Wood Burner conversion

[jinsko]

Hi Everyone-

I'm new here and just bought this wood burner at an auction: Steel Wood Burner, has Fire Bricks Inside 41"x21 1/4"x 31&qu... - Repocast.com?

The plan, obviously, is to convert it to a pizza oven. I assume the most important thing to do is to create a thermal mass on top of it. Does this look feasible? What's the best material to use for the thermal mass?

I've been looking at this one as a model; it looks interesting and successful, yes? http://www.fornobravo.com/forum/f43/...oven-3717.html

Thanks!
Jeff

[Laurentius]

Re: Wood Burner conversion

To make an efficient oven, you'll have to modify, by closing the chimney in the back, unless it already can be closed. Something must be done with the floor vent, its taking up most of your cooking space and will not allow heat saturation of the hearth. The door-ceiling ratio needs to be changed. I don't know if it was better than buying a steel barrel and starting from scratch. Go for it and good luck

[Wiley]

Re: Wood Burner conversion

Hi Jeff,
The steel dome oven to which you link is mine and yes it is successful. There are several WFOs that have been built using the steel dome idea I promoted on my thread. One is a commercial oven built on a trailer and which is local to where I live.

First thoughts are along the line of conventional barrel ovens which vent out the front into either a transition area to a chimney or to the world. If you follow that premise (conventional barrel oven) you will need to close off that rear chimney. This will take some design time. The height of the interior of the oven should be measured and using that figure as 100% compute 63% of that height. If the present height of the door is within a couple of percent of that 63% then no modification of the door will be needed. Otherwise you will probably end up modifying the door/entrance. That need not be as complicated as it sounds.

The thermal mass can be whatever you have available that is suitable. Closely stacked firebrick, basalt/fondu concrete (like I made) or some sort of castable refractory should work fine. That would of course be covered with insulation and then some sort of weatherization. All that is base on the premise that the burner as it sits is a simple steel shell with a firebrick hearth/floor.

That hole in the floor/hearth will need to closed off and the space filled with firebrick. The hearth will need to be insulated, perhaps removing the existing firebricks and rolling the oven over onto its top will permit a rim to be built around the parameter and so form a tray that you can cast with perlcrete or vermicrete. This might be considered first as trying to invert the oven once the refractory is added to the top will mean a great deal more work. Some sort of support for the insulation will then be needed as well as to get the oven to working height.

Sounds like a straightforward fun project. Personally I'd sit down and lay out all the steps and modifications I would want to take and make and thereby create a "critical path" to keep from working against myself.

Good Luck!
Hope this helps,
Wiley

[jinsko]

Re: Wood Burner conversion

Thanks for the replies, Wiley and Laurentius. Wiley, your oven is really cool. I enjoyed reading about how you did it.

I knew I'd have to fill in the hearth. As for the other things, I'm new to this, so please forgive some ignorant questions:

I now understand the door-ceiling ratio. In the event that I need to alter the door height, any brilliant (simple) ideas as to how I would do that?

As for the chimney, a chimney in the back simply won't work? Does that mean I will have to create a new one in the front?

And Wiley, for insulating the hearth, you mean underneath it? So there would be an insulating layer on top of whatever sort of base I create-- is that right?

I'm not ready to throw in the towel before I start, but now I'm thinking maybe I made the wrong call in buying it; it all happened rather spontaneously...

Thanks in advance!

[Wiley]

Re: Wood Burner conversion

Rather than designing for that which is unknown it would be easier to know which one has to do: make the doorway shorter or make the doorway taller (and by how much).

Leaving the chimney where it is in back isn't going to give you all that I expect you want from your WFO. By having the exhaust exit out the front one gets the airflow so that it comes in thru the entrance and flows along the hearth to the rear of the oven, it then flows up and along the top of the arch and ducking down and out the entrance. This is where the 63% ratio comes in; it wasn't achieved by some complex math formula but rather by examination of tens of centuries of constructed WFOs. It's empirical. Build it this ratio and the WFO will most likely work, build it to another ratio and it becomes a case of maybe it will work maybe not. It's your WFO you get to build it as you want, we are simply trying to give as good advice as we can.

