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Mother Earth News 3-way Outdoor Oven/Grill/Cookstove Options
Owen
#1 Posted : Thursday, August 21, 2008 8:23:09 AM
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Hello everyone,

As you've already read, we're looking for ideas for this three-way stove.  Everyone is welcome to contribute suggestions.

It sounds like a great idea, although I've never used a stove like this.  But I do love to barbecue and I plan to use the one I build for this project almost every day.

I'm fascinated with the simplicity of the design and how one stove can be used to cook in many ways.

Here are a few things I'd like to incorporate into the design:
- firewood storage in the base
- adequate countertop space on each side
- a roof so I can cook outside whether it's raining or not
- probably a sink to one side (but we don't need to worry about this right now)

My main concern is getting the firebox proportioned correctly so it has good venting.  I don't want a bunch of smoke in my face.  I've love to get the exact dimensions of the one Cheryl used so we're assured of it working correctly.

Also, we need to work out all the details: what the door will be made of, how to attach the door and so on.

My other main goal is to keep it simple and affordable.  I really hope to see lots of people use our ideas, and an overly complex/costly design will probably discourage its use.

That's all for now.  I look forward to your comments and suggestions.

Owen
jgulland
#2 Posted : Friday, August 22, 2008 11:38:02 PM
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Hi Everyone,
This sounds like a great project. I like the idea of multiple functions. I'll just jump in and offer some initial comments.

If I wanted to build something like this there is a number of design issues I'd want to deal with.
  1. In order to build an efficient combined grille, oven and boiler I'd aim at the minimum firebox size I could get away with. Wood fires burn best when enclosed in a cosy environment, so the fire should occupy as much of the chamber as possible. A small firebox heats up faster because there is less mass to heat. A normal sized grille, like a regular barbecue, isn't very big. Nor is an oven for normal household use. For a boiler, you want the cauldren to cover most of the steel plate so you don't waste heat. So, to build an efficient unit, don't make the firebox bigger than you absolutely need.
  2. I suspect the steel plate will be the most vulnerable component because it must span the firebox walls and gets intense direct heat from the fire. This is especially true when bricks are on it in preparation for baking. The problem in this case is you want the oven to get very hot and the bricks are intended to insulate the steel plate so it doesn't loose heat. When steel starts to get red hot it gets soft and may start to sag. The thicker the steel, the less likely it is to sag because the upper surface next to the brick might not get as hot as the side facing the fire. One way to deal with this is to make the plate sacrificial and be prepared to replace it every couple of years. This is just speculation, of course, because I haven't built anything quite like this before. Mind you, I have seen a lot of mild steel parts fail in wood burners over the years, sometimes in devices I've designed and built.
  3. Sealing that steel plate to the top of the firebox will also be an issue because any leakage around it will suck in air and cool the whole affair. I've struggled with this problem myself when making maple syrup in our little sap boiler. The sap pan sits on a steel firebox and it leaks a bit around the edges. When I take off the pan and look at the bottom, I see evidence of air sweeping from the edges toward the center, suggesting that leakage is cooling the pan. Since I am always looking for ways to increase efficiency so I use less wood, I obsess about such things. Others might not be as concerned. See: http://www.gulland.ca/maplesyrup.htm
Ok, there are probably a dozen other things I could mention, but I've run out of time. I look forward to further discussion.
John Gulland

jerrydon10
#3 Posted : Saturday, August 23, 2008 8:14:36 PM
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Well, it sounds like a lot of work for my little lady putting on and pulling off steel plates and firebricks.

Would it be better to have the fire in a centralized location below, then the oven, grill and stove already assembled above?

She could then vent the heat via 3 stove-pipe type valves (I forget what you call those..lol) to any one of the three she chooses by having one valve open and the other two closed.

I know that idea needs some work, but it's a start.
Owen
#4 Posted : Wednesday, September 03, 2008 2:39:10 PM
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OK, we're off to a good start with several good ideas.  I like the suggestion about minimizing the firebox size.  This would be more efficient, plus I don't need a huge stove.

Thick steel is no problem.  I can easily get 1/4" or 3/8", and maybe other sizes.  Anything above 3/8" though seems too awkward.  We can grind the edges for a better seal.

[Would it be better to have the fire in a centralized location below, then the oven, grill and stove already assembled above?]

