outdoor wood burner who has built one

[MD3]

I built my OWB and installed it more than a year ago. A great project and a big savings in the cost of the unit and with utility bills. It is not as simple as add some wood every few days and forget it but there are a few things that seem to help cut down on labor. I bought plans for my OWB on ebay from Deb Design. The fire box and water jacket is made from flat plate. ( No rolling or forming of material). If you intend on only burning dry wood then I think you are right in not wanting a blower. You might want to rethink about the water filled door. My door is 36肺 36? With that size door water helps to prevent warping. I do not split my wood. When I have a log 20 + in diameter I cut it to a length that I can handle. Boy do they burn for a long time. The water filled baffle and over all design appears to me to be pretty efficient. You will burn more wood with a OWB but look at what you get in return. Domestic hot water year around if you want, Regular base board heat, burner is outside, heat your shed. Do not believe what Deb Design tells you about cost and labor hours. It will be somewhat more but it will not take a rocket scientist to build.

MD3

 

[beenthere]

I have had a round boiler heating my house for just close to 35 years now. It basically consists of two round tubes, one 24" diam just inside the other 28" diam to form the water jacket that is about 9 gal. Easy to make if can get the steel rolled. The draft is controlled by an aquastat that opens and closes a damper to shut the fire down.
The only tough part that I see is getting a door that will seal tight. It has a pressure relief valve and is not much different from a regular domestic hot water heater for controls. As said, it has worked well for 35 years. I figured if it stopped working, I'd have another welded up. The company in northern MN went out of business a number of years ago. The door and frame is a cast part and possibly a substitute could be found.
A couple pics.
The length of the inside tube is about 3" longer than the outside jacket tube. 6" flue thimble in the back. This one takes 20" sticks. The stainless steel tank to the back is a 30 gal pre-heat for water going to the domestic hot water heater. Just pre-warms the cold water. Works by gravity, and this is a closed system.

 

[pat32rf]

I've heard that one of the companies around here will sell you a complete door and latch system, but even with a very good air control system they still burn a lot of wood (inefficient) due to the cool burn temp. Firebricks are a good step in increasing the temp.

 

[badbowtie]

I was not sure what the round gave you I was going to use 3/8 plate for my fire box and was proably going to make it 3' wide 3' tall and 4' deep or not sure if I would be better to go with 4x4x4 instead. Then build another box around it that contains my water. I was not going to have water on the bottom or in the front I was going to have my water be on both sides back and the top. I am not sure what size material the outside of my water tank is made of not sure if I should go with 3/16 or 1/4 plate. I also need to really figure out what size will give me somewere between 150-200 gallons of water and least amount of waste and cutting steel plate. I have a plasma cutter that will cut 3/8 so it wont be to bad.

 

[MD3]

I was not sure what the round gave you I was going to use 3/8 plate for my fire box and was proably going to make it 3' wide 3' tall and 4' deep or not sure if I would be better to go with 4x4x4 instead. Then build another box around it that contains my water. I was not going to have water on the bottom or in the front I was going to have my water be on both sides back and the top. I am not sure what size material the outside of my water tank is made of not sure if I should go with 3/16 or 1/4 plate. I also need to really figure out what size will give me somewere between 150-200 gallons of water and least amount of waste and cutting steel plate. I have a plasma cutter that will cut 3/8 so it wont be to bad.

I used 5/16" for the fire box and 1/4" for the water jacket. the fire box is approximately 48" deep x 36" wide x 42" high. The water jacket is approximately 52" deep x 45" wide x 50" high. It will hold about 200 gallons. The weight is about 2000LBS

 

[beenthere]

I've heard that one of the companies around here will sell you a complete door and latch system, but even with a very good air control system they still burn a lot of wood (inefficient) due to the cool burn temp. Firebricks are a good step in increasing the temp.

My air control system works well, and does shut down the fire so there is less than a 10 deg rise above the set point. There are no firebricks (I see no practical use for firebrick as I want the wood fire heat to go directly into the adjacent water), as the water jacket is all around (except for the ends which are 1/4" plate). The fire temp at the steel doesn't get more than the temp. of the water in the jacket. An ash bed on the bottom and the efficiency is pretty darn good (as long as the wood is dry).

I've never seen a need for more than the 9 gal of water jacket. The water heats up fast, responding quickly to a quick fire when the damper opens for more draft and the house runs call for heat. When the room temps are satisfied, the water temp rises to set point, and the damper closes. No big swings in the water temp that I'd expect in a large gallon supply. Must be the reason the outdoor units try to keep water temps at 180 deg consistently. I'd look at that as a waste of energy.

As implied, it is a simple, easy-to-build system with simple parts and simple control. It works very well.

 

[Jeepnford]

Do you have any firebrick in your firebox? That's the only downside i see to round, but you could weld in plate walls to support the brick and use the small channels on the outside of the walls to support airflow. (Kind of a square peg in a round hole concept)

If you look at the design of modern EPA stoves, you'll find that managing airflow and secondary combustion is the key to high efficiency. To do that you need heat and a secondary source of oxygen that mixes with the wood gas late enough to have a combustible concentration, but early enough that there is adequate heat to ignite it.

I'm placing a flat plate with a grate and ash clean out on the bottom of the tube so I will have a place for firebrick. I'm also putting a draft fan that will blow under the fire controlled by an aquastat. I'm still pondering a waterfilled door. The door is about 18" square made from 1/2" thick steel so it may not be necessary.

 

[RonMar]

I would do the water cooled door just for the added collection surface area. Heat transfer is about temp difference, fluid contact time and surface area. A door can be as much as 1/6 the surface area of a square box, so that is as much as 15% more collection area... I am in the process of converting an old wood stove over to a outside boiler for my garage space. I am going to pull off the cast and glass door and add a water cooled one for the 10% more collection area in my case.

 

[racerboy832]

beenthere, how long a burn time can that stove run? How much baseboard or radiators can it take? I got one last that someone was giving away and have been thinking of using it.

 

[beenthere]

racerboy832, if you have this same unit, you have a good find.
Burn time will effectively run 8-10 hours. I have two levels of house with 1400 sq ft on each level. Three baseboard zones with pumps and this unit will keep up with temp in all but the coldest weather to negative 20 deg. If there is wind with that cold, the temps may not hold up to set point, but have added all new windows so expect that won't be a problem anymore.
I have gas back-up but run this wood unit plumbed separate from the gas boiler.

It is a very impressive burner.
However, if I try to feed it split oak before two years seasoning, I can notice the lack of heat coming from the wood. Dry wood is important. High moisture wood will burn, just won't have enough heat for the house.