OWB Build

[SPD748]

Hello all,

I have a bit of a new project on the board and I'd like some opinions. I recently moved to a home that doesn't have a fireplace. I'm really missing my wood heat, I mean REALLY missing it. I've ran through all the possibilities which have all lead to an OWB. A manufactured unit looks to be in the $7-$8000 range. I believe that I can build one for much less. Now, it won't have the newest, latest, greatest features that commercially manufactured units have however I think I can design something that will be fairly efficient and last many years. Basically a ~15 cu ft firebox surrounded by ~175 gallons of water, aquastat, forced air, solenoid controlled damper, water pump... Unless I've missed something, I think it could be constructed for around $2000.

That being said, are there any opinions here? Someone that build one with advise on how to avoid pitfalls? Someone that attempted and discovered that it wasn't financially doable? General thoughts on the matter?

-lee

[Gearbox]

There was a company in Wisconsin that was using 500 gal propane tank cut in half for the firebox . If you look around in salvage yards you can find them with the valves pulled . You may even find one big enough for the water jacket .

[thecfarm]

I have the older style Heatmor. Just about what you want to build,but no solenoid controlled damper. I myself would forgo that part. That is why I bought a Heatmor,a very simple design. Most parts are just common boiler components,nothing fancy,Honeywell controllers. Two blowers to drive oxygen in the firebox and a bladder. I also have water flowing through the door too.

[SPD748]

Thanks Gear!

I already have a 26" diameter cylinder that's 48" long, rolled from 1/4" steel. I could go with a 44" diameter water jacket which, with the 26" firebox inside, would leave ~47,500 cu in of volume or ~200 gallons without loses such as the chimney, bracing, etc.

-lee

[JSNH]

Search on the web many people have made their own.
Seeing your work on the mill. I know you can look at many boilers and pull the best ideas together for yourself. Your ability is top shelf and you take the time to do it right. You will make an outstanding stove. Document your build here and inspire others as you have done before.

[SPD748]

JSHN,

Believe me, I've been pouring over page after page of web documentation. I will share this build here and, as before, I hope it will inspire someone out there to do the same.

One question has come up... Is there scientific relation between the size of the firebox (i.e. surface area of the heat transfer plate) to the volume of water the tank will contain. In other words, is there a magic number or range of gallons of water to available fuel volume? Is 100 gallons half as good as 200 gallons? Is more (or less) better? I understand that more water will take more energy to bring up to temperature however said volume of water will hold said energy longer. Based on this, and nothing else, the largest possible volume of water would be best however there's always the law of diminishing returns.

I could just mimic a commercially fabricated unit however I'd really like to understand the science behind this project in lieu of going with a WAG. Know what I mean? 

-lee

[dave_dj1]

I believe it all boils down to how often you want to feed it and the load you put on it. A Garn can be fired once a week in the right setup.
There is a (what i feel is) a good design with the tank in tank with the chimney down 3/4 of the way into the firebox.
That is the style I am going to copy if I ever get around to it. I believe in natural draft, at least once up to temp. I feel the blower burns more wood than it should. Just my .02 worth. Good luck. ;D

[thecfarm]

Yes the blower might blow too much,but when the blowers are not on the Heatmor that I have,the fire is smothered out. Open up the door and you would think the fire is out. This allows me to burn cedar and dead wood that with a natural draft I could not control the fire.

[snowstorm]

my taylor holds 900 gal of water. take a look at how the smoke tubes are in a taylor going down and back then up and towards the front

[Gearbox]

I would go with some kind of shaker grates . My Crown Royal with dry oak has only clinkers what will pass the holes in the grate . Why put wood in to make charcoal . Induced draft with variable speed . Upper secondary burn .

[DDW_OR]

here is a link to my Central Boiler 750 install
https://forestryforum.com/board/index.php/topic,92218.0/all.html#lastPost

the outside temps have been around +18 to +38. one load of madrone wood, 22 inch long, in the CB-750 has lasted FOUR days. the inside temp is +72

the next load of madrone lasted SIX DAYS at similar temps.