I think we're after a very simple design like the one Cheryl used.  It may not match everyone's needs (there's no such thing), but keeping it simple and affordable are top concerns of mine.
jgulland
#5 Posted : Wednesday, September 03, 2008 7:54:27 PM
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Owen wrote:
"Thick steel is no problem.  I can easily get 1/4" or 3/8", and maybe other sizes.  Anything above 3/8" though seems too awkward.  We can grind the edges for a better seal."

I'd be inclined to try 1/4". It is fairly beefy, but not excessive. I don't think grinding the edges would help much. It is the roughness of the masonry that might allow leakage around the steel. I'm trying to think of a way to seal the steel plate to the masonry without making a permanent joint. Do you expect to have a lip or ledge for the steel to sit in?

If the masonry mating surface was perfectly level and smooth (very tricky to do), you could maybe use one of the flat woven gaskets that are used to seal stove door glass between the masonry and the steel. That is just one possibility and there are probably others. On the other hand, that gasket might be damaged when the grill is used.

[Would it be better to have the fire in a centralized location below, then the oven, grill and stove already assembled above?]

I don't think so. The simplest approach is to build the fire right on a firebrick hearth. Remove the fire for baking and keep it going for grilling and boiling.

The question of firebox shape is an interesting one. For all three uses (grilling, baking, boiling) I suspect that a roughly square footprint would be the most practical and convenient shape. If you keep the total size reasonable, the fire can occupy most of the floor. As to the dimensions, I would think about 18" square would be in the ball park; an inch or two smaller or larger might work well, depending on your objectives.

I think we're after a very simple design like the one Cheryl used.  It may not match everyone's needs (there's no such thing), but keeping it simple and affordable are top concerns of mine.

Good plan.
John
Owen
#6 Posted : Thursday, September 04, 2008 3:18:51 PM
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Owen
#7 Posted : Friday, September 05, 2008 3:25:17 AM
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John wrote: The simplest approach is to build the fire right on a firebrick hearth.

Yes, this is what we're looking for.

Steel plate: 1/4" steel will be easy for readers to obtain.

Firebrick: standard size is 9" x 4.5" x 2.5"
I made a quick sketch. A 13" wide x 9" high firebox seems about right. This size comes from two bricks on the bottom laid end to end (18"), minus bricks on each side laid on edge (2.5" x 2 = 5"). So 18" - 5" = 13" wide firebox. The 9" height assumes two rows high.

This may be smaller than what Cheryl is talking about, but this size is good for me and matches up well with various pots and pans:
- bath canner = 13"
- bread pans = about 5" x 9" (two would fit side by side with space for air circulation)
- casserole dish = about 9" x 12"
- cookie sheet = 12" x 15"
- many pans/skillets are around 12"-15" diameter

So lots of things fit well with minimal wasted space.

John wrote: Do you expect to have a lip or ledge for the steel to sit in?

We could add a groove to each side so the 1/4" steel top can be slid in and out as needed. This would help seal the edges. For example, we could add a piece of steel to each side and the back that's bent to 90 degrees. Or do think people would rather lift the steel top straight up as opposed to sliding it out? (This wouldn't seal as well.)

TROY GRIEPENTROG
#8 Posted : Friday, September 05, 2008 2:38:49 PM
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Do we need to consider mortar or is this a dry-stack method?

Will any bricks work or do they have to be special to hold up to the heat?

Could a gasket from an old oven door be used to seal the top or will the heat be too much for it?

Just a few thoughts. I don't have answers, just questions.

Troy

Owen
#9 Posted : Friday, September 05, 2008 3:31:13 PM
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Troy wrote: Will any bricks work...?

See my post above about firebrick.  The firebox has to withstand high heat and that's what firebrick is for.

Troy wrote: Do we need to consider mortar...?

There's a special high heat mortar.  My thought at this time is to use dry joints between the firebrick but embed them in high heat mortar.

Not sure about the gasket.  Keep in mind we want to select materials that everyone has ready access to.


jgulland
#10 Posted : Friday, September 05, 2008 8:02:09 PM
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For what it is worth, I used firebrick splits for the floor of my oven. A split is 9" x 4 1/2" x 1 1/8" or 1 1/4", or half of a normal brick. They are the standard unit used in masonry fireplaces and wood stoves. They stand up well and are cheaper than full sized firebricks.

