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mini rocket mass heater

Chris Burge


Joined: Oct 21, 2012
Posts: 88
Location: Spokane, Washington
    
    4

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!


"It is a miracle that curiosity survives formal education." -- Albert Einstein
Andor Horvath


Joined: Nov 28, 2012
Posts: 91
    
    1
awesome build and documentation, thanks for sharing
you might want to cross post this over to Donkey's board too


we CAN build a better world
Chris Burge


Joined: Oct 21, 2012
Posts: 88
Location: Spokane, Washington
    
    4
thank you and... done.
Lucy Guss


Joined: Mar 03, 2013
Posts: 12
Chris: Wow. Thanks so much for posting this. I have a 1970s manufactured fireplace with a hearth that might work with this. It is otherwise, mostly useless and inefficient except in emergency, no other heat source situations.

A couple of questions for you. Is your exhaust pipe run completely up the chimney or just the 12+ inches higher than the barrel?

My chimney is a metal chimney due to the nature of the fireplace. Any thoughts on the degree of heat/creosote coming from the exhaust as problematic for this?

I do not see a mantle in your photos. Any thoughts on how hot the tile behind the unit is getting? Though the hearth itself is brick, the facing on it is some funky faux brick with a mantle above.

Seriously, thanks again for doing this in the first place and sharing your information. This could potentially be a huge problem solver or partial problem solver for me.
Feng Nie


Joined: Mar 02, 2013
Posts: 3
This is soooo cool. Thanks for posting. It is sad thing that you have to take it down, I would have hired you to build one in my house regardless of what my wife would say. Anyway, I am planning to build one myself. I have a question that is general to all rocket mass heater though. Maybe you know the answer well. When the fire die down say when you want to clean the ash out, do you have a lot of smoke coming from the wood feed??
Chris Burge


Joined: Oct 21, 2012
Posts: 88
Location: Spokane, Washington
    
    4
Lucy Guss wrote: Is your exhaust pipe run completely up the chimney or just the 12+ inches higher than the barrel?

I'd love to make it longer, but 12" is as far as I can extend it without cutting a notch in the iron flue of the fireplace-- it only has a 3" opening

My chimney is a metal chimney due to the nature of the fireplace. Any thoughts on the degree of heat/creosote coming from the exhaust as problematic for this?

The barrel exchanges most of the heat into the room. With mass around the exhaust, I could probably have exhaust temps around 140-150F; right now they're ~200F when the stove is at temp.

I do not see a mantle in your photos. Any thoughts on how hot the tile behind the unit is getting? Though the hearth itself is brick, the facing on it is some funky faux brick with a mantle above.

The house is over 100 years old and that's the original fireplace-- well, what's left of it. It's made of brick and is quite stout since the backside of it protrudes 12" into the bedroom on the other side of the wall. There is a mantle about 24" above the top of the barrel. The fireplace actually has a rectangular opening that is a few inches higher than the tile facade. I have to keep the kettle of water on top as a heatshield for that horrible piece of wood trim that some foolish previous owner DIY'd around the fireplace --so, yes, it get's pretty toasty.

Feng Nie wrote: When the fire die down say when you want to clean the ash out, do you have a lot of smoke coming from the wood feed??

With the reducer improvement, now I don't even have to worry when the pellet feeder runs out because the draft is strong enough to suck the smoke right off the top. But, I clean it out in the morning before I fire it up --hardly any ash. Even then, I have to open the feed tube and let some air draft through it for about 2hrs before it's cool enough to stick my hand in there. I went three days without cleaning it, just to see... burned a total of 40lbs of pellets and about 4cu. ft. of wood... pulled out about 1cup of ash.
jacob green


Joined: Jan 11, 2014
Posts: 29
Chris...

F'ING WOW.

I love the small footprint and yet there is sill at least some thermal mass to help moderate temps.

What are the external dimensions of the barrel? And how many gallons is it?

Also, how much space between the riser and inside of barrel? maybe you listed that in your dimensions but I didn't understand which stat represented that value?

What do you mean you kept the exhaust "above the negative pressure plane"?

When you fill the feed tube, and pellet feeder, how long does it on on a burn?

I am assuming this system can be scaled up with no problem.
Sean Printz


Joined: Jul 29, 2014
Posts: 1
Where do you find a small barrel like that? What was its original purpose, size in gallons, dimensions, etc...

I am new to all this, but if the barrel gets to 800 degrees, is that not dangerous/skin melting/scary to have in your house?

I am really trying to convince my wife if an installation like this, so I am curious, and we have small children.

What kind of mortar did you use for the bricks that was temporary?

Overall it is a really nice looking design.

You mention not being able to have a lot of horizontal ducting, is that mostly due to your size constraints? Or due to the 6" pipe used?
Cindy Mathieu


Joined: Aug 19, 2013
Posts: 241
Location: near Houston, TX; zone 8b
    
    1
If you have small children, please find an alternative to a bare steel drum. They can get 800°F; any contact with that temperature metal would do permanent damage to the skin.


www.dragonheaters.com
http://blog.dragonheaters.com/
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 779
Location: Okanogan Highlands, Washington
    
  96
In my experience with the larger heaters, it's rare to get 800 F anywhere a small child could reach. If you get that hot, it will be at the top of the barrel, or within a few inches of the top, and that's over 48" above floor height.

I would consider them much more child-friendly than a woodstove, with surface temperatures at toddler-height on an 8" heater typically in the 200's to low 300's - lower than a hot pan of cookies just out of the oven.
I'd appreciate Chris or anyone's results from working with the smaller heaters - what's the barrel temperature, roughly?

-Erica


Play with nature, make nifty stuff:
www.ErnieAndErica.info
Nick Peterson


Joined: May 25, 2013
Posts: 5
Awesome! I'm really interested in scaled RMH for tiny spaces!

Any idea what the temps are coming out the other end?


Simple, Intentional Living!
http://www.livinlightly.com/
 
 
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