Pellet Stove 3.0 (now in color)

After almost 2 years since the last iteration and my considerations on heating the cabin, I’ve finally gone ahead and made some much needed improvements; particularly related to safety. This version does away with using old gas cylinders (propane etc…) as the burn chamber and sticks to plain, steel, square tubing and flat stock with maybe an angle or two thrown in for reinforcement. This was following some much needed advice I got from a welder who emailed me after reading my previous post (thanks, Mike!)

For comparison, this is the original “automatic stove” idea.

This is a quick sketch of all my ideas for an "automatic" pellet stove

This is a quick sketch of all my ideas for an “automatic” pellet stove

And the 2.0 design.

Stove 2.0 with improvements

Stove 2.0 with improvements

And the new and improved 3.0. Note, the flue/cleanout setup is the same as in version 2.0.

Stove 3.0 with new safety measures and simpler materials.

Stove 3.0 with new safety measures and simpler materials.

For this design, I’ve made using flux core welding wire to put it together a bit easier. Flux core tends to be more beginner-accessible (no gas needed) a tad safer and requires less skill, which is a big deal since this design is meant to be DIY. I’ve also increased the diagram size and font sizes by request. Apparently, a lot of folks couldn’t read my rubbish text without squinting at the screen. Apologies for that. I really didn’t expect any more than the 4-5 regulars who read my blog to be interested in the design, let alone the 300(!) who emailed me.

I’ve separated the interior components to two easily distinguishable sections : The stainless steel pellet hopper made of thinner flat sheets clad in cement board and the burn chamber with its all square tubing and flat stock construction.

Flat stock is almost always easier to weld than curved surfaces; as is cutting it. If your material has the same thickness, it makes switching temperatures, changing welding wire, voltage etc… completely unnecessary within each section. We can stick to one temp, one voltage, one thickness and, best of all, we’re not relying on old gas cylinders which may or may not withstand the high temperatures they were never designed to endure.

The only time any temp changes would be necessary is for the stainless steel hopper. I elected to use stainless here since often, the pellets you get from the store may contain moisture. The pellets in the burn chamber will, of course, quickly dry out making moisture less of a problem. The thinner stainless steel hopper is also separated from the hot burn chamber by the slight gap created by the space needed for the pellet stop. This tiny gap, along with the cement board wrapped around it, greatly reduces the amount of heat transferred to the rest of the hopper and our (highly flammable) fuel.

The grate is now designed to be replaced relatively easily if necessary since it’s in one piece and welded only at one spot that’s accessible by the air inlet pipe. There are two grates to ensure burnt ashes fall away without being sucked back into the burn chamber and without clogging the air inlet. In addition, this allows hot ashes to cool down in the lower chamber which isn’t as exposed to the full heat of the burn grate.

Also, being mildly OCD, I wanted to ensure there’s ample room to put a wide tray underneath the stove to collect all the burnt ashes without making a mess of my floor. The bent steel rods used as feet reduce the heat transfer to the floor, which may be bamboo or hardwood.

I also tried reducing the overall size of the stove. This one is about the same height and is roughly 2 – 3 times the width as a full ATX tower computer case, like the one housing the computer I’m typing this post in. I want it to be safe and stable, produce enough heat while still be “out of my way” as much as possible. The interior of the entire stove case is clad in cement board (such as Durock®) and the case itself is cut 4 – 5 inches short of the front hot exhaust tube with only cement board used to close opening. This reduces the heat transmission from the exhaust to the rest of the case while at the same time allowing me to reduce the interior volume needed for insulation.

If anyone does build this design or find it useful in any way, please drop me a line and let me know. Any improvements or suggestions are most welcome.

Enjoy!

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Pellet Stove 2.0

After yesterday’s post on designing my heating options for the cabin, I got a flood of the usual “you’re doing it wrong” emails. Not that I’m complaining, I actually found most to be very helpful and I’m grateful to everyone who took the time to write them.

Using some of the suggested improvements and a bit more browsing on YouTube, I came up with some modifications. One of the things that struck me after I made the initial post is that a pellet stove is basically a Rocket Stove on its side. It’s pretty much the same principle of thoroughly burning the fuel by mixing the air with the wood gas generated during the combustion process.

As with many pellet stove designs, this means it would benefit from a vortex generator; basically a static fan that sits in the flame allowing the gases moving past the blades to spin up (as opposed to an active fan which does the spinning). Also, the exhaust tube is much too small which can cause a potential backdraft through the cleanout or, more disastrously, out the hopper setting fire the rest of the fuel. The quicker I get the flames out the exhaust, the better it is. The gases will continue to combust in the exhaust tube as well, so really the burning process extends out the stove cabinet.

Stove 2.0 with improvements

It did surprise me that I didn’t really need a second starter burner to get the gases going up the flue. I thought this would have been absolutely essential to prevent a backdraft, but apparently, as long as the air is coming from below in the primary burner, the hot gasses will exit the larger opening. Flames travelling the path of least resistance and all.

One thing I overlooked in the previous design is how I’m going to clean the whole thing. No matter how “clean burning” Rocket Stoves are, I imagine eventually you’re gonna need to clean the tubes In the rare circumstance of an incomplete burn, perhaps due to incorrectly prepared fuel, you would still need the option of a cleanout. This is why I worry when I see elaborately constructed Rocket Stoves built into the dwelling. If things go wrong, you’re in a whole heap of trouble and have a humongous mess on your hands deconstructing the whole thing. A hole or two here and there to clean, couldn’t hurt.

I still have to decide how long I want the exhaust tube to be before connecting with the flue. Obviously, it can’t be too long since that runs the risk of a potential backdraft, but I also want to get as much heat into the living space before it leaves the cabin envelope. I’ll need to do some experimenting.

Now I need some welding lessons.