[SPD748]

Thanks so much for the information and opinions guys! Here's what I have for design so far...

A 26" diameter x 48" long firebox rolled from 1/4" plate. I'm thinking about welding in a flat plate on the bottom so a shovel will scoop out the ash easier. I looked at the grate and shaker grate systems. While there is no doubt this style of ash removal may be convenient, I think a simple flat bottom will work fine. Push the burning material toward the rear with a rake, shovel out the powder, pull the burning material back toward the front then add more wood. I think it would be easier to repair a flat bottom vs a grate/ash drawer style as well. I'm still open to hear that I should go a different route there however so keep the opinions coming!

A 42" square x 60" long tank formed with 3/16" plate. That would allow for a ~300 gallon water capacity. Normal plumbing arrangement. I like the idea of having the chimney extend well into the firebox to trap heat at the top of the chamber. A forced air blower in a well insulated door, which will be 24" square. Wrap the sides and back of the tank with several layers of fiberglass insulation.

This unit will sit inside a building so there won't be a need to finish the exterior with sheet metal, like commercial available units. I may do something to neaten it up but I don't think I'll spend a lot of time and effort there. I've sized the building to hold around 10 cords of wood in addition to the furnace.  I like the building idea to help cut the wind off the unit and to have dry wood storage.

I do have a question about the heat exchanger. My air handler is a condenser on top/down draft style arrangement. The fan draws air down through the condenser then through the floor and into the duct. Could I place the heat exchanger between the condenser and the air handler thus the fan would draw air down through the condenser then through the heat exchanger then into the duct? All the pictures I can find have the heat exchanger on the pressure side of the fan. I can't seem to find an arrangement where the heat exchanger is on the suction side of the fan, exactly like mine.

Thanks guys!

-lee

[hedgerow]

Lee
Just a couple things to think about. I too was going to build a wood boiler of sorts year ago. I also wanted it in a building because I don't want to be out feeding wood outside when it is -20 and the wind is blowing. My insurance company wouldn't cover my building any more with a none UL listed wood burning device in it. That forced me into to buying a Garn that is listed and designed to be installed in a building. I have three furnaces in my house that my heat exchangers set above the AC coil but all three are on the supply side as my are up flow. Shouldn't matter that yours is a down flow you AC coil works Ok?

[thecfarm]

The heatmor that I have they run 4X8 rectangle tubing the length of the top. The smoke goes up into this tubing in the front of the OWB travels the length of the OWB and exit in the back. Make your chimmney out of ¼ inch pipe.

[Ljohnsaw]

I do have a question about the heat exchanger. My air handler is a condenser on top/down draft style arrangement. The fan draws air down through the condenser then through the floor and into the duct. Could I place the heat exchanger between the condenser and the air handler thus the fan would draw air down through the condenser then through the heat exchanger then into the duct? All the pictures I can find have the heat exchanger on the pressure side of the fan. I can't seem to find an arrangement where the heat exchanger is on the suction side of the fan, exactly like mine.

Thanks guys!

-lee

Absolutely NO experience here, but, putting the blower on the "hot" side would mean your lubrication for your bearing is being cooked.  How hot is the exchanger running? 120°? 180°?  That would be my only concern.  Well, that and if it springs a leak - is the exchanger physically above the fan?

[Gearbox]

I would weld in 2 or 3 or4 angled cross tubes across the top . The reason to angle is to reduce the chance of boil in the tubes. Mine are flat and at 190 it will start chunking[boiling ] in the cross tubes .

