design for DIY outdoor wood burner, tell me what you think

[puckett02]

So I like to make things myself, usually it costs less than buying any pre-made product, but is usually more fun and can improve on a current design that manufacturers use because it is more labor intensive.

Any way, I wanted to run this across anyone who has knowledge on boilers and fireboxes. I don't really call it a boiler since I am not boiling the water, just getting it hot to heat my home. Here is the design, you have to picture this in your head:

The water tank is a rectangular box, 2 feet across, 3 feet high, 3 feet deep, sides made of 1/8" (or there abouts) stainless steel. The tank has a pressure relief valve that is an automotive style radiator cap, will vent to atmosphere if pressure in tank exceeds it's pressure setting. The bottom of tank is 1/2" thick stainless, and this plate serves as the bottom of the tank and the top of the firebox. The firebox is a trapazoid, 2 feet across at top, 1 foot across at bottom, 1 foot high, and 3 feet deep. the bottom and both sides are lined with standard firebrick from front to back, the top is left bare stainless. the total inside height from the top of the brick to the bottom of the plate is 9-1/2" high. The loading door is a trapazoid mimicking the shape of the box, 1' 3-11/16" across at top, 6-1/2" tall, and 9-3/16" across at bottom. Under the door are 3 2" air inlet tubes that extend 1" into and 1" out of the front of the firebox. These are about a 1/2" off the top of the bottom layer of firebrick. Being tubes instead of just holes they should inject a good directed flow of air towards the fire from the bottom of the wood. Not sure If I want to use a raised grate along the bottom or just have it directly on the bottom level of brick. The flu is about 6" from the back of the firebox and extends through the water chamber out the top, and is about 6' to 8' long total. The whole thing will be enclosed in a heavily insulated storage shed, which will also house the wood to be burned. The water tank will have it's own layer of fiberglass insulation all around the sides and across the top. I estimate the water capacity would be around 120 gallons in the tank, plus whatever is in the lines.

Now if you can understand the design, I'll explain why I came up with this. The firebox is not surrounded on all sides by the water because this design creates a cold firebox. Wood will not burn well and heavy smoke is the result. Next, instead of a stand alone boiler that is it's own "building" I don't think that would insulate the water as well as putting it in an insulated shed, and also insulating the water box. I can't say for sure, but two levels or insulation is better than one in my mind. This is better because it also keep the wood dry and warm in the shed since the bottom and sides of the firebox can heat the shed. I don't think I am losing much heat to the air in the shed since the bottom and sides of the fire box is lined with firebrick. The thick plate between the fire and water and the flue contained within the water will transfer alot of heat. Remember I don't want to boil the water!

So this is my design I came up with in my head and on AutoCAD. What I need to come up with is a way to automatically control the air inlets to close when the water gets to hot, and open if the water loses temp. The rest is easy standard stuff like water to air heat exchanger (like a car radiator) at my central fan unit of the house, and the pumps, lines, etc., that connect the system.

Let me know what you think of this design and any tweaks you could recommend. I have thought maybe a taller fire box, but I also like the long low box so the fire is closer to the top of the box to heat the water.

Thanks!
Jon Puckett

 

[webbie]

Some others might have more intelligent comments than I, but:

1. A thick plate does not transfer heat better - as you seem to mention. It transfers it worse....or about the same in a case like this.
2. Most outdoor boilers (and we might use the term hydronic heaters instead) are not pressurized at all - zero. They have a tube extending upwards from them a bit and that is open to the atmosphere.

Large flats plates of 1/8" cannot take much pressure. Real boilers would use either a bunch of "stays" - which are reinforcements welded between the inner and outer walls - or they would use designs which are round - vastly stronger!

You also have not really addressed the idea of very clean combustion, which requires much more than just not keeping the firebox sides a bit warmer. The only practical ways for cleaning up smoke and getting any sort of decent combustion are catalytic converters OR a downdraft combustion system which runs the smoke back through a super-hot coal bed (usually fan forced) before going up through the heat exchangers.

