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70chevelle 04-24-2008 06:23 AM

A little different design - what do you think?
I installed a flagstone patio a few years ago, and have a store bought copper/steel fireplace that my whole family enjoys on cool evenings. Last year I built a drystacked stone wall behind it for aesthetics and a wind break, which worked out well on both counts. The fireplace has seen it's useful life an is time for replacement. This is my plan. I've designed a fireplace with a chimney in the rear, and my pizza oven (barrel type) on top. Some of the details are as follows. The foundation of the fireplace is 12" tall and 36" square. I will build the exterior out of brick that matches my house. The interior will be with standard solid red contractor brick. The rear will angle toward the front, and stop short of the top to create a smoke chamber in the rear, with the chimney installed above the smoke chamber. I don't want to pour cement (right now) so my thoughts are to make the top (top of the fireplace, floor of the pizza oven) out of my existing brick. Between the brick top and low duty firebrick oven floor, I plan on installing a layer of FB Board. Then the barrel type oven. From my design the cooking floor is going to end up approx 30 w by 26 d. After the barrel is complete, I plan on laying my house brick around the perimeter to completely conceal the barrel and install some sort of roof. From what I've read here and on other places, the insulation is one of the most important items for the oven to work efficiently. There have been some q's about using fiberglass insulation above the Insulating blanket. Has anyone considered using cellulose insulation? It's insulating properties are much better than fiberglass, and it can be dense packed, and is more fire resistant. Best of all it's about $5 or $6 a bag at the home centers. My thoughts right now are to fill the open area above dome with cellulose and then install my roof.

I don't have any forum friendly designs to post. Is there a huge design flaw in building my oven on top of a fireplace. They may be used at the same time, but I don't see that happening too often. I also wanted to alert you all to the cellulose insulation, if it hasn't already been discussed. Any and all comments and thoughts would be appreciated!

dmun 04-24-2008 06:55 AM

Re: A little different design - what do you think?
Wow Wow Wow. Cellulose insulation is wood fiber, basically recycled newsprint. It's not a high temperature insulation. I know it's treated with a fire retardant, but this is way out of it's design specs. This sounds like a bad idea. Here's an experiment. Put a pan full in the home oven. Heat to 400f or so. See what the house smells like.

I built an oven on top of a fireplace, but the openings were 180 degrees apart. You can do it, but it makes your project much more complicated.

Your design sounds way too small, even for what you plan. Remember you need room for your fireplace flue to circumvent your oven, and room for insulation between the fireplace flue and dome. Also, building an two-flue chimney is a non-trivial exercise.

Some of the barrel vault folks here may weigh in on your plans. Please give us some sketch, even if it's a digital photo of your back of the envelope sketch.

SpringJim 04-24-2008 10:34 AM

skip the cellulose
Perlite and Vermiculite are relatively cheap in bulk....better choices!

Cellulose insulation is made of groundup
or shredded newspaper which is naturally
combustible. In fact, cellulose insulation
is regulated as a recognized fire hazard
by the Consumer Product Safety Council
(CPSC).4 To protect against fire hazards, cellulose
insulation is heavily treated with fire
retardant chemicals prior to installation.
Tests conducted by the California
Bureau of Home Furnishings and Thermal
Insulation have demonstrated that some cellulose
samples failed the standard fire safety
test only six months after installation.5
Additionally, smoldering combustion and
re-ignition problems are concerns with cellulose
insulation should a fire start.6 Even
properly treated cellulose insulations will
burn at about 450įF. Thatís the surface temp of a 75w light bulb!

gjbingham 04-24-2008 11:04 AM

Re: A little different design - what do you think?
XJ - is that you?

70chevelle 04-24-2008 12:12 PM

Re: A little different design - what do you think?
Check! perlite or vermiculite!

What do you guys feel would be the smallest useable oven space? In looking at some of the prefab dome units, the small ones are 31.5" round or 780 sq/inches. I was trying to hit somewhere between that and 900 sq/inches. Is this too small? I guess my problem is that my base is pretty much set at 36" square. I can flare out another 6" around (48" wide by 42" deep) without too much trouble to get to approx 30" square, but that's going to be my limit. I guess I'm saying it's going to be a small oven or no oven.

