efficient design of homemade OWB

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woodburnerdave

New Member
Sep 21, 2011
11
northern virginia
Hello all,

I am a total newbie. I am so new that I thought I sent this thread yesterday and it didn't go through. I am building an OWB in my backyard out of concrete for my 2 zone heating in my 4,000 foot home with full basement in Virginia. I'd like to build my boiler as efficiently as possible. I have been in construction for 35 years and am handy but I am a terrible welder. I will have an air gap between the firebricked concrete wall housing the boiler and a second concrete wall which is the outside of the building, which I see being about 8' by 10' and 8' high.

Because I cut and load my logs with a bobcat they will be 6' long, so I hope an efficient firebox could be narrow (20"h x 20"w) and long (96"), with preheated combustion air entering near the front door and mixing with coals for 8 feet before passing up to a secondary combustion chamber for the wood gases at 1100F. In the upper chamber I will plumb 200' of 1" glavanized pipe running at 33 gpm (less pressure loss) for heat recovery. I will try to stick some water pipes up the flue too. I bought several books on stove design by Jay W. Shelton which will arrive next week. Once I find out how hot my flue needs to be I will try to run the exhaust horizontally or with enough water jacket to absorb heat down to this temperature but not so low that it presents a creosote hazard.

Any ideas how big the secondary upper chamber should be? An easy length for me would be 96" and an easy width of 20" because it sits on top of the firebox but the height isn't set yet. How high should this area be and how should it be designed? Should I baffle it? I could probably plumb the pipes to act as baffles or (poorly) weld something.

I may also put heat exchangers at the ceiling of the concrete room for heat recovery. The water would pass through the cooler ceiling heat exchangers before going through the hotter water and flue jackets. Inside my home I will have about 300,000 BTUs of heat exchangers and 500' of radiant pipes. I will first plumb the hot water through a 120,000 BTU heat exchanger on each of my two plenums, then through a water to water heat exchanger for my domestic hot water, then through basement radiators, then through a greenhouse before cycling through the OWB again. I am using a Taco 0010 circulating pump at 33 gpm and a 1" line in a close loop system. I also will drape EPDM rubber in two used 275 gallon fuel oil tanks as heat sinks.

What am I doing right or wrong? I really enjoy criticism because I know it will save me manyhours of construction time. I promise to send at least one joke to everyone that helps out. Be forewarned, my joke repertoire is limited and dirty!

Thanks for any help you can give.
 
Welcome Dave. I have been a member here for a little over a year and have learned ALOT. I don't mean to sound like a parent to you or a broken record, but you really need to research this first. To give you an example, I don't think you want to use concrete. It will literally explode if it gets too hot. The firebrick would work, but I would prefer to go with a castable refractory. You buy this in bags just like mortar for masonary work. It will mix and you apply it just like masonary, but it will withstand high heat, is much harder, and won't have cracks in it like you would have between firebricks.

The next problem I see with your idea is using 6' long logs. If you plan to use this as a gasifier, the wood may be dry enough for your grandchildren. Most of us use split firewood in the 16-24" range and it can take up to a couple of years to season properly. I would about guarantee you that you could take a 6' unsplit log, put it in the dry for 5 years, then cut it in the middle and you would find it was nearly as wet as it was the day you cut it down. Wood cures from the outside ends toward the center. If you did manage to get a fire hot enough to burn these, you would lose half or more of your btu's from the wood just trying to dry the water out. The trick with a gasifier is that the wood has to be truly seasoned and dry.

The last thing I will mention is the use of galvanized pipe in a boiler. I was thinking about using it in mine until I read deeper around here. Someone can correct me if I am wrong, but galvanized can actually create a form of acid when used in a boiler. I don't remember the details of it, but maybe someone will come in and explain.

Don't get frustrated. I have been in the same boat as you. After a year of being around here, I am building a gasser and still having questions as I go. It can be a headache, but a truly rewarding experience when it is done correctly.
 
Boatboy63, your advice is great! Your two minutes of writing just saved me a month of headaches, thanks a lot. I will get some castable refractory and research how thick to form it. Hopefully it isn't much more difficult than concrete.

