#51  
Old 05-25-2010, 03:49 PM
Mitchamus's Avatar
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Default Re: High Heat Loss

I don't agree.

I have an oven with no insulation under the floor, and it easily hits over the 500 degrees C mark.

It no like the concrete slab is some kind of super conductor that will stop the oven getting hot.

It will just take a little longer to heat up - it's just extra thermal mass.

you just have to fire it properly.

honestly, the way the need for insulation is drummed into people here, has seriously effected their ability to think logically.

Advising a guy to rip down and re-build his oven just because their is no insulation under the floor? - seriously?

I bet the guy who invented pizza didn't have Perlcrete under his oven floor.

YES - you should insulate.
NO - it's not the end of the world if you don't - what you will have is great oven with a high thermal mass. You just have to be prepared to fire it for longer.

ok rant over... sorry.
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Last edited by Mitchamus; 05-25-2010 at 03:59 PM.
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  #52  
Old 05-25-2010, 06:11 PM
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Default Re: High Heat Loss

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

I got the impression he had plans to insulate the floor. Yes, It's not the end of the world. But it is a big source of heat loss. Definitely not the reason why he can't reach 500C. Moisture, correct firing, and dry wood among other things should be corrected. However, maintaining temp will be another story.
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  #53  
Old 05-25-2010, 06:24 PM
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Default Re: High Heat Loss

Quote:
Originally Posted by lwood View Post
"....But it is a big source of heat loss.....
If fired correctly it is "a big source of heat"

Please don't forget that the only difference between using insulation and using thermal mass is the amount of wood that you use.

thermal mass and insulation do exactly the same thing. - keep the oven hot for longer.

The only differences are the firing time and the wood usage.

You're not losing heat - you are storing it.
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  #54  
Old 05-25-2010, 06:42 PM
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Default Re: High Heat Loss

Then why insulate?

It holds the heat better (for the same mass), more efficiency, less wood, faster heat-up. We insulate to optimize the oven, but it's not a requirement. If the temperature of the bottom of the concrete slab is very hot, then you are loosing heat out of that surface.

If the outside temp of your oven is not warm then you are containing the heat and the oven will maintain temp longer. Given a slab with our without insulation, the insulated one will always maintain temp longer.
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  #55  
Old 05-25-2010, 07:00 PM
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Default Re: High Heat Loss

Mitch
Quote:
thermal mass and insulation do exactly the same thing.
not trying to be annoying,, but seriously,,(I consider it) 2 different things,,,
Quote:
Thermal mass, also called thermal capacitance or heat capacity) is the capacity of a body to store heat
Quote:
insulation refers to materials used to reduce the rate of heat transfer
The more insulation you have,, the more heat your thermal mass can retain.. I do agree however he is probably still burning off a lot of moisture... and if insulated the slab will act as thermal mass as you are not storing that much of it now as it is radiating out the bottom...

Cheers
Mark
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  #56  
Old 05-25-2010, 07:01 PM
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Default Re: High Heat Loss

You're both correct. Insulation doesn't store heat - it contains heat. The oven stays at a higher temp longer because the heat can't easily escape.

BUT the reason a slab causes 'heat loss' is because it is in fact storing the extra heat. The release is no faster than in the oven but because there is no insulation to contain it the heat dissipates faster through the bottom of the slab. That high mass also takes longer to bring to high temp and will even out much faster resulting in a slow, steady heat loss at much lower temps. This is why masonry ovens work so well.

So, if you want higher temps maintained you want more insulation. If you want lower temps released you want more mass.
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  #57  
Old 05-25-2010, 07:24 PM
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Default Re: High Heat Loss

Mark - my post (that I deleted) was for lwood - not you.. sorry bout that.

you must have just sneaked your reply in before I had finished mine.

mus remember to quote next time.
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Last edited by Mitchamus; 05-25-2010 at 07:29 PM.
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  #58  
Old 05-25-2010, 08:01 PM
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Default Re: High Heat Loss

It seems to be getting missed that you are firing with your door on and cracked. That is a major part of you problem. Put the door away until the oven is hot and the coals are gone you fire needs to breath. How much thermal mass do you have in your dome? The 7 inches in your floor is pretty extreme and couple with no insulation will take time to get hot.

A point to everone thermal mass without insulation will radiate heat in all directions. The insulation simply keeps it where you want it. An uninsulated oven IS loosing heat fir that very reaso no matter how much mass it has THAT is logic
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  #59  
Old 05-25-2010, 08:15 PM
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Default Re: High Heat Loss

I regard to keeping an oven hot, insulation & thermal mass do the same thing.

If you took an insulated and thermal mass oven side by side and fired them both for about 6 hours:

Both of the ovens internal temps would be about the same for the next 24-48hrs.

This is what I am getting at.

yes it radiates heat..... blah blah blah whatever
- at the end of the day the internal oven temps will be about the same.

getting back to my original point
- a thermal mas oven needs to be fired properly to work as intended.
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  #60  
Old 05-25-2010, 08:46 PM
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Default Re: High Heat Loss

shuboyje - I think you may have it. If you are trying to heat the oven up with the door on, that is your problem. Build a roaring fire without the door and you should be good.
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