Stuck Between two mindsets and need so opinions...

[shuboyje]

I've got a dilemma on my hands and I just can't decided which way to go.

All this relates to the oven you see here:
http://www.pizzamaking.com/forum/index.php/topic,13681.0.html

That thread basically ends last fall when the cold weather set in and I could go no further with the oven's finish.  Over the winter and spring I ran into an issue with the oven I quickly realized would need to be fixed before I could move on.  The stucco finish you see there that was to be the base coat for the tile cracked like crazy, and it was very obviously due to thermal expansion.  The spongy layer of vermiculite concrete was not absorbing the thermal expansion like I thought it would.  After much deliberation I removed the stucco and insulation layer and wrapped the entire oven in 1.5" of encapsulated ceramic fiber to account for thermal expansion and it is not time to put the insulating concrete back on and proceed with the finish.  This all builds up to my dilemma.

I built the oven with a Neapolitan vent, and have now used it as such for about 9 months.  The vent works and draws well, but I have not seen any "magic" from it.  Mind you I had an oven before this with a standard vent so I have a baseline to compare to.  Although I've recently said I'm keeping the Neapolitan vent, I am now seriously considering getting rid of it.  This is my last chance to get the oven the way I want it before I close it up for good.  I'm torn between these two mindsets:

Building a more efficient modernized Neapolitan oven
My original plan was to build a modernized version of the classic Neapolitan pizza oven.  I used castable refractory in place of their cast iron and terra cotta, and I used more efficient modern insulation in place of their natural tuff.  Sticking with this mindset removing the vent is a no brainer.  I would replace it with a standard vent fabricated from stainless steel.  I plan to finish the oven with black glass tile and the black and stainless would look great, but would not look traditional which kinda matters to me.  I will not route the stainless back to the center over the insulation, for me form and function should go hand in hand and that form would have no function other then aesthetics.  I know this would lead to a better oven due to better insulation.   

Building an oven with traditional features and appearance
I love the tradition of the classic hand built Neapolitan ovens.  I want my oven to be very obviously inspired by that.  By taking away the Neapolitan vent I lose one very key attribute to those ovens.  If I leave the vent the oven will look much like the pictures at the end of my build thread, which I think looks very much like traditional Neapolitan ovens with a bit of my own flair.  I would really like to keep this key feature and look, but it may bother my for years to come that I could have improved the oven by giving them up.

Who has an opinion?     

[TXCraig1]

I love the way your oven looked in the last pictures. I would not change it. I don't think you lose enough heat through the top into the exhaust to be meaningful. I certainly don't think it negatively effects the baking performance. Perhaps if you plan to do a lot of baking with residual heat - but if not, I wouldn't even consider rerouting the flue.

[scott123]

Jeff, I need some clarification.  If I had your cad skills, I think we could cover ground faster, but let's see where we can get with verbal descriptions.

A while back, you had spoken about Neapolitan vents being uninsulated.  When you say they are uninsulated, are you referring to lack of insulation above the inner ceiling bricks or are you talking about lack of insulation above the top of the vent?  Isn't the entire top of of the oven (vent area + non vent area) covered with some insulation?  Because the vent area is quite a bit higher, I'm guessing it probably doesn't get as much insulation above it as the area surrounding it, but doesn't it get some?

[Jet_deck]

What about a vent "scoop" on the front that leads only back to the dome?  It should draw since the scoop inlet (at the front of the oven)  and exit (back into the dome) are 6 - 8 - 10 inches different in height.  Maybe the best (looking) of two worlds.

[TXCraig1]

When the oven is running, wouldn't the flue gas above the dome be hotter than the bottom of the flue (directly above the dome)? If so, wouldn't heat be going from the flue into the dome rather than vice versa? If so, isn't insulating that portion of the dome counterproductive?

[Jet_deck]

When the oven is running, wouldn't the flue gas above the dome be hotter than the bottom of the flue (directly above the dome)? If so, wouldn't heat be going from the flue into the dome rather than vice versa? If so, isn't insulating that portion of the dome counterproductive?

If your asking me a question, you either need to rephrase it or draw a picture; otherwise i have no idea wt... your talking about.
In either case it was just a .02 suggestion.

[TXCraig1]

If your asking me a question, you either need to rephrase it or draw a picture; otherwise i have no idea wt... your talking about.
In either case it was just a .02 suggestion.

It was rhetorical.

The gas in the flue flows over the dome right as it exits the oven. I don't know what the temperature is, but I'm guessing it is quite a bit hotter than the bricks right below it. If this is the case, heat will flow from the flue gas into the dome - not the other direction. Moving the flue so that it sends the exhaust straight up may be counterproductive as the heat that was going into the bricks would be lost. Is this not correct?

[Tscarborough]

I do not think that when running the squirrel tail style operates at a loss, but a plus.  After it is shut down you will lose more heat than a direct shot chimney, but not much.  Go with your gut, obviously you prefer the Neapolitan, stick with it.

If you have a picture of the cracked stucco, I can tell quite a bit about what caused them from where they are, how they run, and the size.  One thing you have to remember about stucco is that it cracks, period.  The secret is to convince it to crack in a planned fashion so that you can control it and make allowances for it.