I never thought about the moisture drawback to logs, proving that I am as green as my wood. I will cut and split my logs asap to get the curing process started. I thought of galvanized pipe because it's cheap, but instead I could have a buddy weld stainless or carbon steel plate and use a heat transfer liquid that won't cause rust. I'd guess my system will only be 25 or 30 gallons so any percentage of propylene glycol wouldn't cost much. If I used plate steel the whole secondary chamber could be a wet sleeve, right?

It looks like I need to do some serious reading and I will start now. Perhaps the fireplace design books would have steered me toward the right path, but perhaps gasification is a recent design innovation. Thanks again. If you see some other obvious deficiencies, please point them out.
 
Dave, another thing most of us do is to use an oxygen barrier pex tubing to transfer the hot water. When water gets somewhere around 160 degrees, it keeps the oxygen from releasing and rust will be at a very minimum. The oxygen barrier pex keeps it from entering also. You can use a boiler additive that will also help keep rust away.

I will give you another warning...the castable refractory I am using is around $50 for a 50 pound bag in my area. This refractory (Mizzou Castable Plus) is one of the best and is recommended for boilers and is rated at 3000 degrees. There are "recipes" on the net for making it, but it isn't rated as hot. Considering a good gasser can burn at 1800-2000 degrees or more, you want to make sure you have something that can handle it.

Stainless is also questionable in the build of these. Many companies have used them in the past, but there is talk about problems. These relate to stainless being more prone to cracking over time. Basic steel will handle the expansion and contraction better.
 
Hi wodburnerdave,
boatboy63 is right about the galvanized pipe and the process he refers to is electrolysis. Electrolysis will dissolve stell as fast or faster than aggressive rust. Not a good element for a boiler. Sounds like a big project and I wish the best to you. Many very experienced people frequent this forum and you will learn loads to prevent extending your learning curve. Keep us posted please!
 
boatboy63, I had planned to use pex everywhere but the hot room. Good to hear that mild carbon steel is best because it's also the cheapest and maybe I should build a wet sleeve around the whole secondary burn chamber. I also have 160' of 1.5" ID pipe left from a previous job that would give a lot of surface area. I will research what thickness to use and check a local company to find out their largest tubes and their bending capacity. Now that you've explained that long logs won't work, is a long firebox necessary for more complete combustion? Maybe I should just wait to see what Jay Shelton had to say. I don't mind the refractory cement cost because it doesn't look like it is very thick, it may even be applied in parge coats over fire brick.

Cave2k, thanks for the warning about electrolysis, I should have known that --duh. Mild steel is much easier to work with. When I figure out the dimensions of the secondary chamber I will let you know. In the meantime I am open to suggestions. What's wrong with a very large gasification chamber with a whole lot of steel surface area for absorption?
 
I would beware of using a large diameter pipe for your heat exchanger. If you go too big, you are missing out on what is flowing in the middle of the pipe. I think 1.5" is about right. You also don't want to overdo the amount of pipe for it. You want your flue temp to run at around 300-350 to prevent creosote. If you try to pull too much heat from the exchanger, you will drop your flue temp and have the black soot running back down the pipe. You may also want to research the term "turbulator". It has to do with making the exhaust air circulate and come in contact with the pipe wall to exchange heat better. I have read threads about people using chains to hang down in each pipe to make the exhaust "stir".

As for a longer firebox, I wouldn't go any larger than needed. The way I look at it is with a "smoke dragon" (old school wood burner), you don't really care about the efficiency as long as it burns. With a gasser, you need to keep a consistent burn and it would be easier to regulate in a 24" box as opposed to a 60". The larger the box, the harder it is to keep a consistent amount of air to all areas. The primary burn chamber of the boiler I am building measures 18.5†wide by 26†deep by 32†tall. The secondary chamber is the same length and width, but is only 12" tall.

Be careful about skimping on the thickness of the refractory. The manufacturer recommends a thickness of 3" or more to prevent it from crackng and to tolerate the impact of the wood entering it.
 