[JB Husky]

SPD... A couple of questions for you. Are you building this because you can't afford to buy one? (Not trying to get in your business) I built one about 6 years ago because I couldn't buy one and I'm now looking to buy one. These stoves are inherently inefficient even when engineered and tested by manufacturers who spend big money on R&D. Not doubting your ability just pointing out the obvious. Secondly the heat exchanger can be configured they way you describe, my old furnace was, but is there any way to get it downstream of the fan? It cost me a fan motor by having the heat exchanger upstream of the fan. As for the construction of the stove the dimensions you have listed are close to mine, as my firebox is 30" in diameter and 36" long made of 1/4" steel. The water jacket is a 42" diameter 48" long 1/2" wall pipe. If I were going to do it again I would do as you said and extend my flue down low in the firebox or create some sort of baffle. I used a damper instead and I lose a lot of heat out the flue. Another thing I would change would be sloping the walls of the firebox towards the middle so as the fire burns down your wood in theory will all fall together. I'm sure if I think more about it I can think of some other stuff I would change. Best of luck to you.

[SPD748]

hedgerow,

I haven't thought about the insurance angle. I'll ask them and see. The boiler will be in a dedicated building so the loss would be contained to the structure, wood pile and soft parts on the exterior of the unit. I really haven't thought about the insurance. Thanks for bringing that up.

cfarm,

Based on your suggestion, I'm thinking about a horizontal plate open in the rear of the firebox and making a 90° turn at the front then exiting the top. With a fan in the front forcing air from front to rear, I think it would be wise to have the smoke/heat travel from back to front to collect as much energy from the exhaust as possible. Smoke generated near the door would have to travel the length of the firebox twice in order to exit. 1/4" pipe is an excellent suggestion.

ljohnsaw,

Believe me, I've really been stressing about the heat exchanger arrangement. A solution will be to build a plenum to sit under the air handler. This would eliminate all possible heat related issues and, in the event of a leak, would save the electrical components though the floor might not like it >:(.  My lineset is long enough to allow the air handler to be raised 24 inches or so. That should be plenty.

Gearbox,

Do the tubes you're referring to carry water? If so, are they pressurized or to they just connect one side of the tank to the other through the firebox?

JB Husky,

Though I could shell out the cash for a commercially available unit, I believe (as my Dad taught me) that my hands were made to make things. That being said, I totally agree with you on the point that companies spend tons of money on the research and development of any product in order to make said product as marketable as possible. Now, that marketability may be in the form of efficiency. It could be in the form of lower maintenance. Ease of use perhaps. Some integrated combination of all, and more, of the above. In my experience, efficiency and marketability don't necessarily go hand and hand. Take my sawmill for example... I could have bought a brand new bandsaw. Said bandsaw would most certainly fit the style/type/frequency of sawing that I do much better but then, where is the journey?  The time I was able to spend with my Dad working on that Frick is beyond priceless to me. In the end the equation is simple for me. I'm building vs buying for three reasons: 1. It's what my Dad would have done. 2. It's what my Dad taught me to do. 3. My daughter will be partaking in the project. In fact, she will be welding some of the less than water tight areas of the boiler (she just doesn't know it yet  :D). Please don't take my response as rude or assertive. You are correct. A commercially available unit most likely would be more efficient. On to the heat exchanger. I think a plenum under the air handler will eliminate any issues there. I like the sloping firebox walls idea. I'll modify my drawing and post it here. Stay tuned!

This sort of exchange is exactly what I was hoping for! Great ideas and information that, I'm sure, will lead to an excellent result. You guys are the best!

-lee

[SPD748]

The first of what I'm sure will be many drawings/revisions...



 

cfarm, were you talking about something like this?

-lee

[Ljohnsaw]

SPD748,

And don't forget, on a commercial unit, one of the most important aspects is price.  While the price of OWB is staggering (to me), you better believe that sound engineering will loose out to economics every time. :-\

I had a wood stove that had what appeared to be an air tube high up in the firebox.  It was 1¼" or maybe 1½" diameter with a series of small holes.  You could see how it was providing fresh air to the smoke.  There was a clear area right by the holes followed by a nice flame.  It was allowing the smoke to combust.  I understand you have a fine line between long burn time vs. efficient burn but I would consider this afterburner an essential part of your design (to keep you from smoking out your neighbors!).  Perhaps having this air supply fixed and a damper on the remaining air input thermostatically controlled?