GJ - no, not XJ.

dmun 04-24-2008 12:37 PM

Re: A little different design - what do you think?
James has talked about cooking in really small pre-fab ovens, and how little room there is to work, and how hard it is to have decent heat retention because the door opening is so much bigger in proportion to the cooking space. This is less of a concern if you are mostly doing baking in your oven, and not trying to juggle a fire and floorspace for pizza.

You also have to think about your fireplace flue placement. With a rectangular oven, you are not going to have a corner to sneak your flue through.

This said, I think most people would agree a small oven is better than no oven at all.

70chevelle 04-24-2008 01:29 PM

Re: A little different design - what do you think?
dmun - thanks again for the reply. My flue for the fireplace would be placed behind the oven, an not interfere with the oven at all. At least that's my goal. My main use would be for pizza, although I'm sure after I got my wood fire oven education, my wifes homemade bread would be done there also. It's a little disconcerting to think about all the insulating via insulating blankets, perlite, and insulated cement to have heat loss issues thru the door. I guess I'm having second thoughts due to my size constraints, and may just build a typical fire place, and buy an ceramic cooker for pizza's and smoking?

james 04-24-2008 04:57 PM

Re: A little different design - what do you think?
I'm not sure from the dimensions if you can run the fireplace chimney behind the oven, and still get a 30" cooking surface. If you can, I would be tempted to build a traditional round 30" oven on top. That gives you just enough space for pizza and baking.

Because the vent of the oven is in the front, you can cantilever that and any landing area and still work with your footprint.

Here is a sample photos -- though I can't tell how that did the oven vent. Is it pulled back to meet the fireplace vent, or did they skip the all together?

Pizza Oven Photo San Louis Obispo

In terms of insulation, I would recommend buying FB Blanket (or another ceramic blanket) to be safe. The oven will get hot and burn traditional insulation. Been there; done that. I tried a low-end insulating blanket in Italy (Rock wool) that I am guessing was designed for fireplaces, and my oven scorched it to where it was black, smoking and smelled up a storm.


70chevelle 04-26-2008 06:07 AM

Re: A little different design - what do you think?
Well, I rethought my plans and came up with an idea to increase the size of my oven. I took some pics yesterday and today. I cut my flagstone patio to set my 800#, 5000 psi chunk of concrete. It was a tough one man job wrestling that around, and getting the lifting straps out from underneath. Anyway, to get the size I need, I'm going to attache a lintel around the perimeter of the concrete, and then place 6" block on the lintel. this will pick me up 12" of width and depth, plus I can get a little more if I need it when I'm laying the bricks. So as of right now, I should be able to get about 50" w & d, so I feel pretty confident I can get a 30-32" cooking floor.

Now to another Q. I am using low-duty fire brick for the oven floor. If I lay the fire brick on the FB-Board, do I still need to use the concrete/vermiculite insulating concrete underneath the FB-board?

So my next steps are to attach the lintels and 12 6" block. Layout the fireplace. Lay the firebox, then the outside walls. After that's done, I'm sure I'll have more questions, a few pics to post, and then start the oven. Still not sure if I'm going to go dome or barrel yet.

gjbingham 04-26-2008 09:47 PM

Re: A little different design - what do you think?
Good question regarding the vermiculite/concrete in conjunction with the insutating board. It is a pretty well accepted fact that one of the biggest difficulties with a WFO is keeping the floor hot. The dome retains heat very well, the floor cools much quicker.

I think you mentioned your wife's bread somewhere back a couple of posts. If you plan to do bread all the time, I'd absolutely use the equivalent of 6 inches of insulation under the floor of the oven. That's 6 inches of vermicrete or three inches of ceramic board, or any combination thereof. One inch of ceramic board = two inches of vermicrete. You might also want to consider using the floor bricks laid on their sides to increase the thermal mass in that area.

I do a fair amount of bread. My oven drops from pizza temps to 450 degrees in a half hour to 45 minutes depending on how long I burn a fire in there - giving me one good load of bread, which is plenty, but it also requires that I stay closeby and monitor temps until I hit my bake window. It holds 425degrees down to 300 degrees for another day. Not hot enough for the bread I want to make, but great for general cooking.

You and your wife should discuss what you would really like to do with your oven and modify the plans based on your needs.

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