of your system? Thanks.

I would love to see more of this project that Bradley has going on. Thank you for the photos. Please keep us abreast of your progress.

Thank you , Mike and thanks for the pics. That helps give me an idea of what to expect . I suppose a double layer of plastic might also help.

I am seeing in some threads on the Rocket Mass heaters that there is some suggestion that they need fed quite often to keep them burning in order to get the mass to temperature that will maintain heat inside the greenhouse say throughout the night. This could be a real drawback for this idea of mine. I can see where if you are inside a home tending a fire throughout the day would not be a problem but, inside a green house you may not want to be inside there all day to tend the fire continuously. Any thoughts on this? Thanks.

S Bengi wrote:

The exhaust has as much H2O(steam) as there is CO2. Even though the RMH is really good, you still have Nitric oxides/acid, some sulphur oxide/acid and carbon monoxide, VOC and whatever did not get completely burnt. You never really want to inhale fire exhaust the carbon monoxide and other stuff is heavy and will settle to the ground possible killing pets, childrens and even you if you stay in there too long. I just dont think that it is worth it.

Thank you. Glad I asked. The thought I had was the CO2 would be good for plants in a green house. Thanks again for clarifying.

Just thought of another question. Is there a limit to the length of flue pipe running through your mass heat sink?

I see in one of the videos on this site there is were plans to build a RMH inside a cold sink pit type green house. Has that ever been completed? If so, how does it work?

Another question on that same line: If the exhaust is mostly steam can that be vented right into the green house? Is there any co2 coming out with the gases? Thanks.

I can understand the science behind these heaters. However I do see a couple potential problems.
1. If the gases can cool to a point where there can be condensation inside the flu before the gases exit the chimney, couldn't this cause the flue pipes to rust out in time? Perhaps this isn't a problem?
2. Could this be eliminated by placing a T right where the flue starts up the chimney to allow condensation to drain down underneath to a bed of gravel under the floor?
3. Is there a certain length of pipe that would be optimum for this type of heater that might keep the gasses say at 100 degrees C at the chimney so the gases would exit as steam and not cool too much?
4. Is there any limit to the length of pipe you can use say 20 ft./30ft/ 60ft as long as you have a clean out every so far?
5. What size of flu pipe is the best to use, 4",6", 8"? Does it matter?

I would love to see the internal paths of the flu pipes on some of those masonry heaters that KNelfson shared on that site. Thanks.