Feedback on custom design

Hi all,

Following this thread and some of the mods there, here's my design which I surely could use some feedback on. In any case, I will report back on whether it worked.

In short:

it's an outdoor rocket mass heater

maximizes exhaust

The image might explain a bit more.

The Top View shows that there are two exhaust sections.
Section A: the main exhaust, which is already rather lengthy given the sizing of the stove.
Section B: the secondary exhaust, which is (as shown in side view) raised to heat the back of the outdoor 'cob sofa'.  This exhaust passes through the bell, which is supposed to heat the exhaust and create an additional pull
Bypass starter: is an opening directly into the bell, to start the RMH and is closed once heated enough to push the RMH

As one can see in the side view, the idea is to insulate the bench entirely except from the seating part and the back. This is to maximize the heat going to the seating area when outdoors.

What I'm concerned about is the dimensions of CSA and whether it can push/pull enough air to bridge the length of the first exhaust. Honestly, I hope the second works, but that is less crucial. It would be simply be cool to get the second going as well. If the first would work, I already would be happy enough.

Hi Dirk;  Welcome to Permies! And welcome to the wonderful world of rocket mass heaters!
It sounds like you have a solid plan.
Is this going to be a J Tube rocket? 6" or 8"?  
Building in a bypass will help get things started when cold.
Tell us more about your dimensions and the building materials that you are planning on using.
We would love to see photo's as you build!

I see Exit 1 and Exit 2... are you planning two different chimneys? Are you planning no chimney? Is your Section A the lower section in the side view? How does the bypass relate to the chimney or other parts of the flow path? I presume the 20/40cm part to the left is the J-tube core. With the whole bench only about 5 feet long, I don't think you will have enough duct length to extract most of the heat from the fire. I think you would get much better performance with a bell cavity in the bench instead of just ducts.

Thanks for the feedback. And sure, I will take pictures as I build along.

The feed is indeed a J feed, of either 6'' or 8''. Given that I worry that my 6'' might not push enough, I thought of having a 8''. The dimensions are as follows:
- Feed of 8''
- tunnel of 16''
- masonry bell with metal top to cool the air down
- exit 1 total length of 59''
- exit 2 with total length of 59''

Both exists are connected. I can close exit 1, and let the flow pass through exit 2. Both exists do not have a chimney indeed. Would that be a problem?

Would you make the duct longer? I thought you could not make the duct too long, for the bell would not have enough power to push the air through...

No chimney(s) would most likely be a problem.

Early developments in RMH technology were done by Ianto Evans in his settlement on the Oregon coast, where there is a constant breeze from one direction. This coupled with putting the outlet on the downwind side of the cottage allowed operation with no chimney. For most of the world, even a prevailing wind direction is not always reliable, and a breeze from the wrong direction can push the draft back into the feed with nasty smoke and flames coming up. It is always recommended to have a chimney; even a short one of 5 or 6 feet will give positive draft and keep your fire going strong. When you start experimenting with an uncovered J-tube core, it will rocket vigorously. As soon as you put a barrel over the riser, the draft will slow drastically and maybe even stall. Adding a short chimney from the base of the barrel will restore the draft and make it strong again.

A 6" J-tube is good for 30-35 feet of horizontal duct in a standard system, minus 5' for each 90 degree bend. So your bench with two 5' runs is only equivalent to 20', and will extract maybe half or a third of the heat you could get from it. A bell cavity in the bench could trap most of the heat to warm the bench instead of the sky. Then a short chimney rising next to the core could give decent draft in all weather and keep hot exhaust from hitting legs or faces.

Thanks for the reply.

The chimney makes sense. Will add a chimney indeed.

The bell is not so clear to me. I thought the draft was pushed by the temperature difference between the heat riser and the bell as such. An exposed bell allows the heat to drop dramatiically, hence create the draft towards the exhaust. However, inserting it into the 'sofa' would that not hinder the necessary heat difference?

Dirk, bells do not operate on draft.  They operate on gravity.  The exhaust from the firebox stratifies in the bell and the cooler gas drops down.  As this goes on and on, the cool gas escapes from a low exit at the end.

Alden Banniettis wrote:Dirk, bells do not operate on draft.  They operate on gravity.  The exhaust from the firebox stratifies in the bell and the cooler gas drops down.  As this goes on and on, the cool gas escapes from a low exit at the end.

Again, I do apologize if I'm not getting it, but...

If the operate on gravity, I assume that means that the cooled gas/air which is lighter that the hot burming gas, will flow to the exit. What I do not get then is that a chimney would aid the cooler/heavier air will exit a vertical chimney, or even more that a vertical chimney would help the air flow to exit. I would think that a clear exit that does not go up would then be better.

And I thought that the clash of temp would increase the flow, but maybe not. If not, and if the difference between the secondary burning chamber and the exposed barrel is not the key, than the barrel could indeed be integrated in the mass of the sofa.

Dirk, my experience is limited to masonry bell heaters utilizing the 'free gas flow' principle.  Draft is of little concern in these stoves.  By the time gases flow into the chimney, they are relatively cool and worthless.  These cool gases are not pulled up the chimney so much as they are pushed up.  Indeed, no vertical chimney is needed- a horizontal pipe exit from the bottom at the end of the bell that extends to the outdoors allows for the exit of the stove gases and the continuing flow of same.  Of course, the bell must consist of enough mass to absorb all of the heat that comes out of the firebox.  The bell that I am about to build is some twenty feet long, five feet high, and made mostly of cob.  The firebox, well-insulated, will generate a lot of heat, but by the time the gases will arrive at the opposite end, they will be entirely cooled.  Since there are no channels meandering through the inside, there is no need for draft and therefore no need for any chimney in the conventional sense.  Chimneys are utilized for draft.  When I built a model of this stove design, there was never any exhaust gas coming back at me out of the firebox.  As you mentioned, no chimney is better- but only if you do not employ draft in the stove's operation, because you save the time and money building that chimney.  I have little experience with 'rocket' stoves, so I cannot really say much about their use of a chimney.  There are people on this forum who have a lot of expertise in rocket stoves and I am sure they could explain the science to you much better than I can.  My personal preference is for kiln-bell stoves because I hate dealing with draft, chimneys, etc.  My stove will be going down the center of a rectangular cordwood house, the firebox at one end (in the kitchen) and the exhaust at the opposite end, under the floor and out.  Hope this helps.