My EKO uses 2" or the closest metric size to it for the 8 heat exchanger tubes. I have cleaned them several times and built a tool to speed things up. The EKO40 came with built in turbulators in each tube. ie suspended, near diameter blades, mildly twisted (2-4 twists per foot) that run nearly the full length of the tubes attached to a handle that allows you to raise and lower the blades to help keep the tubes cleaned between serious cleanings. Remember I used my EKO without storage so the need for cleaning because of creosote from idling was more prevalent. If a gasifier is working correctly (not bridging, wet wood etc) the only time you will experience creosote build up is at start up or shut down when there is still viable wood in the primary combustion chamber as secondary combustion will ideally burn any potential creosote. Some with storage have used chains instead of blades to create turbulence as mentioned above and have claimed clean tubes even after a month of standard storage operation.

If my EKO were 10" or 12" taller in the primary combustion chamber I think I would like it even more than I do. That being said I would suggest shortening the length of the primary combustion chamber and raising the roof for longer burns. 60 ninches is long and will be difficult to to load according to your storage peaking out without getting idling from a fuel overload (more wood than needed which will cause idling). As well an elongated primary combustion chamber may cause loads of bridging and burnouts/cold ineffective fires as a result from unbalanced burns. Look up some of the EKO40/60 threads concerning bridging and you will get an idea of what I mean as some 60 users have even closed off one of the two nozzles in the EKO60 to great effect. By design a shorter length box will force the fuel into the nozzle region for a more consistent burn if designed properly and a taller box will allow you to go for longer burn times with fewer loadings. That may only be 2 cents but in my experience of going for the longest burn times it makes a whole lot of sense. So to cap on your heat needs maybe a 24" box with a single nozzle or up to a 40" box with two nozzles would be more in line with your heat needs. If you go with a longer box and two nozzles I would recommend cutting your wood at 16-18" to frustrate any potential bridging. Getting caught short on time will allow you to buy pre-cut, split and seasoned wood that will fit your boilers design.
 
boatboy63 said:
I would beware of using a large diameter pipe for your heat exchanger. If you go too big, you are missing out on what is flowing in the middle of the pipe. I think 1.5" is about right. You also don't want to overdo the amount of pipe for it. You want your flue temp to run at around 300-350 to prevent creosote.
Click to expand...

Thanks for the good advice again. I'll take the square footage of exposed steel times the temperature difference and figure on 2 BTU's per hour of heat exchange per sq.ft./hr/degree difference F. This will give me the length of pipe needed to drop the temps down to 300 but no lower. Of course I have to guess the BTU/hr capacity of the stove but after measuring the exhaust temperature I can adjust the amount of water pipe or horizontal flue until I get it right.

I'll research the turbulator, I don't know if this is meant for water or air pipes but I will figure it out.

How many BTUs per hour is your setup? I'm aiming for 200,000 so I may need a proportional scale up in dimension. I'll take your advice that a longer firebox doesn't create a more efficient burn. I will be sure that the refractory cement is a full 3" thick.
 
"I'll research the turbulator, I don't know if this is meant for water or air pipes but I will figure it out."

How many BTUs per hour is your setup? I'm aiming for 200,000 so I may need a proportional scale up in dimension. I'll take your advice that a longer firebox doesn't create a more efficient burn. I will be sure that the refractory cement is a full 3" thick.[/quote]

Turbulators work inside the heat tubes to keep creosote and ash from building up on the tubes and robbing them of efficeincy and of course clogging the pipes and sice the tubes are in a water jacket there will always be a tendency for condensation. The chains or blades that act as turbulence devices simply force mor heat to the tube walls as the exhaust gasses escape towards the chimney. Check www.cozyheat.net and you will get some very helpful information about boiler btu/kwh ratings.

No matter what size you build be sure to stick with your plan to have the secondary combustion chamber in a water jacket to prevent warping your boiler as some have found even very thick refractory or firebrick was not enough insulatoin to protect the steel from warping and ruining their work. It might be labor intensive but a design concept I have is 1/4" or 3/8" fins on the secondary chamber roof and side walls to absorb more heat than a flat surface will transfer. The real trouble with that concept is the eventual effect on the flue temps and of course not making the fins too large or "tall" and too plentiful. To facilitate easier cleaning I would not use fins on the bottom of the secondary chamber. Please keep us posted on your progress.
 