The dome of my oven is just standard cheap house bricks. After two years of use they show no signs of deterioration. I used them because the book I used as a guide said you could use firebricks if you want, but the author didn't.

A friend who is now building a brick oven ran a test in which he put regular house bricks in a fire and burned it for a while. They fell apart. That tells me that 1/ you should use firebrick for the floor and 2/ that the walls of a brick oven don't get as hot as the floor with hot coals on it. To be clear, though, I'm not recommending standard bricks, but just that I used them and so far so good. See:
http://www.gulland.ca/homenergy/brickoven.htm

You are planning a firebox that is longer than it is wide. That may not be a bad strategy from a combustion point of view, although It means you'll be reaching in a long way for things cooking in the back of it. I suspect you will end up with a fair difference in baking temperature from front to back, just because the fire will be flowing that way as it burns.

Regarding gaskets, almost all wood stoves have glass doors these days and the gaskets they use to seal the glass to the door is a flat woven tape about an inch wide, usually with a tear-off backing with adhesive under it. The tape is wrapped around the edge of the glass and the adhesive holds it there. I wonder if it would be worth sticking that tape to the underside of the steel to help seal it to the masonry. It might stand up to the heat and it might not. Just a thought.
John

Owen
#11 Posted : Saturday, September 06, 2008 3:56:23 AM
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Good work John!  Thanks for contributing.

I think several of your oven ideas can be used on this project.  (Check out the link posted above.)
- the insulated door seems perfect
- splits would work fine
- insulating around the firebrick is a great point.  This agrees with what I've read about rocket stoves, etc.  It prevents the surrounding masonry from drawing the heat away.

Questions:
Did you use regular cement mortar for the dome?  If so, have you observed any cracking on the inside?
Did you set the splits in the freshly poured insulated concrete or did you place them after the concrete had dried?
How would you secure the gasket to the steel?  It doesn't seem like tape would hold up.

Owen
TROY GRIEPENTROG
#12 Posted : Tuesday, September 09, 2008 3:55:59 PM
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From John Stuart:

Cheryl Long pretty much had the old stove description about right. It is 12"  wide X 32" long by 15" high.  For convenience, the actual floor is built up in the air a ways, so we don't have to lean over too far.  There is about a foot of rockwork below the actual stove floor.  We didn't want the bread pans to sit directly on the masonry so we found a couple of nice stainless steel racks (either refrigerator or stove) and designed the width of the stove to fit the racks.  There are two levels for the bread pans.  One set rests on the floor rack and we inserted a few metal tabs in the masonry walls to hold a second level rack.  The one layer of firebrick on the sides and rock outside that is 12-14" thick.  With a piece of heavy metal to cover the top, we could then cover that with a stack of firebrick.  We figured that having the movable upper firebrick would give us the ability to adjust the heat by leaving or removing some of the firebrick. 
 
With enough masonry mass, we did not have to have room for the fire while baking.  Like Cheryl said, we just scraped the burning coals out of the oven, waited a bit until it cooled down to 350-400 degrees and then went to baking.  Our favorite set-up is to plan the baking for meal-time, put a pizza in the oven at about 500 degrees, first.  It only takes about 10-15 minutes to bake, and by that time the oven has cooled to bread-baking temp. With the bread loaves ready to go, we pull the pizza out for dinner, put the bread loaves in. 
 
This set-up was good for only the one baking.  After one hour of baking, the temp was down to about 300 degrees.  So for longer bakings,  a stove with more masonry mass or the adjacent fire-chamber mentioned by John Gulland would be needed.
 
I will try today, to get you a picture of the stove.
 
John Stuart
jgulland
#13 Posted : Tuesday, September 09, 2008 5:06:50 PM
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Owen wrote:
Questions:
Did you use regular cement mortar for the dome?  If so, have you observed any cracking on the inside?

Yes, I just used regular cement mortar. The plastering of the outside of dome cracked immediately, although nothing serious. For the first couple of firings I noticed a few pieces of mortar falling from the dome, but we never found any in food. I think the reason I got mortar falling is that, while I intended to get inside and point the dome joints, when I tried I suffered immediate claustrophobia and gave up. Since then I have seen no deterioration of the dome bricks or mortar at all. I judge the temperature in the oven by the pattern of carbon deposits on the dome. Lighting the fire always blackens the dome bricks, but when it is really hot, almost all of the carbon has burned off. When hot and clean the bricks and mortar look pretty much like the day I put them in.

