#41  
Old 10-27-2009, 02:27 PM
Peasant
 
Join Date: Sep 2008
Location: australia
Posts: 29
Default Re: High Heat Loss

FWIW, I really think demolition and a rebuild would be the best option. I have been using my oven now for about 9 months and everything is fine except that heat retention is a bit weak.

Basically I used the FB design for a home built dome but the only thing I skimped on was the floor - instead of a concrete supporting base, then a layer of vermiculite concrete, then a firebrick hearth, which all seemed to be a bit time consuming (ha!) I used AAC panels as a support for the firebrick hearth. Kills 2 birds with one stone, yes? support and insulation? Well, no, not really. The oven works OK but heat loss is far too rapid for my liking, mostly through the floor rather than the dome. The dome is standard FB style and never even gets warm on the outside.

If I fire it up to pizza heat, then rake the fire out, the temperature seems to drop from 350 C down to about 100 C in only an hour or so. Makes long slow cooking a bit problematic. It is OK but only just. Seriously, if this problem was any more severe I would demolish it and start again.

Already I am planning my NEXT oven!
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  #42  
Old 10-27-2009, 02:57 PM
Mitchamus's Avatar
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Location: Australia
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Default Re: High Heat Loss

I think the problem seems to be to too short a burn time.

You guys are forgetting that the concrete should work as heat storage.
With that amount of thermal mass, once hot, the oven should stay hot for days! or at least 12 hours....

You should try a burn of 6+ hours. In fact, keep a fire ticking over ALL DAY.
and try again.

You could still be driving out moisture from that amount of mass, which will tend to sap heat from the oven.

Or...

The fire bricks used were insulating rather than refractory bricks.
Which is a knock-down & rebuild situation, but at least you wont have to build the stand!


A good way to test would be to take a temperature reading every 30mins after removing the fire. That's the only way to really see how the thermal mass of your oven is performing.


Don't forget that logically thermal mass and insulation do the same thing,
keep the oven hotter for longer.

Thermal mass stores the heat and radiates it back into the oven - keeping the oven hot.

Insulation stops the heat from escaping - keeping the oven hot.

A well insulated, and well thermal massed oven should perform with similar results as far as heat retention in the oven goes.

The only trade off between the two methods (in terms of heat retention) are fuel efficiency!

(and obviously in turn - heat up time)

Last edited by Mitchamus; 10-27-2009 at 03:04 PM.
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  #43  
Old 10-27-2009, 06:44 PM
Peasant
 
Join Date: Sep 2008
Location: australia
Posts: 29
Default Re: High Heat Loss

Yes, must do a continual measurement to see just how fast the heat goes. Firing time could be a bit of an issue - mostly I fire it up and allow about 3 hours which seems to be plenty; have tried a bigger fire for an hour and a half and no, that was NOT long enough. The surfaces were hot but the heat had not diffused sufficiently into the rest of the structure. So - maybe even 3 hours is not enough either.

One of the best results with our oven was an occasion when I was away and our 25 year old son had a fire in there big enough to smelt steel; he rang me in a panic because he thought he might have broken something; and it was hours before the fire died down enough to get near the oven to cook a pizza. It worked perfectly! and nothing broke! Maybe I should learn from that.
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  #44  
Old 05-03-2010, 10:21 AM
cjt cjt is offline
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Join Date: Feb 2010
Location: west yorkshire
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Default Re: High Heat Loss

