Heat break plan

[ThermoJax]

I just purchased everything, including super insulating firebricks for my inner arch. As noted by others, they are soft and kind of prone to crumbling. I also purchased firebrick splits to mortar onto the face of the inner arch, so that the logs going in, and the pizza going in and out, as well as the door being placed onto the surface of the arch will not make it mess up. BUT, my plan is to make a 42 oven with a 20 wide door/arch 12 high, that flares out to 23 wide. The landing depth will be 15. I have my indispensible tool on my plywood and the whole thing traced on it, but haven't cut it yet, as it occurs to me that if I want the final product to be a 20 wide opening, that mortaring splits pieces (1 1/2 in thick) onto the arch face will bring my opening inward to 17 inches. So, perhaps I should make my board 23, not flaring outward to the outer arch, also at 23, but the "gluing" of the 1 1/2 splits will bring it in to the desired 20 inches for the inner arch.

What say you all?



Tom

[GianniFocaccia]

Re: Heat break plan

I believe I read (dmun) that insulating bricks do not hold a mortar bond so will not prove sufficient to support your plan. I am planning a heat break like the one in drseward's build. It allows contact of 1/2" of a standard entry firebrick to the face of the inner arch. Because insulating board is twice as efficient as insulating firebrick/vermicrete, I'll be going with that.

[Tscarborough]

Re: Heat break plan

A 1/4" air gap is as good a thermal break as you need.

[SCChris]

Re: Heat break plan

Tom, I think you could get the Firebrick to mortar to the Insulated Firebrick but it might take some doing to get the best joint. If you've decided on this, here is what I'd try. See what happens if you saturate the IFB in a bucket of water. Does it hold up? If so can you mortar a split to it, laminate it, before final placement by soaking the IFB and letting it dry just enough to get the mortar joint set? This might work. This said, I wonder about any expansion differences. I guess you could laminate and try running a torch on the FB until it gets to temp and see what happens to the joint. If there are significant differences in expansion the joint will fail.

These are the questions that I'd be asking.

Chris

[david s]

Re: Heat break plan

"A 1/4" air gap is as good a thermal break as you need."


1/4" of vermicrete would be better IMO but substitute lime for cement. An unfilled gap will fill up with cheese and subsequently cockroaches, although being the worlds toughest creatures they're unlikely to find the intense heat particularly comfortable.

[SCChris]

Re: Heat break plan

If you can find a foundry supply or shipyard, I'd bet you could find scraps of Cal. Si. board for free that would fill your 1/4 inch or better gap. The only spot in the break that you'd need to protect the material would be where the hot gasses exit into the flue area and I think you could just run the material back a 1/2 inch in a trough. If you felt like filling the trough, you could mortar this joint and not take much of a penalty. I guess you could do the same around the break area and use IFB. I also thought that If you cut and placed the IFB such that you allow for a split to cover but to be mortared to the full firebricks on the back side. Sorry, my sketch skills suck, but if you were to place a split to cover but on the backside of the split half of this face actually attaches to a full firebrick and the remainder bridges over the IFB.

I hope this makes sense, lack of sleep and a lack of coffee doesn’t really help my communications..

Chris

PS you might try these folks for scrap. I don't know that they can help, but the search indicated that they use ceramic fiber insulation. Most folks are really fasinated and just want to help any way they can.

Agr Fabricators, Inc. - Jacksonville, FL
Thermal Products Co., Inc. - Norcross, GA

[GianniFocaccia]

Re: Heat break plan

My plan calls for a 1" gap and to stuff it with Insblock pieces I saved. A standalone entry that contains the flue will allow a 180* ring of insulation to separate the dome and entry bricks. I'm hoping to minimize dome-to-entry heat transfer by limiting the contact point between the two to 1/2". I wonder if a bead of high-temp caulk (red) on the backside of the entry brick where it touches the dome would be advisable.

[ThermoJax]

Re: Heat break plan

the box of IFB was $80 at our local supplier of ceramic insulation, Atlantic Firebrick (we are a port city and they used to do all the boilers in the port), and now with this new information, I will take the box unopened back to them on monday. Kinda delays my build. I like the idea of the gap instead. I had to purchase the INSBLOCK19 by the box, so I have leftover to use. Apparently my pi R squared skills are not up to snuff, as I did not buy enough of the 12x12x2 firebrick tile (shiplap tiles), so I cannot continue this weekend anyway. 4 years in the planning, stopped in my tracks. The IFB seem so brittle. The guy at Atlantic Firebrick said he thought they could be snapped by hand, no saw needed. Doesn't sound tough enough to last for years of use.

