Stove for a housebus (revisited)

[woodman]

With the wood burning at the back of the fire box the combustion is taking place up in the riser. Is this effecting the quality of burn initially?

[Donkey]

.. Dunno. Shouldn't be a problem. The Aprovecho cook stoves all place the wood directly below the riser. Their stoves don't really have a burn tunnel, it's the wood feed. Also, their feed uses a divider plate to divide the horizontal feed into two. They place a divider about 1/3 from the bottom. The top 2/3 is for the wood and the bottom 1/3 for air. They're calling it secondary air, as some flows along the wood, though I'm not too sure it's really acting as "secondary" air. This divider insures good airflow and acts as a pre-heater.

Something like this might be helpful with the potbelly conversion.. It's worth looking at.

[jgreen]

When I used this stove as it was designed (as a regular cast-iron stove), I welded up a grate that fit inside and held the wood slightly above the bottom of the stove (to insure better air flow and cleaner burn).

I considered using something like this in the conversion to a rocket stove, however I think I've decided against it because it seems slightly redundant and inconvenient. Redundant because I believe with careful loading of the wood and the right shape of burn tunnel/feed tube that there will be both good air flow and turbulence and the air should be effectively preheated as well. It seems inconvenient because when I used this stove before, the messiest job was cleaning out the ash because I first had to remove the grill (sending ash everywhere no matter how carefully I did it). I like the idea of a relatively flat bottom of the tunnel so that I can make a tool shaped to drag all of the ash from the rear of the tunnel forward and scoop it up without a big mess. And it seems like something I can play with later as an insert.

That was me just thinking out loud.

[canyon]

Nov 10, 2009 10:45:52 GMT -8 Donkey said:

Canyon,
With your water heater, there's probably a similar air control to what jgreen intends to use.
So - I'm sure there's a "sweet spot" in opening/air intake size when you use it...
Have you measured the size? And if you haven't will you, and will you post it here?

Well Donkey, I gave this some thought and observation over the last two fires and I think it depends. It depends on fire intensity, wood load, moisture content, system limitations etc. I measured the average "sweet spot" and it is 15 in^2 with the flue size roughly 30 in^2 in other words half flue size. But on some fires that spot has brought my radiating keg to cherry red while still ramping up and I've been worried about it collapsing or deforming with the 14 gallons of water bearing on it. So I do think the basic range is 1/3 to half of flue size but adjustability is sure nice as I find I have to tweak it frequently to optimize. I would like to try an aprovecho shelf with adjustability on both primary and secondary.

[Donkey]

I've wondered if there is away to make a passively adjusted airflow vane thingy.. Some kind of way to feedback loop stove demand to an aperture for optimal airflow...
Dunno.

[jgreen]

.... like a carburetor for a rocket stove?

[woodman]

Like the old clock spring on some of the old dampers that would open when the thing cooled down?

[Donkey]

??

It would need to be pretty sensitive to open for the draw of the fire, automatically adjust as the fire grows or dies to provide JUST the right amount of air at all times.
It might be something as simple as a spring loaded door with just the right tension on the spring.

Dunno.. It's kind of a tall order.
I think that there are some HVAC automatic air dampers. I think I've seen one, it's a flapper with a counterweight on it. The weight is on a threaded bar for adjustment. Maybe one of those could be used, or some like that could be made to better fit this application.. ??

[gorbull]

This is similar to a project I'm cotemplating. I am building a sauna and I was hoping to heat it with wood via a rocket heater. Unfortunately while I was busy pondering someone gave me a free wood stove. Damn! Undeterred I would still like to use principles of the rocket heater in conjunction with the woodstove. Anywho...I'm thinking it's going to look quite a bit like Jgreen's masterpiece. Difference being a sharper U turn down to the ground after the 4 foot insulated heat riser into a cob bench, then a 10 foot run and out the wall.

This woodstove is homemade out of 1/4" plate steel and I would like to weld a box on the top for the sauna rocks to heat. So I want this heater to act as a hybrid with enough quick heat escaping as in a traditional sauna with the efficient burn and storage of a rocket mass heater.

Question 1: Will this function as a rocket stove if there is refractory on the bottom and sides but not above the fire?

Question 2:
Can the "U-turn" pipe and down pipe be ordinary stove pipe or should they be insulated as well?

[canyon]

I only had firebrick on the bottom and sides at first on my 1/4 inch steel plate woodstove into rocket conversion (I call it my "tester" rocket) but I added firebrick on the ceiling as well ultimately. It worked fine without but better with it. You still get plenty of radiant even with the firebrick it just takes a little longer. The key is the insulated heat riser which it sounds like you are doing.
I've learned a few things about using regular stovepipe to pipe the gasses after a heat riser without burying in cob etc. One is you will get some leakage because of the positive pressure generated. I RTV'd all joints really well, but the first elbow and next joints expand/contract so much that it doesn't work for more than a few firings. So you will get CO etc. in your confined space! We really need gas tightness for this stuff. Another thing I've learned is the condensate can be significant so think about where it is going to go and how you are going to deal with it.

[gorbull]

Thanks for that information. Is it possible to find some kind of "one piece U-turn pipe" that will negate the connections as much as possible?

[gorbull]

On re-reading this post and others I'm lead to believe there is a good possibility of burning out the woodstove if the interior is not fully lined and protected....something to do with oxygen rich fire... Is this correct? If so, how do I go about applying firebrick to the ceiling of my woodstove?

Thanks!

[canyon]

You're correct. Where oxygen is plentiful with the intense heat of the rocket, steel will burn (oxidize) over time. I lined the bottom of my stove with fire brick first then the sides with bricks that I cut to fit with enough vertical room to support the ceiling bricks(1/2 thickness hard firebrick).

[gorbull]

Thanks Canyon,
You wouldn't have any pictures of the inside of your stove, would you?

[canyon]

Sorry gorbull, I don't have a digital camera and my wife who does have one isn't around. Last time I tried that business it took Donkey a bit of work to upload 'em for me and I still haven't learned how to do it. Silly me.