Cave2k said:
My EKO uses 2" or the closest metric size to it for the 8 heat exchanger tubes. I have cleaned them several times and built a tool to speed things up. The EKO40 came with built in turbulators in each tube. ie suspended, near diameter blades, mildly twisted (2-4 twists per foot) that run nearly the full length of the tubes attached to a handle that allows you to raise and lower the blades to help keep the tubes cleaned between serious cleanings. Remember I used my EKO without storage so the need for cleaning because of creosote from idling was more prevalent. If a gasifier is working correctly (not bridging, wet wood etc) the only time you will experience creosote build up is at start up or shut down when there is still viable wood in the primary combustion chamber as secondary combustion will ideally burn any potential creosote. Some with storage have used chains instead of blades to create turbulence as mentioned above and have claimed clean tubes even after a month of standard storage operation.
Click to expand...
Cave2k, any idea what the length of each heat exchanger tube is or the total length combined? Also, what about the wall thickness of these? Are they thinner like exhaust pipe on cars as opposed to a thicker pipe? Thanks
 
boatboy63 said:
Cave2k said:
My EKO uses 2" or the closest metric size to it for the 8 heat exchanger tubes. I have cleaned them several times and built a tool to speed things up. The EKO40 came with built in turbulators in each tube. ie suspended, near diameter blades, mildly twisted (2-4 twists per foot) that run nearly the full length of the tubes attached to a handle that allows you to raise and lower the blades to help keep the tubes cleaned between serious cleanings. Remember I used my EKO without storage so the need for cleaning because of creosote from idling was more prevalent. If a gasifier is working correctly (not bridging, wet wood etc) the only time you will experience creosote build up is at start up or shut down when there is still viable wood in the primary combustion chamber as secondary combustion will ideally burn any potential creosote. Some with storage have used chains instead of blades to create turbulence as mentioned above and have claimed clean tubes even after a month of standard storage operation.
Click to expand...
Cave2k, any idea what the length of each heat exchanger tube is or the total length combined? Also, what about the wall thickness of these? Are they thinner like exhaust pipe on cars as opposed to a thicker pipe? Thanks
Click to expand...

Without doing a little disassembly work no I can't tell you the exact length of the heat tubes but with a tape measure I came up with an approximate length of 32" per pipe and as I said there are eight of them. If you need me to open up the boiler plate for specifics I can. Wall thickness is at least the same as regular 2" steel black pipe but I am inclined to think it is closer to the thickness of steam pipe whick I am inclined to think is called schedule 40. In any event the heavier pipe would seem to provide a longer life expectency for the heat exchanger.
 
Hi Dave

It sounds like you're not afraid of jumping in head first.

You've gotten plenty of solid advice, but I'll add my two cents more.

After a minute of reading the search results for the author you mentioned I found one solid fuel encyclopedia from 1982 and a book on homeopathy. I won't touch the homeopathy, but the early 80's was the dark ages of a lot of american technology, so I'd take the books with your 70's sun glasses on.

If you want to do a gasifer, then Professor Hill and his research is required reading. Specifically, he states somewhere that a water jacketed PRIMARY chamber is required to keep the wood from burning too fast.

The garn is another great example that you should understand. It's big, but in some ways simpler than "true gasifiers", it manages this by using the storage to not have to idle.

Also don't be afraid to use firebrick and masonry, coal burning boilers used them for centuries, and they'll work fine for a wood burner if you figure out how to do it right. I can't point you in any direction for wood boilers built with fire brick, but you could look up some antique books on coal boilers and translate that knowledge over to wood boilers.

Also, you might want to consider adapting a junk boiler to your purposes rather than building from scratch. I have a small commercial cast iron boiler core that I've used on top of a firebrick combustion chamber, it has serious drawbacks but when I finally abandon it I can haul it to the scrap yard and get my $50 back. A steel fire tube boiler could be adapted to a gasifer easily enough if you could scrounge something like that.
 
benjamin said:
If you want to do a gasifer, then Professor Hill and his research is required reading. Specifically, he states somewhere that a water jacketed PRIMARY chamber is required to keep the wood from burning too fast.
Click to expand...

