DIY design for outdoor wood burner, looking for comments and suggestions

Old 10-18-09, 02:32 PM
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DIY design for outdoor wood burner, looking for comments and suggestions

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.

Jon Puckett
Old 10-18-09, 03:25 PM
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Actually, this would be classified as a hot-water boiler and there are significant safety hazards involved.

The "water box" worries me (which is about as far as I got in reading your description). Under normal circumstances, it would be designed in accordance with ASME codes. It is a pressure vessel that should be designed and stress analyzed by a qualified engineer. Rectangular pressure vessels have major stress risers at the edges. I doubt there are too many of us, if any, on this forum that would consider ourselves qualified to design your boiler. Those people mostly work for boiler manufacturers.

What design pressure were you thinking of? What hydrostatic test pressure were you thinking of? There are many other questions that I'm not competent to ask.

Your idea of using an automobile radiator cap for over-pressure protection is totally inadequate and suggests to me a dangerous lack of understanding.

I can't help wondering if this is possibly a joke.

Last edited by Mike Speed 30; 10-18-09 at 04:14 PM.
Old 10-18-09, 04:16 PM
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Mike, I sure hope he's joking because what he has described is nothing short of a bomb just waiting to kill someone.
Old 10-18-09, 06:31 PM
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Dude, your nuts.
Old 10-18-09, 07:17 PM
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Jon, I'm sorry, but I've got to close this thread. The advice you've gotten is spot on. Please cease and desist before you kill yourself.

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