DIY Outdoor Wood Boiler

With winter rapidly approaching, I decided to try a real simple outdoor wood boiler build. I had an old water heater and air compressor tank to use as raw materials. Here's what I came up with:



With the heavy insulation (the outside of the boiler for the most part is under the boiling point of water) and ample air supply, the burn is quite clean. It can also manage several hours between reloads. Here's a short video that shows the exhaust:



The real expense with this was hooking it up to a simple hydronic system. Fortunately, I had most of the stuff lying around from previous projects. I'll show how to hook it up in a future video. I also have several planned modifications that I'll document as well.

At any rate, it works quite well as is, so I'll have cheap heat this winter

Here's the simple circulation system I designed for this boiler:



Earlier today, I measured the internal temperature at 1246 *F and rising. My thermocouple is only good to 1400 *F, so I didn't push my luck and removed it before seeing how high it would top out at. The exhaust 343 *F, so the boiler manages to extract a lot of heat from the system. I'd like to get that down to 275 *F if I can.

The homemade insulation works really well. The highest temperature I've measured on the stove body after several hours of use is 250 *F. This means a temperature difference of around 1000 degrees! I plan on using this waste heat to preheat combustion air to further increase efficiency.

Very interesting! I stumbled onto your rocket boiler videos a few months ago while searching for heating options. What happened with the rocket project? Is this one going to be more efficient?

I have radiant tubing in the slab of my berm home that I'm currently not utilizing. I'd like to do something like this in conjunction with solar thermal collectors to heat a 500+ gallon storage tank. Do you think it would be feasible to heat that much water with a system like one of yours?

Geo : My old family motto is, if it works don't fix it, I really like the Float, easy to make and replace if and when needed ! Big AL

Jared Blankenship wrote:Very interesting! I stumbled onto your rocket boiler videos a few months ago while searching for heating options. What happened with the rocket project? Is this one going to be more efficient?

I have radiant tubing in the slab of my berm home that I'm currently not utilizing. I'd like to do something like this in conjunction with solar thermal collectors to heat a 500+ gallon storage tank. Do you think it would be feasible to heat that much water with a system like one of yours?

I liked the rocket stove, but the feeding system was a chore. The extra steps of bundling the wood and dropping wood down the chute was not worth the effort. Because of that, I wanted to try a different way of achieving clean combustion. So far, I've been very happy with this system and have made several modifications already to make it even better.

A system like this or the rocket stove will work great for heating a large storage tank. I, too, am interested in using solar for heating. The sun doesn't have to be felled, cut, split, and stacked for use

My old family motto is, if it works don't fix it, I really ike the Float, easy to make and replace if and when needed ! Big AL

Thanks, Al! We must me distantly related

I liked your how-to approach - good use of the camera, nice pace, a lot of easy-to-understand explanation along the way.

I watched the first vid on Youtube (trying it here didn't allow my browser to put the visual onto 'full screen"). Being a DIYer myself, I posted a few questions in a reply on Youtube. But for all I know you may be logging-on here at Permies more often than over at Youtube. So here are my small tech questions:

How difficult or easy was it to remove the sono (concrete pillar) tube after you'd put your refractory mixture into place (the casting) and let it cure?

In final assembly, how difficult or easy was it to place the boiler tank and get it secured in place, way "down" along the now-insulated water-tank combustion chamber?

How easy will it likely be to remove the old insulative material and replace it (as you've said your cured mixture WILL need to be replaced every so many years)? and how do you plan to remove it? or will it likely simply be a patch job on the existing material?

And as to the previous question, when it comes time to re-line the inside of the combustion chamber, do you think it might it be easier to just get another old hot-water tank and start again?

Thanks. Nice job on the boiler, and on the vids.

Joel Russ : We use a similar technique to make up an insulated Heat Riser Cores for Rocket Mass Heaters, they are used there as a sacrificial form,
they just get burned up !

There is a major failure point between 450-550 degrees C or ~1000degreesF~ as the Portland type cements disassociate! It will be interesting to
follow this build, The skim coating of Furnace cement will really be tested and its ability to sustain abrasion and hard knocks to its shell will govern its
future

As this was an experiment, Geo has already had success ! Big AL

Bump !

Thanks for the bump, Allen! I've done a lot of modifications to the boiler since I posted in this thread and have documented it all on video. Here's my youtube playlist from start to finish: https://www.youtube.com/playlist?list=PLq-ETrB2scGH8-PKQOL0MPwEeEro-gC_T

Video #7 shows the old insulation after I removed it from the boiler. It held up well except for the very top, the constant abrasion of putting in firewood was too much for it.

This year, I switched to a more proper material. I used a ceramic fiber blanket, coated with a hard liner to protect it. So far, it is holding up so much better. For those who don't have broadband to watch videos, I've got a lot of pictures on my blog: http://greenenergyexperimenter.com/wp/?cat=8

You'll notice the heavy inspiration from batch burning rocket stoves in the videos and pictures. This setup creates more of a side/down draft combustion than a down draft found in commercial smokeless wood boilers. The firebrick in the back and secondary chamber all glow a brilliant orange to yellow when it is up to full temperature. The secondary injectors bring air back to that chamber whether or not the induction blower is going.

Since shooting my last video, I've been able to procure some well seasoned wood. What a difference it makes! It burns smokeless within 5 minutes of startup. When the induction blower kicks off, it doesn't smoke. When wood is added, no smoke. I have yet to see a spark or ash make it out of the stovepipe. It has been the most successful wood burner I have built, and I believe it to be the cleanest. No wood burning odor at all, and unlike my first rocket boiler, no fly ash is released(I know that's a coal term, but what I mean is the ash that gets swept along with the exhaust, to be deposited outside the stove). I plan to do a new update video next Friday showing how well it burns with all the tweaks I've done since the last video and using good seasoned oak.

The biggest negative is the small firebox. If it is cold out (20F or below), I do have to fill it every 4 hours. That will be my project next year, along with building an insulated shed to house the boiler in for better efficiency.