Benjamin,

Can you give me more info on this person and his research, and where I might find it ... specifically, Hill's full name and the titles of any of his publications? I would be interested in reading this material.

TIA.

DC
 
Dick's paper is here:

http://hotandcold.tv/wood furnace.html

Make sure you use the whole line since this website seems to not underline the whole thing.
 
Thanks Benjamin, I'm not afraid of jumping right in but I'd rather copy an efficient design from anybody in the 200,000 BTU/hr range. I am not sophisticated enough to build a high tech gasifier, but I think my construction skills can make a functional one. I will also look up the guy you recommend. I got a book on rocket mass fireplaces, but this does not look like the direction I want to go.

I'll look into the Hill guy you recommend, although I think I can run enough pipe through the secondary combustion chamber to transfer heat. The theory you mention is different from the theory to keep the firebox hot for greater efficiency, but I don't know enough to weigh in the debate. I'm just learning a lot.

Am I right in thinking that people are talking about a wet sleeve with 2" pipes for the exhaust with turbulators. I think I can do something like this but the homemade turbulators may be very crude.

I'll look around for a used boiler. Good idea.
 
OK, so I read it and it looks like a very good paper. The problem is that I probably absorbed about 20%, the burn theories are a little complicated for me. I seem to be stuck on a traditional firebox with the secondary chamber above it. These designs with a pipe connecting two seperated chambers just look more challenging. But it appears that the secondary chamber is twice the size of the firebox, maybe I should increase it's size.
 
I misunderstood your description. I thought you were talking about copying a typical gasifer with different materials and on a larger scale.

I think what your talking about resembles a larger version of a greenwood/adobe/? at least as far as a firebrick main combustion chamber and a relatively simple water tube boiler section in a separate chamber.

I agree that the precision and control needed to make a gasifier work aren't always worth the effort. The garn is a simpler alternative. There is at least one thread here of a homemade boiler that is based on the garn, and many garn installations.

It's actually really simple to burn wood efficiently, just split it really small and feed another piece every couple of minutes. It gets more complicated when you try to load it up and make it the heat last, and that is why we're so obsessed with storage and gasification.
 
If you want to do it right, you won't waste time with logs. Get a wood chippper and a stoker boiler designed for wood chips and you won't have any smoke or a smoldering mess. Something such as: http://www.youtube.com/watch?v=IR5-KHOnihQ
 
Dicks Hill's paper is an early version of his design. After he had sold the rights for household units, he developed a much larger version for a Maine Forest Service Nursery in Greenbush Maine. It looked the same but was much larger. If I remember correctly, at full fire it required 40 pounds of wood every 20 minutes. The wood was standard splits. I do not know if there was a paper published.

Trying to combust a whole log effciently is very difficult, the logs tend to be wetter and the log can form a char layer on its exterior slowing the burn. There were several outdoor boilers that advertised they could burn green logs, in theory they could but most of the heat went up the stack as CO and creosote with a lot of water vapor. Most states have banned them.
 
OK guys, I have redesigned the firebox and secondary burn chamber and I will cast them out of refractory cement. The firebox and the gasifier will each be 24" by 26" and the gasifier will have the 1" pipes laid out as alternating up and down columns to act as baffles. I'm thinking that the baffles will be about 6" apart, does this sound too close or too far away? They'll be created by turning the 24" pipes 180 degrees with two elbows at the end and running them back. Twelve or 15 will create a wall for a baffle and I'll weld a thin piece of flat steel to them so the air has to flow the hard way instead of between the pipe.

If you see any major flaws with this design please advise me now. I will take a picture in a week once it is formed up from plywood.
 
The cheapest refractory cement I've found is 25 pounds of premix for $17. I would guess that 3/4 of this is sand, which means that I'm paying about $3 per pound for cement. Does anyone know a cheaper source in the mid-atlantic area?
 
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