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.



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.

Heating the Cabin

This is just me thinking out loud. I may or may not use these ideas in my own cabin, but in case someone else is also looking for a place to start, they’re welcome to use them.

The space is so small, I can either easily turn it into a sauna or expend the same futile effort of a candle trying to heat a concert hall, so moderation is a big issue. Above all else, whatever method I choose has to be relatively self-governing since I don’t feel like waking up every 4 hours to replenish the heat (although I wake up every 4 hours anyway due to insomnia).

The first place I looked was, naturally, solar.

Electrical = No Dice

I quickly eliminated an electrical approach due to the excessive drain. No matter the output of any photovoltaic cell + battery combination, in-floor electrical heating or some sort of space heater would be too much of a drain on the power supply for the size of the cabin.  According to my current estimate of the South facing roof, I would have approximately 120 square feet to play with and even if I covered every inch of it with PVs, I don’t think it would be enough.

Also, if there is an extended period of overcast in the Winter, I’ll be in serious trouble trying to maintain the heat while toning down the drain of any other appliances. This sort of juggling feels like too much of a hassle and the whole point of this was to get away from stress. I want to enjoy those little luxuries like bright lights, induction cooker, a microwave and maybe a toaster every once-in-a-while without having to resort to Watt-pinching.

Solar Hot Air?

If this is supplementary to another method, then it works the best. There are countless examples on the web of solar heaters for the home, all with varying efficiencies and ease of construction. What I wanted was something that I know for sure I can build, won’t cost a fortune and will be reasonably effective without having to turn one whole wall into a collector.

And above all else I don’t want to have to notice that it’s there.

I then looked at solar heaters using Aluminum downspouts as collectors and came across the best iteration of it so far. What I really like about this concept is how low-profile it is. Granted he’s trying to heat a full house, basement and all, but I think I can get away with less than half this size since the overall cabin is small. Best of all, I don’t always have to look at it.

It would be a fairly simple affair to wire up a fan or two, a thermometer inside the cabin and one in the collector to a microcontroller and power the whole ensemble with photovoltaics. Computer fans typically run on very low power, most are fairly quiet and are designed for variable speeds making them the ideal choice, I think. Also, I can easily replace these parts without too much expense when they break down.

But as I mentioned above, this will only work as a supplement to something else for days that are overcast for long periods or (heaven forbid) if the cabin got struck by lightning and my arrester and breakers failed frying all the electrical stuff.

Small Stove

I’m a bit ambivalent about all the DIY stove designs floating around out there. On one hand, these are improvised designs, but on the other hand, these are improvised designs. There’s a reason why commercial stoves aren’t made of used air, Freon, LPG or similar pressurized tanks.

Regardless of origin, the traditional approach to a stove is basically a container to burn the fuel, a one-way air input control, a baffle to delay the hot gases existing before heating the surrounding air and finally, a flue. I think it’s time we started moving away from the traditional arrangements for this combination.

I want my stove to not look like a stove.

Traditional stoves have a quaint appeal that works well in rustic settings, but that’s not the look I’m going for. Besides the efficiency issues, traditional stoves take up more room and are fairly dangerous (especially for a clutz like me). In this vein, I’ve looked at these allegedly hyper-efficient Rocket Stoves which are all the rage these days.

They turn an ordinary flame into a twirling vortex of hot gasses inside the burn chamber, shoot inside an outer chamber warming the ambient air, then in turn can be vented under a bench for warmth before being exhausted outside. That’s quite a long list of opportunities to use the hot gasses for actual heat as opposed to conventional stoves where most of it is lost to the outside air.


Alas, the Rocket Stove also has the same problem: It looks like a stove.

If you need a demonstration of how one would construct such a stove simply, this gentleman had done so with ordinary household items. The demo is in Japanese, but the explanation is very well done visually so it should still be understandable.

It also takes up too much space and it feels like there’s way too much effort and mess to get an efficient heating system going. Besides that, it still needs that tendering that I don’t like so much as mentioned at the beginning of this post and the style is all wrong. For some reason, every Rocket Mass Heater I see reminds me of the structures on Tatooine.

I’m also more inclined to go with a biomass option for fuel since it gives me the greatest flexibility for automation. If I want my stove to shut off or limit itself by automatically (I’m fine with having to start it), then conventional — therefore odd sized — wood and sticks ain’t gonna cut it. I need a steady stream of fuel in a predictable size range that’s easy to manage while at the same time being environmentally friendly.

Pellets it is!

Not only do they offer the most flexibility in terms of fuel (I can use everything from grass, hard or soft wood to even plant fiber derived cellulose), it gives me the ability to control the flame without too much handling. More fuel + more air = more heat. Less fuel + less air = less heat. And that’s about it.

I’ve been thinking of applying the rocket stove idea to pellet fuel, but most of the rocket stoves I’ve seen require manual feeding of some sort. Besides, there’s the mess problem as mentioned above. I thought this can all be improved with something very simple like a chimney connected to a burn chamber by a tube of some sort that will act as the horizontal leg of the rocket stove.

Lo and behold, someone already came up with the same idea ages ago!

This video is also in Japanese, but the product demo is very visual as above.

What I really like about this is how simple it is and how inoffensive it is visually. Most of the hodgepodge rocket stoves I’ve seen look like they belong on Serenity, the spaceship, and not in a good way. While this is a testament to the improvisational capabilities of the builder, I don’t feel like I should improvise on safety.

By combining the Rocket Stove idea with some of the concepts of the above product, I figured I can build myself a stove that ran on pellets, doesn’t look like a stove (or at least is easy to hide that fact) and isn’t cumbersome to operate. So I set about designing a stove that does just that.

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

If the pellet hopper has a lid, I can hide the whole thing as a piece of cabinetry or even use the top of the hopper as a small table. The air supply can be a feed from under the cabin so I don’t have to drill holes in walls and other nonsense. Also, by putting the air supply inside the cabinet, I can reduce the cold air leakage into the cabin from there.

Now that I look at this, I think the only “control point” necessary would be on the air supply. If I turn down the Oxygen, the burn rate goes down and so does the temperature. Though I’ll still need a shutoff for the pellets to completely stop the burner in an emergency.

I think I can hide the starter burner in a cabinet as well or I may not even need it if it turns out the length of the flue is enough. And the best thing about this design is that I don’t have to touch anything inside the cabinet until it’s time to clean the ashes. I’m sure I can automate this too somehow, but for now, this gives me the least amount of “homework” while operating the stove.

Just load with pellets, light it, walk away. Of course, I say that now, but we’ll see how well this will turn out.