mini rocket mass heater

[chrisburge]

Even though this system has been developing under the posted heading 4.5" rocket stove with pellet feeder and ashbox , I felt it necessary to re-post it under this new heading because it has truly evolved into a successfully operational mini rocket mass heater. Plus, for those who are struggling with the need for small footprint/low mass/limited output applications, I think it might be more useful to post something that concisely presents all of the pros and cons of such a system, design parameters, and a clear illustration of what I've put together.

To begin with, I would consider this system experimental, but completely functional. In addition, although my application is somewhat unorthodox, I think there are some aspects that can be generally applied to smaller systems --if not simply encourage others that such a system is possible. However, there were a few definite operational challenges that I suspected during the design and mock-up phases of this project, that have been confirmed during installation and optimizing...

--> The exhaust must have a very limited horizontal run.
--> The exhaust requires a vertical chimney that is at least 12" taller than the top of the barrel.
--> 4" is the lowest limit to which a properly drafting mini-rrmh system can be scaled.
--> The smaller combustion area requires a grate, ash collection, and adjustable bottom draft to prevent occlusion from cinders and maintain proper burn temp.

As far as the limitations on a horizontal run: I would have to say that a lot depends on how tall the vertical chimney is as to how long a horizontal run can go, but not a lot can be expected from a 4" system; plus, you can't extract all of the heat or the gases will stall out when they try to go up. On the whole, you have to sacrifice a little bit on the efficiency side of the equation in order to have enough heat leftover for the vertical chimney to do its job.

Bottom draft is a tricky subject and can easily backfire on you (pun intended ). Sor far, I would have to say that a good rule of thumb is that the total CSA of the combined feed tube and bottom draft openings cannot exceed the mean CSA of the system. As a practical example, this system has a feed tube opening that is 4" x 4.5" and a draft opening that is 4" x 0.5", but they both can't be fully open at the same time. The only times I use the bottom draft is either when I'm using the pellet feeder (which takes up half of the feed tube) or when I'm shutting the stove down-- whereby I close the lid on the feed completely and leave the bottom draft open to burn out whatever is left. I'm definitely getting good temps because this is my grate after 1 week:

This spalling is not an issue as i have designed this to be cheap and easily replaceable. A short piece of HSS 2x4 is easy to come by at the scrapyard... 10 min with the angle grinder... done.

Here's a SketchUp of what's going on... [the fireplace is only represented by the dimensions of its interior]

As you can see from the drawing, what makes this a "mass" system by default is the fact that I've integrated it into the existing fireplace --which happens to be constructed atop a 3'x3'x5' concrete block. Unfortunately, the major detractor to this huge block is that it's in direct contact with the ground-- essentially making it impossible to fully 'charge' it with thermal energy. But, since concrete is so slow at conducting heat, I've found that after a long (6-8hr) burn, I can sort of charge up the top half and get some residual heat, but it's mostly delivered right back into the core of the stove. I have a pile of brick rubble and big pieces of basalt that are supposed to fill in the exhaust side of, as my wife refers to it, “the big brick box” (-- she's not too fond of my design), but I was waiting until I'd worked out all the final bugs with draft.

Here's the list of critical dimensions (in inches):

feed tube opening: 4 x 4.5
bottom draft opening (w/ ashbox in place): 4 x 0.5
burn tunnel: 3 x 4.5
total burn tunnel length (from front of feed tube to back of riser): 13
length of burn tunnel ceiling: 4.5
bottom course of riser flue: 4.5 x 4.5
second course of riser flue: 4.25 x 4.25
third course of riser flue: 4 x 4
riser: 4 (diameter) x 17
total riser height: 27.25
interior barrel dimensions: 14 (dia) x 18.25
top gap: 1.25
manifold opening: (circle segment) 10 (chord) x 4 height
exhaust: 4 (dia)


The original intent of this system was supplemental heat, but so far, when I go toe to toe with the gas furnace, I win. Prime example: got home last night from picking up the wife and kids; my wife went straight to the thermostat and turned it up a little to take the chill off the house; I went over to the other side of the living room and fired up the rocket; then, about halfway through what would be expected as the typical run-time for the gas furnace to bring the temp up 3 degrees, the furnace shuts down-- I was able to heat up the living space of the house faster than the central heating system!

