Heat retention versus oven size

[UtahBeehiver]

You should post pics of IT, pretty or not, there are some key design factors that affect the IT and now is the time to make sure they are right and not when you start the build.

[NCMan]

If you want it to stay warm for 4 days then you need to design it with that purpose in mind….thick mass, ceramic fibre insulation, low mass gallery, twin doors and a chimney plug should do it…

Can you explain what you mean by a chimney plug and why it would be used? Also, I'm a fan of thermal breaks. If someone really wants to retain heat, in addition to the other suggestions, I'd use them. If heat retention isn't a big deal, I say skip 'em.

[fox]

Chimney plug = anything that stops heat escaping out the chimney.
This is more important if you have a hight mass gallery and only really important if you want to store heat for a long time.

[mongota]

Can you explain what you mean by a chimney plug and why it would be used?

The board insulation under the floor limits heat loss through the floor. Blanket insulation over the dome or a dog house full of loose-fill insulation does the same for limiting loss through the dome.

The dome's opening is now the weak point...add an insulated plug door and now the edges of the door, where the door lip meets the brick, are the weak point. Add a gasket to seal the lip of the door to the brick and you're getting closer. But the weakest path is still heat escaping from around the door and getting to free air through the tunnel/landing. Adding a second door to cover the front landing opening helps, even if it's not insulated. But there is still a path up the flue. Add a plug to prevent air from going up the chimney and that's about the best you can do.

Your landing tunnel is now a closed off "vestibule" of sorts, with the two avenues of escape (the front opening and the chimney flue) closed off.

[NCMan]

The board insulation under the floor limits heat loss through the floor. Blanket insulation over the dome or a dog house full of loose-fill insulation does the same for limiting loss through the dome.

The dome's opening is now the weak point...add an insulated plug door and now the edges of the door, where the door lip meets the brick, are the weak point. Add a gasket to seal the lip of the door to the brick and you're getting closer. But the weakest path is still heat escaping from around the door and getting to free air through the tunnel/landing. Adding a second door to cover the front landing opening helps, even if it's not insulated. But there is still a path up the flue. Add a plug to prevent air from going up the chimney and that's about the best you can do.

Your landing tunnel is now a closed off "vestibule" of sorts, with the two avenues of escape (the front opening and the chimney flue) closed off.

Sorry guys. I still see no need whatsoever for a "chimney plug". Sealing up the doorway is great. Closing off a chimney at that point just makes no sense to me. If you're concerned w/losing heat to a flue gallery, you use thermal breaks and of course, a well sealed door. Now, if a chimney is built over the oven chamber, then it might make a difference, but a chimney shouldn't be there. If someone is that concerned w/heat loss that they use a second door out side the gallery area, and use a plug in the chimney, I sure hope you went to great lengths during your build to retain all the heat, including but not limited to thermal breaks in the floor, around the inner arch and between the arches, if applicable. I do see the use of a second door more as a storm door to keep out weather, but using one to retain heat and then actually plugging up a chimney (?)

[fox]

Like I have already said it depend on the heat loss though the gallery and if the gallery is made from a heat sink martial.
If you can feel the inside of the gallery is still warm one hour after you have fitted the inner door then there will be a continuous heat loss, or if the floor bricks continue out past the inner door you will have a heat sink there too.
From my experience a thermal break has to be a bit more that a piece of string to work effectively so you either seal in the gallery with a outer door and chimney plug or apply a very effective heat break to prevent any of the dome heat traveling into the gallery section.
The door its self can often get very hot on the outside unless it is very well designed.

Of course none of that might not matter to the majority of wood oven owners but, this thread is about heat retention .

[NCMan]

Like I have already said it depend on the heat loss though the gallery and if the gallery is made from a heat sink martial.
If you can feel the inside of the gallery is still warm one hour after you have fitted the inner door then there will be a continuous heat loss, or if the floor bricks continue out past the inner door you will have a heat sink there too.
From my experience a thermal break has to be a bit more that a piece of string to work effectively so you either seal in the gallery with a outer door and chimney plug or apply a very effective heat break to prevent any of the dome heat traveling into the gallery section.
The door its self can often get very hot on the outside unless it is very well designed.

Of course none of that might not matter to the majority of wood oven owners but, this thread is about heat retention .

I'm not sure who uses a "piece of string" for a thermal break, but carry on. I'm trying to have a meaningful discussion about someone using chimney plugs. To each their own, as the saying goes. I'm all for saving all the heat we can. Naturally, if you or someone else feels the need, have at it. My main point still stands, though. If someone feels the need to do so, I really hope they took all the other measures during construction to save every last bit of generated heat, that's all. If not, using a plug is like fartin' in the wind.