My grandfather used to say " there is never enough time to do the job right, but there is always enough time to do it twice" Twice and thrice seems to be how my build is going.

[SCChris]

Re: Heat break plan

I'm with your Grandpa. I feel that there are materials that are going to last and those that are suspect. I'd avoid the suspects even if it means a bit more loss of heat. I have to say that I'd do more insulation in my next build, but my oven's 100F loss per day after the first 24 hours just amazes me. That it takes something like a week to drop to ambient. These ovens and how they compair, or don't, to what the modern world considers an oven is fasinating. 99% of modern homes or more have these disposable ovens that only hold heat for the time that they are in use, what a waste of energy to build these, disposable, modern ovens, install them, use them and replace them again after 10 years of poor service.

Chris

Sorry for the soapbox.

PSS. Tom, trial and error is kinda the rule, or it was with me. Some relationships of materials and how they relate to the space and then how they will be used are hard to fully understand. Stress points and transistions from a dome to an arch and what to do about these places just takes time to decide what to do. Sometimes it comes down to deciding to decide.

[GianniFocaccia]

Re: Heat break plan

Chris,
No apologies needed. I'll bet the majority of residential ovens are bought primarily on appearance instead of functionality. -100F/day? Are you kidding me? That's way cool (hot)! Can you tell me, are you measuring each day's heat loss with an IR gun on the dome, floor, both? Or are you reading a thermometer through a door perhaps?
John

[SCChris]

Re: Heat break plan

John I'm using a through the door FB thermometer for the readings. It's consistent with the IR gun for loss over time but the specific internal oven air temps from the through door does differ from what I get from the IR reading the floor. I'm not concerned by the differences between the two floor isn't going to be exacly the same as the air temp at the door. I know I said it before, there is an interesting period of time from closing the oven after the pizza burn that lasts between 12 and 24 hours where the temp loss curve is steeper. I think this is where the oven mass is still charging with the higher inner oven temps. The 100F loss per 24 hour slope sets in when the oven gets to about 500F.

I was thinking about how to calculate the oven's mass and the loss of temp and try to calculate the amount of energy that it might take to keep the oven at 200F. I'm wondering about running a solar panel that powers a heating element inside the oven just to hold these low temps. To what advantage, I bet the first 24 hours of accelerated heat loss would be less accute, also I wonder if it's feasable to keep the temps at the 250 to 275 area where roasting, or reheating, anything, is always a quick option.

It'd sure be a interesting way to get the curing started.
Chris

[david s]

Re: Heat break plan

Chris,
The higher the temp the greater the heat loss, so it's not a straight line graph. Firing a kiln works the same way. It's easy to get to 1000 C but to go over 1200 C takes more than double the energy because of the heat loss. Cooling down works the opposite way rapid heat loss to start then slower as the temp drops.

[SCChris]

Re: Heat break plan

I noted that the heat transfer through the Cal Si board shows faster flow at higher temp. It seems that the first 24 or so hours or down to about 500 - 550F flows the heat about 150% of the lower than 500F temps, about 150F over the first 24 hours rather than 100F. David are you a ceramics person? I was focused on ceramics in college for awhile, some cooking was done on these ovens. I remember a bit of cooking over a tunnel kiln years after my college. The job I had at the time was refinishing slip cast items to refire. Some of the ladies would put a pot of pork stew on top of the exit area, heat a few tortillas and lunch was served. I loved the food that came off that kiln...

Chris

[david s]

Re: Heat break plan

Hi Chris,
I've just picked up this thread, been on holiday in Europe for 10 weeks. A thing we have in Australia where everyone who has worked at the same place for 10 years gets 10 weeks holiday on full pay. Yes I am a ceramics person and have built about 6 kilns over the years which has given me a fair understanding of heat and fire. Built one out of mud brick as an experiment and the inside face of the mud brick sheared off in melted sheets because of the fluxable material of the brick, at about 1100 C