Optimizations after installation consisted mainly of futzing with the exhaust; shortening the horizontal run, removing a 90degree turn, and keeping the smoke path above the negative pressure plane of the system. But the biggest improvement to the draft velocity was installing this reducer plate at the bottom of the burn tunnel:

The bevelled edge catches coals and pellets that fall into the burn tunnel and holds them against the slots on the side of the grate where they are burned completely, but more importantly, the height of the plate reduces the burn tunnel from 4.25" x 4.5" to 3" x 4.5". Decreasing the CSA at the point in the system where the gases are expanding increases their velocity. This is the basic principle behind any rocket engine used for propulsion. So far, this one change has provided the greatest improvement to the system's overall function.

Here's a video of it burning with the new reducer installed:


Now for the bad news.... it can't stay. I promised my wife that this was an experimental installation to prove the viability of such a system and that as soon as the burn season was over, I would disassemble it and rebuild the pseudo-rumford insert that I had mocked up in the fireplace over the holidays (she misses the aesthetic of an open fire). BUT, now that I have proven to her that I can effectively heat the house with a scaled down unit, I have been given the go-ahead to develop a 6" system that can go in the basement and tie into the central heating system. WOOHOO!

[satamax]

Chris, i have good news for you, you can make it open fire. Re read the horizontal feed thread. donkey32.proboards.com/index.cgi?board=experiment&action=display&thread=511

If there's enough room you could also fit the barrel into the chimney. Well, you'd have to block the funnel part of the chimney. Well, if you want, later i'll do a drawing of what i have in mind. For the moment, it's the morning and i don't have much time. Wouls be good if you had the internal dimentions of the fireplace and chimney handy.

[chrisburge]

In my opinion, the issue with a horizontal batch feed is that it doesn't seem to fully incinerate all of the fuel (there's an awful lot of cinders left in the firebox in the photos). As a batch burns, ashes build up in the bottom of the firebox and effectively insulate what they cover. The bottom corners and edges become dead zones where ashes bury fuel, starve it for oxygen, and reduction temperatures are never reached.

That was the problem I found while experimenting with small J-tubes: there needs to be enough fuel burning at once to achieve "clean" temps, but, by doing so in such a small firebox, the fuel couldn't burn fast enough; system chokes out, bottom of the firebox builds up coals; smokeback; flameout; end of test. That's the main reason that I started to incorporate some form of induction and ash collection into the design: to allow oxygen to reach the entire burn area without ashes slowing down the process in the corners.

And believe me, I went through many design iterations with the idea of putting the barrel in the fireplace (including the thought of using a 55-gal barrel to fabricate a heat shroud that could have forced air blown through), but there are a whole host of problems with that... mainly that, due to the radical taper of the flue, that's about as high as as I could get the barrel-- were it in the fireplace. Plus, the 110-year-old iron flapper only gives me a 3" opening, so there's not a lot of room.

...and, remember... temporary:(

[Deleted]

Mar 6, 2013 14:24:10 GMT -8 chrisburge said:

--> 4" is the lowest limit to which a properly drafting mini-rrmh system can be scaled.

Combining a 4" burn tunnel with 3" feed and chimney works nicely for cooking.
I cannot see any reason why this should not work for a rmh.

[pinhead]

That's a neat 4-inch system you've got there! I've contemplated a similar system, though I'm not nearly as good with brickwork as you. Ever since I built a Peterberg stove, though, I've moved more and more in that direction.

Just FYI, though, my Peterberg stove doesn't leave any coals once it goes out; just fine ash...

As long as it is lit as Peterberg suggests -- kindling started at the base of the throat and fuel stacked on top of that -- all of the wood is burnt.

[peterberg]

Mar 7, 2013 6:23:42 GMT -8 chrisburge said:

In my opinion, the issue with a horizontal batch feed is that it doesn't seem to fully incinerate all of the fuel (there's an awful lot of cinders left in the firebox in the photos). As a batch burns, ashes build up in the bottom of the firebox and effectively insulate what they cover. The bottom corners and edges become dead zones where ashes bury fuel, starve it for oxygen, and reduction temperatures are never reached.

Chris, I agree with that, as far as it is done with a normal rocket stove layout. BUT... It's entirely possible to build a rocket equipped with a horizontal batch feed which produce very good results, challenging the best standard RMH's around. Please read in the experiments corner the thread "Adventures with a horizontal feed". Not only I would think it's doable, I've also used the proper equipment to measure and prove the result of 6 months work.

Two of the 14 finalists of the Next Generation Wood Stove Design Challenge are going to use a batch box rocket of this design. Namely, Walker Stoves and Dragon Heaters.

[Donkey]

Neat stove..

