Outdoor Wood Furnace Info
All-Purpose OWF Discussions => General Outdoor Furnace Discussion => Topic started by: newmod on October 03, 2011, 05:49:18 PM
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Hello,
I am new to this site. Looks like great site. I am planning on building a OWB. What wall thickness steel should be used for the firebox. I used to own a Central Boiler, so I am familiar with OWB, just not with building them. I have seen some others on here use a propane tank cut in half. Looking to go that route, but will it hold up as a firebox. Thanks all>
Newmod
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There is a lot of heat stress in this area. A thin firebox wil give better heat transfer but may not stand up well over time. My firebox is 3/8 inch. It should be around for a while.
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Mine's 1/4", I wouldn't go any thinner on a fire box....
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Airgap,
I just finished looking at your pic from your OWB that you made. Very impressive. I have been contemplating a round or square firebox. How long did it take you to make yours.
Newmod
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Thanks. Mine's pretty simple compared to some that have been built by people on here.
Anything round will be less welds, which saves time and is less work, and if you are not a good welder, it will reduce the chances for leaks.
I probably have around 80 hours in mine spread out over a couple of months. Give or take.
It's not something for a novice fabricator, but if you have the skills it will be very enjoyable.
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3/4'' thick on mine, propane tanks are pretty thick.good choice ..what happened to CB furnace.
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3/4 inch thick, are you serious?
I know of a lot of furnaces around over 20 years old, some have mild steel 1/4" boxes and
others have 16ga sooo, go figure!
I'm really not sure the actual metal thickness is important as the quality or the person welding it.
My cousins fire box is 1/16 of an inch an this makes it's 23rd season
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i think 1/4 or better is good the pipe i used is 7/16 and that is the way it came from the junk yard and i think a lot of propane tanks are about 3/8
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o yea im serious,3x5 firebox,1''end plate 5/16 thick chimney,1700lbs just in firebox steel. hold heat for alooong time
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Wow, you are losing a lot of heat using a firebox that thick.
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Rosewood,
The CB i had went with the house that I sold. THAT WAS A MISTAKE, SHOULD HAVE KEPT IT, (the OWB ). I have no problem with the fabrication part. I just found a good price in 1/4 in x 4' x 8' steel plate. So i will be making a road trip soon. I really like the simplicity of the way AIRGAP constructed his. I have an indoor WB and hate the mess, smoke etc. BACK TO OUTSIDE !
AIRGAP,
Did you put a baffle ? or whatever it is called to keep some heat in the firebox, so it all doesn't go out the chimney.
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I would recommend you go with the 1/4" plate.3/8" at the Max do to the heat transfer rateno point in spending extra money on steel when its not really doing you any good.use the money for other important parts.as long as you maintain the inside in the off season it should last for many seasons.
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Rosewood,
The CB i had went with the house that I sold. THAT WAS A MISTAKE, SHOULD HAVE KEPT IT, (the OWB ). I have no problem with the fabrication part. I just found a good price in 1/4 in x 4' x 8' steel plate. So i will be making a road trip soon. I really like the simplicity of the way AIRGAP constructed his. I have an indoor WB and hate the mess, smoke etc. BACK TO OUTSIDE !
AIRGAP,
Did you put a baffle ? or whatever it is called to keep some heat in the firebox, so it all doesn't go out the chimney.
No, I didn't put a baffle in mine. My flue travels about 18" horizontal through the water and out the back. This gives me more surface to heat water. I don't know if It'd be better with a baffle or not. It's doesn't get very hot where it comes through the jacket on the back. It would be fairly easy to go back and weld one in.
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yoder, why would i be losing a lot of heat? have you ever heated steel before? what cools faster thicker or thinner steel?
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yoder, why would i be losing a lot of heat? have you ever heated steel before? what cools faster thicker or thinner steel?
Rosewood,The btu's needed to heat your thick firebox could already be heating water if it weren't for the extra steel.I think your steel holding the heat longer is not worth the lost burned wood.When I am fabbing with 1" material with a rosebud the steel still cools fairly quickly,and if it surrounded by water your still cooling it quickly.You might hold heat a short time longer than a thinner box.You know it takes alot of heat to get a 1" piece of colled roll up to a required temp than a thinner guage.But poor cold water on it and it cools off quite fast.A thinner box is already transfering heat to the water while the thicker box is still trying to heat soak.
Same reason you don't weld aluminum lying directly on a steel work bench.The thick table will suck the heat right out of your work piece before you can even think about welding it.Wasted time and material to heat the piece.
Just my thinking on the subject ,and no boilers were hurt in this crazy explanation. ;D LOL!
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yoder, why would i be losing a lot of heat? have you ever heated steel before? what cools faster thicker or thinner steel?
you are losing lots of heat, the 3/8 firebox for ncb
tested 12-14% less efficient than the same stove 1/4 inch thick.
