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SCChris 06-29-2010 08:05 AM

Heat transfer metrics - under door heat loss
Here is an update on how heat retention changed in my oven when the entry materials changed. The first number is at the close of the oven after the last wood has burned significantly down. After Pizza. The second number in each set is 24 hours later, the third is 48 and the forth is at 72 hours.

The first set of numbers are my first high temp fire at the end of curing. What I’d like you to note is the quicker loss of heat related to the second set of numbers. These second set of numbers are a few firings removed from curing. I attribute the change here to the continued drying of the base.

850 400 230 140
850 500 385 245
850 350 200
850 500 400 250 - ? - ? -and 108F 7 days later.

Now compare the second set of numbers to the third set and you’ll note that the heat retention drops significantly in the third set. This change is attributed to a few things, but the major change relates to replacing the front entry floor brick with soapstone. This floor continues under the insulated door and up against the oven floor but does not continue into the oven.

Now compare the fourth set of numbers to the second and third set and you’ll note that the heat retention is now better than any of the preceding set. The soapstone that ran under the insulated door has been replaced with the original firebrick, essentually back to the same original, as built, configuration.

I think that the positive change in heat retention relative to the second set of numbers comes from the additional drying of the concrete base. When the forth and third set are compared the retention change is due to the much higher heat transfer rate of the soapstone, under the door, when compared to the firebrick. Soapstone has a 6X faster heat transfer rate when compared to brick.

My next set of numbers will be after replacing the firebrick under the door with a 1 inch break of rigid insulation board.


texassourdough 06-29-2010 09:41 AM

Re: Heat transfer metrics - under door heat loss
Nice data Chris!

Stone is generally much more conductive than bricks and your data clearly illustrates that. I tend to come to your conclusion also about refractory humidity being a likely source of variation. My experience with my oven makes me a big believer in heat breaks around the hearth.


wlively 06-29-2010 01:43 PM

Re: Heat transfer metrics - under door heat loss
1 Attachment(s)
I think heat breaks are worthwhile as well. Easy to do and can be done without anyone knowing the difference. I made 45deg angle cuts to the side face of each brick on either side of my break joint and filled the gap with very loose perlcrete. The brick contact area between the joint is only a couple of mm.

Side view below.

Food With Legs 06-30-2010 01:08 PM

Re: Heat transfer metrics - under door heat loss
I'm now at the stage of laying my floor and this idea of a heat break sounds intriguing.

Obviously insulation board is not rated for direct flame exposure so how do you protect the top of your floor's heat break from fire, food, and wear and tear?

fxpose 06-30-2010 01:22 PM

Re: Heat transfer metrics - under door heat loss
I used insulating bricks as my thermal break and veneered them with firebrick veneers so they appear as full bricks.


SCChris 06-30-2010 01:39 PM

Re: Heat transfer metrics - under door heat loss
I'll run the board a bit low as a 1 inch break. I also have been toying with the idea of using the board and then facing it with a veneer of firebrick, like fxpose has done. Since this area doesn't get direct flame and as long as the surface is a bit low rather than high I think it'll be fine. As for food, hmmm? With the solid surface in the entry area, I'm still much better as far as cleaning than the firebrick surface.


SCChris 09-09-2010 08:14 AM

Re: Heat transfer metrics - under door heat loss
I now have a 2cm slab of granite in the entry and a break made out of 2 inch rigid insulation board. I profiled the board to create a bench for the granite and still allow a half inch of separation between the backend of the entry slab and the internal brick hearth. Below this bench the board retains the full 2 inch width. The board fits snugly against the oven brickwork in the entryway and the top surface is slightly lower than the granite and hearth floor levels. This slight drop in height allows cookware to slide over the break without actually touching the board material, this is working as intended.

On Sunday I ran a good long fire and cooked a few pizzas. After cooking the last pizza a chocolate chip desert pizza on a 700 degree hearth, I shut the door and was registering 650F on the FB door dial thermometer.

Here are the readings over the following 72hours.

650 450 350 240

The break definitely makes a difference with regard to loss through the entry floor. At 24 and 48 hours, I could feel that the granite entry floor at the outside base of the doorway was warm to the touch. What this tells me that I haven’t shut down the heat transfer as far as I’d like or as I had hoped, but warm is better than hot. The numbers are better with oven heat loss in the 100F drop per day area on the second to third and third to fourth day periods. I’ll look tonight and check the numbers, but I don’t expect much good info since I loaded the oven with wood to dry for the next run. I don’t think the break needs further tinkering and at this point the door’s, less than tight, fit in the doorway rebate is now the weak link and where I think I can make the biggest efficiency advance.

I’ll be tightening up my doorway, with standard oven gasket material, to minimize any airflow in and out of the oven. I think I can find free gasket material at the local appliance store recycled from ovens that have been replaced with new units. This material is stable at the oven's "self cleaning" temperatures of 900F degree or so area. I’ll only be closing the oven with the door after a “completed” pizza run. The oven will have had some time to moderate heat since the pre-pizza big fire stage. I’m betting that the bricks in the top of the arch will be in the 850F or so area and well within the working parameters of the gasket material.

Related to the firewood, I’m burning oak at the moment rather than the almond that I had been able to get. I much prefer the almond over the oak. My feeling is that the bark on the oak tends to produce smoke without much caloric benefit and the almond has a very thin covering of bark. The oak is harvested from what looks like some sort of beetle kill so I don’t feel too bad about using it, but I do prefer the almond.

PS Photos to come later

david s 09-10-2010 04:41 AM

Re: Heat transfer metrics - under door heat loss
I agree about the benefit of a heat break. Advice about insulation being so important with the oven build also tends to ignore the significant amount of heat conducted to the outside, both through the floor and the oven entry. I believe that the entry should also have a heat break. I use 10 mm of vermicrete to insulate the outer arch from the inner parts of the oven. This has an additional benefit of reducing cracking problems of the outer arch from mechanical thermal expansion because of the inner parts expanding and causing pressure against the outer arch, as well as the problem of thermal differential.It also leaves the outer arch cooler to touch because of the reduced conductivity.

SCChris 09-10-2010 09:59 AM

Re: Heat transfer metrics - under door heat loss
One part of the heat curve over time that is interesting is the first 24 hours. In this period the temps really drop. My expectation is that although the oven seems to be saturated it's not. That the oven is clear of soot doesn't really indicate the oven being fully charged. I wish I had temps taken over the period in 15 minute intervals, I'd love to see the curve. I know that the higher the material temps the faster the heat flow is going to be. I wonder how long it would really take to charge the oven to the point where the first 24 hours looses half as much. I also wonder if the oven could be kept in the 200F area with some small heater what that might do for the curve. The 200 to 250F area would sure open up the wood curing and low and slow cooking arena. This last burn, when the temps got down to these levels, I did some spare ribs and a sholder roast, so good. Tonight it's tacos with the roast.


ThermoJax 09-10-2010 04:12 PM

Re: Heat transfer metrics - under door heat loss
early next week I will be going to my firebrick dealer to purchase everything. While I was there last week, I saw that they had a insulating firebrick, very light weight. Kind of like firebrick and styrofoam. would you suggest that I use that for my inner arch, as well as the heat break on the transition between the oven floor and the entryway?

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