Mar 6, 2013 14:24:10 GMT -8 chrisburge said:

But the biggest improvement to the draft velocity was installing this reducer plate at the bottom of the burn tunnel:

The bevelled edge catches coals and pellets that fall into the burn tunnel and holds them against the slots on the side of the grate where they are burned completely, but more importantly, the height of the plate reduces the burn tunnel from 4.25" x 4.5" to 3" x 4.5". Decreasing the CSA at the point in the system where the gases are expanding increases their velocity. This is the basic principle behind any rocket engine used for propulsion. So far, this one change has provided the greatest improvement to the system's overall function.

Interesting.. Nice function stack there.
In a way, it's kind of like a "trip wire" except that it takes up a good deal more space and it's on the floor rather than the ceiling. It will not only create a higher velocity, it should kick up some turbulence as well.

[pinhead]

Mar 7, 2013 8:25:05 GMT -8 Donkey said:

Neat stove..

Mar 6, 2013 14:24:10 GMT -8 chrisburge said:

But the biggest improvement to the draft velocity was installing this reducer plate at the bottom of the burn tunnel:

The bevelled edge catches coals and pellets that fall into the burn tunnel and holds them against the slots on the side of the grate where they are burned completely, but more importantly, the height of the plate reduces the burn tunnel from 4.25" x 4.5" to 3" x 4.5". Decreasing the CSA at the point in the system where the gases are expanding increases their velocity. This is the basic principle behind any rocket engine used for propulsion. So far, this one change has provided the greatest improvement to the system's overall function.

Interesting.. Nice function stack there.
In a way, it's kind of like a "trip wire" except that it takes up a good deal more space and it's on the floor rather than the ceiling. It will not only create a higher velocity, it should kick up some turbulence as well.

I was contemplating combining the narrow "port" of the Peterberg stove with the feed of a downdraft rocket stove. The effectiveness of this "reducer plate," as chrisburge calls it, seems to strengthen the theory behind the effectiveness of such a modification.

[ringoism]

Mar 7, 2013 6:23:42 GMT -8 chrisburge said:

That was the problem I found while experimenting with small J-tubes: there needs to be enough fuel burning at once to achieve "clean" temps, but, by doing so in such a small firebox, the fuel couldn't burn fast enough; system chokes out, bottom of the firebox builds up coals; smokeback; flameout; end of test. That's the main reason that I started to incorporate some form of induction and ash collection into the design: to allow oxygen to reach the entire burn area without ashes slowing down the process in the corners.

Really appreciate your post. I've been test-firing my experimental 3-1/2" J-tube the past couple days and agree with your assessment of the basic challenges; I'll probably be trying to integrate a couple of your ideas. Mine's running fairly well with a 3ft high riser, 11" diameter barrel and no chimney as yet - usually no smoke on the output (unless fresh wood has just been added or some pieces get into a particularly bad location), and doesn't stall even after some hours - but yes, coals are building up, and I'm having occasional trouble with smokeback / fire creeping up the feed, which is the one thing keeping me from installing it inside (where it really needs to be real soon - snow forecast for tomorrow).

Thanks again.

-Eric

[erikweaver]

Mar 7, 2013 9:47:37 GMT -8 pinhead said:

Mar 7, 2013 8:25:05 GMT -8 Donkey said:

Neat stove..



Interesting.. Nice function stack there.
In a way, it's kind of like a "trip wire" except that it takes up a good deal more space and it's on the floor rather than the ceiling. It will not only create a higher velocity, it should kick up some turbulence as well.

I was contemplating combining the narrow "port" of the Peterberg stove with the feed of a downdraft rocket stove. The effectiveness of this "reducer plate," as chrisburge calls it, seems to strengthen the theory behind the effectiveness of such a modification.

I too have been wondering about combining Peterberg's "port" with a standard down-draft rocket stove feed.  Have you tried that? 

[pinhead]

No I haven't; I've moved towards the Peterberg batch box due to the amount of heat needed in my situation. I've used the P-Channel with success and am in the process of building an S-Portal for the new stove I'm building.

[erikweaver]

Nov 21, 2014 12:15:15 GMT -8 pinhead said:

No I haven't; I've moved towards the Peterberg batch box due to the amount of heat needed in my situation. I've used the P-Channel with success and am in the process of building an S-Portal for the new stove I'm building.

Yes, I want to say I recall a thread discussing the port + J-style rocket stove, and that while maybe it could work on paper, in theory at least, that in the real-world application, the primary air in-take is to unpredictable, constantly changing as the fuel burns away.  But I don't know what thread that may have been, so maybe I'm remembering incorrectly. 

[patamos]

I recall the 'P-channel' evolving in the thread history of 'small scale developments'

could be wrong