It is much harder to transfer heat into the water with thicker steel, besides that, it's
not the goal to store heat in the steel, you store your heat
in the water
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Yea, hot steel out in the yard doesn't do much to heat your home. You have to be able to transfer that heat into the water to do any good. Your furnace will be unable to capture as much heat as it produces and so it will escape out the chimney. I had a chart showing the efficiency of different thickness of mild steel and stainless steel but I don't know what I did with it. I do remember that it lost a lot the thicker it got.
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It's about transferring heat to the water, not storing it in steel. I never realized folks seen
it that way. An example would be would u rather hold your hand on the back of 2" piece of steel while ur buddy waves a torch over it or a 1/8 piece of steel, answer is neither, but your going to get burnt a lot faster with the thin metal lol
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jackel , good points..but dont forget that the water on the other side is still hot 165,not stone cold 65 deg water my on cycle time is about 2-3 times longer then the average 400 gal owb.but i have 3 times thewater capacity at 1200+gals.with that said i dont see how im wasting heat.thicker material has more durability benefits. thick steel and large capacity of water has also resulted a really clean furnace,little to no smoke after a hour or less of burning . i am just giving my 2 cents based on my real world results and not by speculation or theory.
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this isnt theory, but if an OWB would last equally as long with the same material, you would want the thinnest material you could possibly get! Thats saying it would last equally. look at radiators, everything is made really thin in them for maximum heat transfer, what your doing with your stove is no different, transferring heat through a metal into water...
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when you say radiators are you talking about cast iron rads,baseboard rads,or automotive?anybody thats been in a old house iron rads knows how long the steel holds temperature.thinner baseboard rads expell heat faster but require more flow of hot water thru them. a automotive rad is designed to expell heat . for friendly debate,i believe you said it was harder to transfer heat with thicker steel,to the water and not the goal to store heat in the steel. were do you thinkthe absorbed heat in the steel goes when its submerged in a tank of water? water doesen't absorb heat? thick stock is radiating heat in water after the fire stops. we all know how wood furnaces work ,when air flow stops entering firebox it locks heat in thats why smoke stops flowing from chimney , therefore any thing hot inside stays hot for extended periods of time.thicker stock stores more energy and can release it into water longer after the fire is off.thinner stock would release the same way but would run out stored energy quicker . what purpose does fire brick serve in wood stoves?2 things..keeps fire from burnning thru thin wall stoves and stores heat energy then radiating out to room .mass holds energy ..thats theory.
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jackel , good points..but dont forget that the water on the other side is still hot 165,not stone cold 65 deg water my on cycle time is about 2-3 times longer then the average 400 gal owb.but i have 3 times thewater capacity at 1200+gals.with that said i dont see how im wasting heat.thicker material has more durability benefits. thick steel and large capacity of water has also resulted a really clean furnace,little to no smoke after a hour or less of burning . i am just giving my 2 cents based on my real world results and not by speculation or theory.
That's what I love about this site! We all have our own ideas and theorys. :thumbup:We know what works for us and thats what we try and build.
I can see where you would have long burn times to heat sink the thicker steel to bring up your water temp.More efficient burn during wide open heat output then short on and off again cycles.
Maybe we are doing it all wrong.....Perhaps we need to use the 1/4" material for the burn chamber then use a 1" out jacket.Then super insulate it to hold the heat...That would quickly bring the temp up in the water and then the large mass of the thick tank walls to slowly give up its BTU s as it cools.
Dang I would hate to buy the steel for a tank out of 1" plate.Heck I would need Versa-Lift truck to move it. :o
http://www.versa-lift.com/ (http://www.versa-lift.com/)
I use one at work all the time to set machines.Might have to pour concrete all around the poutside of my shop to hold it up.LOL!
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jackel your right on about theory and ideas. as a fellow homebuilt owner you can see real world results and tell people your info , accurate info first hand and not from a sales advertisement.
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Steel transfers heat at a set rate that is reduced the thicker it gets. At some point if you made it thick enough it would no longer transfer any heat. Any heat produced by your fire in excess of the transferring ability of the firebox to transfer is wasted energy. Using steel that is twice as thick would require either a fire that produces half as much energy or twice as much area for heat exchange.
The firebox is a obstruction in the path of the energy. What if we take the thicker firebox theory to the extreme? If 1 inch is better that 1/4, would 10 inches be better than 1inch? What about 3ft, would it be even better?
These theories have all been tested by furnace companies many many times and I can assure you none have come to the conclusion that thick fireboxes transfer heat better.
I think Jackel has a good idea, a thick water jacket would be much more desirable because you would not be fighting the heat transfer issues but may gain some with respect to holding heat.