Did you set the splits in the freshly poured insulated concrete or did you place them after the concrete had dried?

After the concrete had hardened. On the advice of a friend who has lots of ceramic kiln experience and who had just built his own brick oven of a very different design, I laid the bricks dry on a very thin dusting of mason's sand. I just poured some on the concrete pad and raked it until it was even and placed the bricks on it dry with nothing between them. The idea is to provide a level base for the splits, although the splits themselves aren't completely regular so the floor isn't perfectly flat and smooth. I placed a bed of mortar around the splits to hold them in place while the dome was built. This approach allows a brick to be replaced if it breaks, but that hasn't happened yet and there is no sign of deterioration of the splits.

How would you secure the gasket to the steel?  It doesn't seem like tape would hold up.

I don't know. It might not stand up. Plus when you remove the steel for grilling, there is a good chance the gasket would be knocked off. I was just trying to think of a way of reducing leakage around the edges of the steel top plate. It may be that there is no effective way aside from making the mating surfaces as even as possible.
John
Owen
#14 Posted : Thursday, September 11, 2008 1:54:51 AM
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Setting the firebrick on a bed of sand makes sense.  This would allow for differences in expansion and contraction.

We could suggest the gasket seals as one option.  But a little smoke on an outside stove seems tolerable.

Size of firebox: I've explained my preference in a previous post, although I didn't mention the depth.  Assuming a 13" wide firebox, how deep should we build it?  Is there a general rule of thumb for width versus depth?  I'm in favor of choosing a size that has good draw, requires no brick cutting and is no larger than necessary so as to save on firewood.
jgulland
#15 Posted : Saturday, September 13, 2008 9:49:48 PM
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Owen wrote:
We could suggest the gasket seals as one option.  But a little smoke on an outside stove seems tolerable.

It wasn't so much the leakage outward that I was concerned with, but the leakage inward. If your chimney is at all effective, outside air will be sucked in, not leak out. Air being sucked in would cool the fire and the underside of the steel top plate.
John

Owen
#16 Posted : Saturday, September 27, 2008 12:38:54 PM
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Any suggestions on the depth of the firebox and how to attach the chimney?
jgulland
#17 Posted : Tuesday, September 30, 2008 11:42:12 PM
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"Any suggestions on the depth of the firebox and how to attach the chimney?"

Sorry, I've been busy for the past week.

I'm not sure about firebox dimensions. Everything mentioned here involves a very narrow and long firebox. I have not built or used anything like that for baking so I don't know what to suggest. I think a width of 13" has been mentioned. That seems very narrow to me, but then, I haven't experienced one like it. My instinct in this case would be to go with a rectangular footprint, but that would produce something very different than what you visualize I think.
John
Owen
#18 Posted : Wednesday, October 01, 2008 1:17:51 PM
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The depth hasn't been agreed on yet.  Some people may want a deep firebox, but I prefer just enough for standard baking pans, etc. to fit.  (See previous post.)  Again, my plan is to use this regularly and so saving wood is important.

First of all, which is most efficient -- a deep firebox or a rectangular one?

Can readers adjust the depth and not change the basic performance too much?  A versatile design would appeal to more readers.

And again, any input on connecting the chimney?

Troy Griepentrog
#19 Posted : Wednesday, October 01, 2008 7:52:23 PM
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Cheryl and I recently discussed the possibility of using "urbancrete," used or waste concrete, for some of the structure or base of the oven. (But still lining the oven with firebrick.) Used concrete has one flat side and is relatively easy to chip off pieces to the size you'd want. It should be inexpensive and readily available.

Any thoughts on this? How does the heat resistance of concrete compare to stone?

Thanks,

Troy

Owen
#20 Posted : Saturday, October 04, 2008 1:56:03 PM
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Hi Troy,

The base is the easiest part.  Urbancrete, stone and brick
(preferably recycled) would all work fine.

Stone and urbancrete would be very similar in terms of transfer of heat, but it's not much of an issue since we'll be using firebrick surrounded with insulated concrete.  In other words, most of the heat will never reach these other materials.

My plan is to use brick and create a storage space for wood under the firebox.  An arched opening would look good and be structurally sound.
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