High Heat Loss

[mateo.scoggins]

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

[texassourdough]

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

[Mitchamus]

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!

[ThisOldGarageNJ]

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

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

[lwood]

Re: High Heat Loss

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

[Mitchamus]

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.

[lwood]

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.

[Mitchamus]

Re: High Heat Loss

"....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.

[lwood]

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.

[ThisOldGarageNJ]

Re: High Heat Loss

Mitch

thermal mass and insulation do exactly the same thing.

not trying to be annoying,, but seriously,,(I consider it) 2 different things,,,

Thermal mass, also called thermal capacitance or heat capacity) is the capacity of a body to store heat

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

[Archena]

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.

[Mitchamus]

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.

[shuboyje]

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

[Mitchamus]

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.

[lwood]

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.