By having the exhaust exit the door and into the world at large or into a transition chimney area you can also close up the oven and retain the heat inside for such things as overnight baking of pulled pork, baked Thanksgiving turkey, roast beef etc etc. Otherwise with the chimney in the back the heat will escape out the chimney when you shut the door.

And yes, I mean beneath the hearth...no matter that the bricks sit upon a steel plate all of the plate will be heated and will actually help bring the oven to working temperature faster than if it were just brick (this will probably cause a bit of a stir but frankly the steel plate allows more rapid transfer of heat to the refractory thru better conduction of the steel on the top of the inside to the refractory though perhaps a bit less so on the bottom as it is beneath the brick hearth...and you do want the brick hearth!). When you are designing your tray or whatever you use to contain the insulation I would suggest you keep in mind you want as little direct steel pathway from inside in the heated area to the outside. Build the form so it is removable or whatever.

I don't think you made a poor decision in your purchase. You can have a WFO in short time, a few weekends if you have a welder or have a friend who does. The local commercial steel domed oven of which I spoke... the builder didn't know how to weld when he started. Basically welding 1/4 steel plate takes far less skill than welding the thin stuff.

Hope this helps,
Wiley

[jinsko]

Re: Wood Burner conversion

Thanks again, Wiley. You're great and very helpful and I appreciate the encouragement. I've been reading around and now understand the 63% ratio and the reason for the chimney placement; I should have read more before asking.

I'm going to pick the thing up tomorrow and will then have a better sense of what we've got. I'll post new pics and, I'm sure, new questions.

On the bright side, the last spontaneous purchase we made for my wife (this is not joke) was an arc welder! So we've got that part covered!

Best,
Jeff

[jinsko]

Re: Wood Burner conversion

Picked up the stove today. Can't tell whether the news is good or bad. Turns out that it is wider than it is deep. I assume that is less than ideal? It is 39.5 inches wide and 20.5 inches deep (front to back). When I dream of WFO, this is certainly smaller than what I'd want, but does it sound too small? Could I cook, say, 2 pizzas in there at once?

As far as door to ceiling goes, the ceiling is 21.5" and the door opening is 12.5". That's 58%. However, there is about a 2" drop from the lip of the door to the hearth. If I put down a new layer of fire brick (1.25"), that would get me to just under 64%. Is that a reasonable plan or stupid?

To my untrained eye, it appears to be a very well built stove. The steel is quite thick, 1/4 inch. Is that a good thing?

I guess what I'm wondering whether it's worth trying to modify. The original idea (hope) was that it would be relatively simple. (Of course, I know all of that is relative...).

Here are a couple of pics. Maybe not all that helpful.

[cobblerdave]

Re: Wood Burner conversion

Gudday
64 per cent......recon you on a winner....that oven should breathe like a WFO should.
Oven width....best to have a deaper oven than wide. But consider bricking in the internal sides. 4 in and a bit firebrick each side that cuts 8 in off the width. So your now to 30 in by 21in roughly thats a lot closer. Brick externally back and roof and you have the thermal mass covered. insulated that ( dont forget the base) and I recon you'll have a working oven!
Width of door to oven width....Can be as wide as you want, the ratio of roof to door height is the most inportant thing. Check out WF bagel ovens they have a door that runs the full width of the oven.
Space for 2 pizza.....you'll have heaps. think of it this way with 3 min pizza.
It takes longer to make and longer time to eat than to cook. 3 pizza is about as many as I can handle without burning one. You have got to enjoy cooking them as well.... you can drink beer and talk to the guests and cook pizza at the same time you know. Sometimes the journey is just as enjoyable as the destination.

Regards Dave

[Wiley]

Re: Wood Burner conversion

Jeff,
I'm with Dave on this one. The only suggestion would be when you are bricking up the insides to create a ledge on each side such that you can place a grill (sort of like a shelf resting on each the ledge at end). Instant Tuscan Grill.