[JRPizza]

My steel door was frequently around 90 to 110 degrees in the morning after a fire. I made a plywood outer door that fits snug in my vent arch and kind of butts against the semi-reveal created by the rear of my serpentine opening, so it inhibits free exchange of air from both the oven opening and the chimney. Was not even thinking about blocking off air from above, just wanted the wood door to sit as close to the metal door as possible. The large wooden handles on the inner door project almost to the start of the serpentine vent. Inner door is much hotter with outer door in place frequently sitting around 140 degrees F! Not sure if this would change if I only plugged the opening from the front, but what I have seems to be working well. To the points above, I have a gasket I have not installed in my door yet as I have been too lazy to clean all the creosote off the lip, but all these little things ha vesome impact on heat retention and without a good set of "before and after" data captured at both similar internal and external temperatures it's hard to say exactly what those impacts are.

[fox]

With the last two ovens I have built, I have made the gallery from insulating fire brick.
By doing so the dome can not lose heat through that area and can therefor be sealed with a single door, however before I realised the benefits of a low mass gallery I was building high mass galleries, cast formed from a single casting.

I found that while the high mass gallery was easy to install, even when thermal breaks were fitted like stove rope, they had limited effect and the heat from the dome and floor was still traveling into the gallery walls that were in turn exposed to the air causing considerable heat lose.
So for next oven I built, I used a chimney fitted with a butterfly damper, with the damper closed and an inner and outer door in place I noted a massive improvement in heat retention!.
So I have now told my previous customers with the high mass galleries, that they can improve heat retention by keeping any heat loss from the dome, locked into the gallery by blocking off the chimney exit.
This is very simple if you have an insulated chimney it just needs an insulated cap placed over the top.

So from my own experience I have found that by by using two doors and a sealed, caped, chimney can dramatically enhance how long the dome can store heat.

Obviously if an oven is ‘not’ losing heat through the gallery walls or fire brick floor, then it would be pointless sealing the chimney or using an outer door for heat retention purposes …..
However, many if not most, pre cast or modular pizza ovens don't allow for thermal breaks as the gallery is cast into the design.

I built a very well insulated oven last year with an insulated gallery (featured elsewhere on the forum) even though it has a quite low mass of 275gk it takes four days to go from max temp back to ambient temperature.

[paulkjrobbins]

Russell, I'll try to remember to take a picture of my IT after work today. Here are some calculations I did for determining my raised arch curve. Hope to put together a template tonight.

[UtahBeehiver]

12.5" is right in the range for a 39" ID dome. Rule of thumb is 63-65% of dome height so you are there. Depending on the type of chimney material, ie brick, double wall SS, etc. buttressing of the side walls may be required (tall brick chimney). SS should be fine sans buttressing. To help with the arch bricks on a axed arch I did a quick design using some software I have called the Anglelizer. It is no longer available from General but I think a forum member posted a link to a public domain software archive.

[JRPizza]

I built my 39" oven with a 12.25 inch high opening - a number I arrived at by interpolating the FB plans between their 36 and 42 inch oven recommendations. A 12.25 opening in a 39'er works out to be 62.8% which was close enough to 63 for me Also if you build a hemispherical arch (not what you are showing) the 9.75" radius that gives you a 19.5" opening sits perfectly on a 2.5 inch brick at the bottom of the arch.

[paulkjrobbins]

I can never get myself to use software for math calculations. I use to teach Calculus & Physics & I guess I got used to writing everything out by hand. I use my TI-89 for just about everything.

[paulkjrobbins]

Here are pictures of my IT tool. I just recently realized that I will need to cut off the bottom support so it is the same width as the top so that I can get a good profile to trace when doing the inside parts of my inner arch bricks, but I will trim those down when I get to that point. I was trying to get something that would work with what I had and in the end just had to buy a threaded rod for it. The bottom pivot is a 1.5" caster. I am actually going to do the full 42" oven, my wife wants to have the option of roasting a pig in it in the foreseeable future. I plan on adjusting the IT for each level taking into account the distance from the brick to the IT (1/2" plywood plus 3/4" to pivot of caster) to keep the radius close to 42" for the whole dome. Any advice on this? Would it just be better to dial the IT in and then just keep it at that length except when adjusting to use with a pencil? Thanks for your help.

[UtahBeehiver]

I can't see the detail from the pics you attached. Now that you are doing a 42", this changes your inner arch height, 65% of 21" is 13.65" instead of the 12.5" in the previous post. I see you are also aware of how the vertical offset at the pivot point changes the dome height. You could do like Gulf did and make a wood block the size of the fire brick and low the pivot point down to the floor elevation. Several builders have use Gulf's idea with much success.