Rocket-stovin’ is tricky business. Witness the evolution of my own backyard experimentation…
I used to have a neighbour named Chris. He moved away and Scott moved in. Chris worked with concrete, so he had his yard landscaped with all varieties of concrete blocks and bricks. Scott did his own landscaping and said I could have all the bricks that he removed. And so here we are…
The inital effort shows promise, even though I proceeded without any regard for vital details like how to construct a heat exchanger. That’ll come soon enough. Still, how do I cover this thing?
Back to the drawing board…
Overall the experiment was a success. This second prototype will likely lend itself to the final configuration. Now I need to find a steel drum to cap it all off. Also, how am I going to give myself access to the burn chamber? In addition to priming the furnace, I’ll need a way to remove ashes.
The goal of this third round of prototyping was to cap off the chimney with my lovely new steel drum. If I know my rocket stoves (and I don’t) containing the flue gases this way causes them to reignite, which produces even more efficient heat and lowers emissions.
I want the furnace to be self-fueling. That is, I want to place a log vertically and have it burn like a cigarette. As the log is consumed gravity pulls the unburned wood into the furnace. With this is mind, it made sense to use some of this nifty little bricks to start building up a foundation.
My backyard is very small, but thankfully my backyard survey revealed sufficient space. I didn’t think I’d need this much, but my furnace keeps getting bigger and bigger.
The foundation also kept spreading. I needed to allow enough room to set the barrel into place. As such, distance from the fueling chute to the burn chamber kept growing.
I left an opening at the base of the furnace to allow the required access to the burn chamber. Everything is basically sealed up tight and ready for the final component.
The curved fire pit bricks are the exact diameter of the steel drum. That was a pleasant surprise.
By the time I was done, it was too dark to test the deployment. Upon reflection, I realized I still need a way to seal off the burn chamber access hatch while allowing exhaust to escape.
The exhaust port needs to be sealed off from the fueling chute, burn chamber, and chimney. If it isn’t, smoke and flue gases will accumulate and snuff out the flame. To accomplish this, I had to expand the base (good thing I still have room).
Then I sealed off the passage from the chimney to what would become the exhaust port.
With the steel drum in place, I realized I’d needed to expand the base further for support and stability.
This brought me right up to the edge of where I’m allowed to build.
After stacking the firepit bricks around the base of the drum, Prototype 4 was ready to be fired up.
Alas, it wasn’t meant to be. The old problem of actually getting the fire going required that I dismantle the fueling chute and light the fire directly at the base of the chimney.
Once I finally got the fire got started, smoke started billowing out at the base of the drum. The bricks do not contain the smoke. The exhaust port does nothing.
The smoke escaping from the sides is a problem for a couple of reasons. First, I want to keep a low profile so that my neighbours have no reason to complain. If the rocket stove is working properly, it should only be emitting CO2 and water vapour. Second, the smoke kept getting sucked into the air intake. I suspect this is why the fire never really got very hot. I could rest my bare hand on the top of the steel drum for several seconds comfortably.
This is all very disappointing, because winter is half over and I have no outdoor furnace to show for my efforts. I’m thinking I may need to take up welding and get some proper steel pipes to act as the fueling chute and chimney. Until that happens, I may have a viable hack with some standard HVAC ducts (coming soon)…
After some soul searching and heavy lifting, I’ve decided that I need to rethink my approach toward the rocket stove furnace. I dismantled Version 2 and started from scratch:
I was trying to address the problem of exhausting the steel drum. My intention was to see if I could build a sand box around it, and indeed, fill it with sand. This would trap the smoke and force it through the out hole.
The foundation was laid to make it easier to clean up sand in the spring.
Those curved bricks have come in handy in all sorts of unexpected ways.
These are the incipient chimney and fuel chute (top).
This all got built up into this:
It wasn’t wide enough to accommodate the drum and a bunch of sand, so I tore down the walls and added some bricks for spacers
At the end of it all (before placing the drum), it looked viable.
It was not.
I’m going to have to revisit my approach to building this outdoor furnace. It’s too smokey, too hard to light, and gets choked with ash in no time. The drum got hot, but I could still rest my hand on the surface. The water in the pot never even got luke warm. Building my furnace from recycled landscaping bricks alone is not viable.
With winter about halfway over, I also need to rethink my goal for spring. I want to try burying the drum in my garden when the ground thaws, just to see if I can get the rocket stove to work. My new goal though, will be to try recycling some new material (like steel) into a water heater that would also charge a battery. I guess I’ll have to think of a new name…
Anyhoo, concrete may be viable, but I decided several months ago that steel would be better. My old friend Trav was kind enough to weld me up a new prototype in exchange for a donation to his company’s Christmas-time adopt-a-family initiative. Unlike myself, Trav’s a good dude and he builds a mean rocket stove:
Winter’s coming and reality is quickly setting in. I do not want to have to traipse through the snow to fuel this bloody thing. Calgary bylaws say it’s okay to have this thing on my deck as long as it’s on concrete. It’s not totally clear, but it also looks like I need to have it beyond two meters of any structure (i.e., my house and the wooden railings on my deck). That leaves me this much room:
Given the nature of the rocket stove, this is about three times more space than I need to remain compliant with city bylaws. The wood combusts in about the space filled by one of those walkway bricks pictured above.
The challenge, as always, is getting the hot air generated from the stove in through my window. Putting on the patio is going to make that whole thing a lot easier.
The following images depict the basic idea of how clean cold air is passed through the heat exchange and pumped through the window. Obviously, I need more duct.
The metal frame holding up the drum came from my old BBQ.
I got that staircase thing off our community’s Facebook auction site. This is precisely the purpose I had in mind.
When all is said and done, I’m a bit closer to my goal of heating my home with an outdoor wood-fired furnace, but there’s still much work to be done. Unlike previous prototypes, this one is easy to light. It’s still not getting very hot though. I tried boiling some water, but it never quite got to boiling point.
Though the barrel is about the right distance from the rocket stove’s chimney, the exhaust is not fully contained and I suspect I’m not benefiting from the flue gases reigniting. Smoke is still venting from the top of the furnace instead of the bottom. I’m still in search of a way to contain this all effectively so as to generate the desired catalytic converter effect. Nonetheless, this has been the most viable prototype so far. Apparent inefficiencies aside, this prototype has the potential to lower our gas bill this winter, which could arrive any day now, it being August in Calgary.
Common rocket stove lore states that the chimney needs to be insulated. It actually took a bit of effort to track down fire bricks in Calgary. I eventually found what I needed at Brock White.