You can create the ledge by staggering the bricks. I would suggest dry stacking on the inside so you can then modify things if needed and easily replace them if damaged.

Bests,
Wiley

[jinsko]

Re: Wood Burner conversion

Great suggestions, Dave and Wiley. Thanks! I'll do it.

So some quick new questions: I'm pretty sure this won't be my permanent oven, so I think in the near term I'd like to try and do something quick and cheap, just to see what happens. That means closing up the chimney hole (can I just put some kind of cap on it?) and maybe creating a new entrance. Could I just do some sort of impromptu thing like Dave's here?(http://www.fornobravo.com/forum/43/o...-16229-17.html). That rig just sort of sits near the entrance, right? It's not permanently attached?

Lastly, what would be the quickest and cheapest (non-permanent) way to create some kind of thermal mass and/or insulation on the outside? For that matter, what if I did nothing at all? Would I get enough heat from the steel itself to cook some pizzas?

[Wiley]

Re: Wood Burner conversion

Jeff,
Expecting the wood burner is in your work area, as opposed to cooking area:

I would weld some sort of plate over the rear chimney hole.

Then (after removing existing bricks from inside), roll the whole oven over on it's top and deal with the vent in the bottom. Cut out and weld a plate over the opening. You can level the interior with some fire clay when you get to the stage of reinstalling the interior firebricks. Then make some sort of retaining structure and pour some 3-4 inches thickness of percrete to insulate the bottom. This form could be as simple as 2x4s screwed together to form a box. This is to create at least a modicum of integrity so the insulation will not fall out or be damaged when you roll it back over. Form should be removable so some sort of wax (as a release agent) should be rubbed over inside before pouring insulation.

Move to cooking area and carefully roll WFO onto a support pate of steel or cast slab. Whatever you use to support the WFO should extent out a couple of inches beyond the edge of the cast insulation.

Elevate the WFO to working height. Easy enough with levers and some blocking and cribbing and when at correct height replace cribbing with cinder blocks. I would expect total weight at this point to be around 1000lbs or less, (1000lbs if using concrete slab and significantly less if using a 1/4 inch steel plate). Plate/slab (which supports percrete insulation with WFO on top) should extend out from sides and back by a couple inches. Remove form which kept insulation from being damaged. Cover the exterior and back with inexpensive Quikcrete mixed to a consistancy stiff enough to form and hold shape. Use as little amount of water as possible to be able to work the material. If you want to get fancy you can embed some expanded metal to hold the whole together when it cracks. Try for a uniform couple of inches of thickness over sides top and back.

Replace interior bricks in hearth and in the inside side areas.

Don't mess with the entrance if you can get 64% ratio by bricks you will probably be as good as you can expect. What with the wider than deep proportions of this oven you are already working in the experimental anyway. You will be exhausting thru entrance. For a temporary WFO this is not a problem, many people use WFOs without chimneys.

Allow to cure for a few days... a week would be nice. You would be wise to start with some curing fires to drive out what moisture in remaining in concrete. Fire up, I would suggest starting with a small fire and increase its size as the WFO heats. See what temperatures you can get and of course keep us posted. Wear eye protection!

This whole thing is an experiment and by going with plain portland cement concrete the overall life/integrity of the refractory will be shorter than proper castable or some sort of home brew using calcium aluminate cement. However, you still aren't sure at this point the WFO will work as planned and by going Quikcrete you can always replace it with better stuff.
Forget insulating the outside until you know the WFO will work as desired.

Excepting time spent securing materials, a couple of weekends with intervening weeks to cure insulation and concrete you should be able to be enjoying pizza on Sunday of the third weekend.

When finished and after WFO has cooled cover to keep dry with a blue tarp. If satisfied with performance we can suggest changes and worry about insulating exterior.

An argument can be made to build and test the WFO in the work area before moving to the cooking area. Reason I suggest the above is that it will weigh the least until it is in position. If it were a failure busting it up (concrete exterior) into manageable pieces would be possible. Your WFO, your call.

The above is offered without liability and simply as a suggestion.

Bests,
Wiley