 

[Nofossil]

Welcome to the boiler room. Building your own boiler is a pretty ambitious project - especially a pressurized one. If you're going to be spending the time and effort, you might want to at least consider a gasifying design. My brother built one based on the EKO 25 geometry, with much more fire tube surface area and additional heat exchangers. Gasifiers are deceptively complex, but get much higher efficiencies. Best bet would be to find one and study it carefully.

As far as conventional boilers, I don't personally have any knowledge. Are you experienced at welding pressure vessels and specifying steel alloys for this sort of application? The reason I ask is that even the established manufacturers have had problems getting it right.

Best of luck!

 

[puckett02]

These are good replies so far, webmaster, I initially came up with this idea from reading other's ideas and using what skills I have. I can weld stainless but no formal training or stress testing besides what I have done hasn't broken yet. The idea for the radiator cap came from someone else, an idea to keep the water from boiling as easily, but I did not think of the pressure against the water tank walls, so good catch. I guess running at zero pressure would solve that structural problem.

Nofossil, the gasification seemed too difficult for me to tackle, and I frequently lose power over the winter months. I am literally the last home on the power line, so every interruption from my house to the distribution center gets me. I did not put this in my original post, but I wanted to design the burning process to work with no power dependency, so if my power goes out and the home becomes too cold, I could take refuge in the shed until power returns. Hooking up the pumps and house blower to backup power is ideal, but that would have to come later.

 

[kabbott]

Some others might have more intelligent comments than I, but:

1. A thick plate does not transfer heat better - as you seem to mention. It transfers it worse....or about the same in a case like this.
2. Most outdoor boilers (and we might use the term hydronic heaters instead) are not pressurized at all - zero. They have a tube extending upwards from them a bit and that is open to the atmosphere.

Large flats plates of 1/8" cannot take much pressure. Real boilers would use either a bunch of "stays" - which are reinforcements welded between the inner and outer walls - or they would use designs which are round - vastly stronger!

You also have not really addressed the idea of very clean combustion, which requires much more than just not keeping the firebox sides a bit warmer. The only practical ways for cleaning up smoke and getting any sort of decent combustion are catalytic converters OR a downdraft combustion system which runs the smoke back through a super-hot coal bed (usually fan forced) before going up through the heat exchangers.

I would add that even thick steel will not safely handle much pressure without stays. A tank the volume of 2x3x3 will need a lot of
stays or as web said make it round.

I also think you will not have enough heat transfer area sending most of the heat up the stack. I have a home built boiler that works but it is
very inefficient. I have tossed around the idea of building a gassifier but I am not convinced I can build it better. I know I can build it cheaper
but if I have to modify it 2 or 3 times the savings will be lost not to mention the aggravation.

There are a few threads around that discuss DIY boilers, You should search for them and do some homework before starting this project.

It is doable but there is alot to go wrong.

 

[Gooserider]

I agree with the previous posts - pressurizing a tank like you proposed brings to mind the technical term "BOMB"... I also don't see a lot of advantage to home building a boiler that basically copies designs that are known to work inefficiently... I would look for some way to do a reasonably efficient design if at all possible. If you don't think you can do an EKO style gasser, have you considered trying to build a Seton style? I'm not really up on all the details of how they work, but a Seton style boiler claims to have efficiency close to that of a gasser, and be much simpler and easier for a home builder to put together... Another option if you can handle the large parts involved would be to try doing some sort of Garn variant...

Gooserider

 

[Singed Eyebrows]

Hi Jon; I was also going to build a boiler awhile back & was probably going to do a Wood Gun copy. By the time I found out that A516 70 steel plate alone would be almost $1000.00 I went & bought an Atmos for $3100.00. You have other options like a boiler off Ebay for around $1000.00 + shipping. Or you can still build your boiler. To regulate the flaps according to water temp you can use a Samson, Honeywell/Braukman type control that you thread into the water jacket & run a chain from the arm to the flap. I believe Zenon at New Horizon can supply this. Welcome to Hearth & good luck, Randy

 

[jpowell1979]

You better take a look at this site "Experimental Masonry Hydronic Heating"
http://heatkit.com/html/lopezs.htm