[scott123]

The gas in the flue flows over the dome right as it exits the oven. I don't know what the temperature is, but I'm guessing it is quite a bit hotter than the bricks right below it. If this is the case, heat will flow from the flue gas into the dome - not the other direction. Moving the flue so that it sends the exhaust straight up may be counterproductive as the heat that was going into the bricks would be lost. Is this not correct?

Craig, the flue gas travels out the door and then up the flue, so, by the time it reaches the area above the oven, it's going to be a bit cooler than the dome bricks, and thus draw heat.  Because the gas is moving, the convection will increase that slight cooling effect. This being said, I don't really foresee the flue channel being that much cooler, especially if it's insulated above it.

The looping Neapolitan vent has to be giving the Italians something or they wouldn't be doing it- and I'm sure it's more than just cosmetics for them.  I think we've confirmed that it gives them better fuel efficiency, but, at the same time, if it did cost them in some area, such as peak temp, I don't think they'd pay that price for fuel efficiency- and I've never seen a Neapolitan oven with peak temp issues.  I also don't see residual heat issues in Neapolitan ovens either.  I don't know, perhaps there's a slight hit in residual heat with this kind of vent, but I don't think it's enough to do away with it.

Lastly, there could easily be an unknown facet of the thermodynamics that we're overlooking here.  I don't worship the ground that Stefano, Mario and Gianni walk on, but, at the same time, unless we have cold hard data revealing the inferiority of the Neo vent, I think we should give them the benefit of the doubt.

[TXCraig1]

Craig, the flue gas travels out the door and then up the flue, so, by the time it reaches the area above the oven, it's going to be a bit cooler than the dome bricks,

I don't know. We're talking about a few tenths of a second. It's not going to cool much, and an oven that is not used every day is not going to be all that hot up there.

[Tscarborough]

The flue gases are generally hotter than any surface temp in the oven at any time.

[shuboyje]

Wow, I posted this then left, figuring I'd maybe get a response if I was lucky, lol. 

Let me try to lump a lot of answers in one here.  I do not think the Neapolitan vent has any negative effect on the oven when it is running full blast kicking out sub 60 second pizzas.  Like Craig stated and I have stated recently elsewhere the flue gases will be hot enough to functionally insulate the dome.  My concern is when the fire within the oven is not full bore.  When I cook New York style for instance I use a bed of coals only.  I guarantee my hot flue will draw more air then the coals are producing, meaning ambient air will be drawn in in a larger quantity.  At that point I am convection cooling my oven's dome from above.  Same goes for retaining heat(which isn't my biggest interest, but worth mentioning).  The heat in the dome will cause the air in the Neapolitan vent to heat.  That heated air will rise and lead to draw again convection cooling my oven. 

It's a one trick pony, and I'm not convinced it really adds anything to that trick.  I've posted recently why I think the Neapolitans use them here:

http://www.pizzamaking.com/forum/index.php/topic,20117.msg199524.html#msg199524

[Tscarborough]

You get the same effect in a straight flue (convection of cool air into the oven), and retained heat aside, it can't be enough to matter.

[shuboyje]

I don't know. We're talking about a few tenths of a second. It's not going to cool much, and an oven that is not used every day is not going to be all that hot up there.

The flue in a black oven doesn't differentiate between combustion gases from the oven chamber and ambient air.  It will draw in ambient air and mix it with the combustion gases.  This will cool the flue gases, but how much nobody knows.

[TXCraig1]

Notwithstanding, I don't think the heat loss through the dome into the flue is material if at all.

CL

[Tscarborough]

Going by my observations and even the video you posted, the air really does not mix that much.  Air from the door enters the oven and being cool stays low, while the superheated flue gases exit the upper part of the oven.  Cool air and warm air have no real incentive to mix.  They will stay laminar and flow along lines of dis-similar temperatures rather than across them.

[scott123]

Jeff, if you're concerned about the flue drawing air when baking with a bed of coals, add a damper to the chimney. Depending on how tall you are and how tall the chimney is, this could be as simple as covering the chimney with a clay tile, or, if the chimney can't be comfortably reached, you can add an exhaust flapper with a metal plate and a hinge that can opened and closed with a pole. You can drill a hole in the plate so it still vents a little bit. You could go whole hog with an adjustable damper, but I don't think you need to spend that much money.  All you need is two settings- completely unobstructed for Neo bakes, and almost completely obstructed for NY.

[scott123]

Even if ambient air and flue gas don't mix (personally, I think they do), the lower position of the door will create a cooler exhaust gas than the superheated gas in the top of the dome. Take a look a the diagram below (courtesy Jeff).  The gas exhausting out the door isn't going to be hotter than the gas in the dome.

Also, as the gas flows through the vent, the top of the vent will be hotter, while the bottom, where the bricks that are a part of the dome are located, will be exposed to cooler gas.

Exhaust gas dropping to exit door - drop in temp
Exposure to ambient air/possible mixing - drop in temp
Differential between top of vent and bottom - drop in temp

In an active oven, for any dome brick in the vent area, the bottom of the brick (inside the dome) will always be hotter than the top of that same brick. Always. I will put any amount of money on it.  Because the top of the brick will be cooler, heat will be lost.  I agree with everyone that the loss should be marginal, but the concept that the vent area might somehow be hotter than the dome is off the mark.

[shuboyje]

Thanks for all the conversation guys, it's just what I needed to get back off the edge, lol.  The vent is staying.

I like your damper idea Scott, somehow I never thought about that.  If I have issues down the line I will go that route.