I have a similar problem although not quite as bad as yours. I can get the temp up and cook quite a few pizzas. My oven floor is firebrick sat on top of 2" of castable insulation under that is a 4" conrete slab with re bar i am finding that when the oven is hot the underside of the slab gets up to 80 C I have been advised to keep on cooking to get rid of moisture but it doesnt seem to be getting any better. is there any harm in me lining under the slab with a sheet of kingspan to keep the heat in ? that way i would be creating a heat store I know it takes longer to heat but once there if held ??? Any advice any one on this similar problem
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  #45  
Old 05-06-2010, 10:35 AM
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Location: nc
Posts: 2
Default Re: High Heat Loss

i also have high heat loss in the floor and im just finishing up....how devistating....(first build) anyways had a local pizza oven guy just stop by and offer some advice. He suggests building forms from 2x6 and making a vermiculite slab in 4 sections. letting them cure for a week sliding them in the wood storage area and sticking them up with firerock mortor. hows that sound.. i guess it couldn't hurt.............
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  #46  
Old 05-24-2010, 11:13 AM
mateo.scoggins's Avatar
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Join Date: Dec 2009
Location: Austin, TX
Posts: 3
Default Re: High Heat Loss

Hi all:

This thread is old, but hopefully someone out there is still listening. I have what appears to be a heat loss problem as well. It might be a curing problem, or maybe some other design flaw that I don't understand. Anyway, when I fire the oven, I can get it up to temperature (500+), but have not had a really nice clearing yet (where the black all turns white and falls away) and have pretty fast heat loss (down to 300 within an hour, 200 within 2 hours). This is a "modified" Pompeii, with 4" firebrick hearth, a 19" dome, 30" diameter floor, and 10"x16" door. On top of the brick dome, I have one inch of mortar, followed by a mix of vermiculate for the bottom half of the dome and 4" of insulfrax over the top. Then stucco over the whole thing. I haven't insulated the floor yet, which is a 3" concrete slab, with the firebricks sitting on top of that (see photo). When I fire the oven, the outside top of the dome gets barely warm, and the bottom of the slab gets warm/hot, but not more than 50C or so. The area around the flue gets quite hot, but I don't know that this is avoidable, right? I have a metal/wood door with 2" of insulfrax, which I use during firing (cracked, to allow air in) and after pulling out the fire (cutting off the flue). I have only fired this thing hot 4 times, and had rain in between, so it could just be that it is too exposed to the weather and is not drying out ever. I also might need to fire longer or hotter or both. Any other thoughts, other than getting that floor insulated?

Thanks for any input y'all might have.

Mateo
Austin, TX
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  #47  
Old 05-24-2010, 01:18 PM
Il Pizzaiolo
 
Join Date: Nov 2006
Location: San Antonio
Posts: 1,719
Default Re: High Heat Loss

Hi Mateo!

Your temperatures are confusing. If Centrigrade they might be okay but you indicate your dome isn't clearing which implies you aren't reaching 750 F which means you aren't even coming close to getting your oven hot. And, it is impossible to meaningfully comment if you don't give us an idea how long you burned.

I am willing to guess your oven is somewhat damp, but it also sounds like you aren't firing your oven hard enough or long enough. But tell us more and we can give you more meaningful comments!
Jay
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  #48  
Old 05-24-2010, 03:50 PM
Mitchamus's Avatar
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Join Date: Aug 2008
Location: Australia
Posts: 369
Default Re: High Heat Loss

Mateo -
keep a fire going in it ALL DAY.

just a couple of logs every hour or so, no need to create Mt St Helens in there - just keep it burning nicely.

this will accomplish a couple of things:


1) Drive out the remaining moisture, which I suspect is what is keeping your temps down.

2) Get your oven to definitely hit 750!

I suspect that you're not loosing heat - you're just not storing enough of it to begin with!
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Last edited by Mitchamus; 05-24-2010 at 03:52 PM.
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  #49  
Old 05-25-2010, 02:55 AM
Il Pizzaiolo
 
Join Date: Mar 2009
Location: USA
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Default Re: High Heat Loss

Quote:
I haven't insulated the floor yet, which is a 3" concrete slab, with the firebricks sitting on top of that
Hey Mateo,,

Sounds like your concrete slab is sucking all the heat out the bottom.. Does the bottom of the slab get hot ?? Usually there is up to 4 inches of insulation between the firebrick floor and the slab..

Mark
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  #50  
Old 05-25-2010, 04:30 AM
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Location: philippines
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Default Re: High Heat Loss

I agree Mark, that slab is sucking out the heat.
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