[SPD748]

SPD748,

And don't forget, on a commercial unit, one of the most important aspects is price.  While the price of OWB is staggering (to me), you better believe that sound engineering will loose out to economics every time. :-\

Agree 100%. As a toolmaker, I watched our engineers toss great ideas because upper management decided it wasn't "cost effective" id est a simple change in design would make servicing the product many times easier however, "We're not in the business of making things easier for the customer later. We're in the business of making money now. Got it?" When I designed my log splitter, I considered repair and replace aspects long before I cut the first piece of metal. That design changed many times as it progressed. My Dad often proclaimed, "The way things are built these days, a mechanic needs three arms and six elbows to work on the thing."

I had a wood stove that had what appeared to be an air tube high up in the firebox.  It was 1¼" or maybe 1½" diameter with a series of small holes.  You could see how it was providing fresh air to the smoke.  There was a clear area right by the holes followed by a nice flame.  It was allowing the smoke to combust.  I understand you have a fine line between long burn time vs. efficient burn but I would consider this afterburner an essential part of your design (to keep you from smoking out your neighbors!).  Perhaps having this air supply fixed and a damper on the remaining air input thermostatically controlled?

I'll research the afterburner concept. I can't seem to get a grasp on what you're describing at the moment. Perhaps more coffee is in order...

-lee

[Ljohnsaw]

I'll research the afterburner concept. I can't seem to get a grasp on what you're describing at the moment. Perhaps more coffee is in order...

-lee

That's ok.  It happens quite often to me ;)  IF the smoke/gasses are hot enough, all they need is air and they will combust.  Injecting air to the (hot) smoke and it will burn.  That will give you heat and lessen creosote buildup.  I somewhat understand that OWBs are trying to maintain a certain water temperature so my concept my not apply - it might be too efficient and over heat the system.

[gspren]

  My background is also tool & die and later experimental fabrication. On my older, conventional Portage & Main OWB the smoke enters what I call a smoke shelf in the rear comes to the front and makes a 180 to exit the rear. This is a rectangular tube about 20" wide by 6" tall with a baffle stopping about 10" from the front, so each passage is 6" x 10". Above the main loading door is a separate door to clean the shelf of all the little ashes that settle there, this is surrounded by water. Two huge benefits are that with all that area to cool the smoke I never see sparks coming out the stack and in the roughly 10 X 20 x 6 tall area just inside the front smoke shelf door the temp rarely exceeds 350 degrees so you can do roasts and baked taters. Since all threads mention food I wanted to take care of that.

[snowstorm]

a easy way to make a smoke shelf  use a piece of well casing . run it out the back and it will fit a stove pipe

[Ivan49]

 I have built 3 of them about 13 years ago and all 3 are still in use yet today. Go like the plans you drew and instead of a shelf get a chunck of sq tube and weld it to the top of the firebox on the inside. Cap one end and run the stack up about 3/4 the way back out the top. If you run it out the back you may have problems as I know some friends that went that route and they had to add a cleanout on the back. The sq tube becomes you baffle and you can weld you stack to he top on one end. To  seal your door up use 3/4 inch chanel and make it so it over lap when the door is shut. In the groove on the stove place your rope for a seal. Use silicone spaced about 3 inches apart all the way around to hold the rope in place. DON"T fill this gap up with silicone to hold the rope in place. I did this and I said a lot of nice words getting the old one out. I know of one person that filled the gap up full of silicone and this became his seal. Roll your fire box, my first one has a sq box and it works but the other 2 had the rolled fire box and they work better. I added grates in the first one but I took them out later, they worked but the heat warped them. I used some heavy steel as that is what I had, but if you use cast iron it will work. The door on mine is a 1/2 thick plate and it has a air box over the outside where the fan draws air in the fire box and it keeps the door cool. I used 12 inch thick blanket Insulation on mine but some friend built one and put it in a building. They used steel studs and built a small building around it then they used blown in fiberglass insulation and fill this room full. Theirs will hold a fire for about 20 hrs when the temp outside is  10 degrees. If you don't count thee wire welder I bought I figure I ha about 1000.00 in each unit. My back ground was a machinst for 25 years building die cast machines