Heat retention versus oven size

[paulkjrobbins]

Hello Everyone,
I am getting ready to lay my dome floor for my 42" pompei and am questioning going down to a 39". We want our oven to retain heat for as many days after firing. I know this depends on a lot of different factors. I have read a lot of posts regarding heat-up time for different oven sizes. But my question is this, Is there a difference in cool down time? If two ovens have the exact same volume and kind of insulation would you expect them to stay hot the same amounts regardless of interior diameter? Thanks for any help!

[Petter]

The total wall thickness (mass + insulation) and insulation efficiency is what matter. Not the oven size.

[david s]

The total wall thickness (mass + insulation) and insulation efficiency is what matter. Not the oven size.

There are other important factors other than wall thickness and insulation. As a large amount of heat loss occurs from the oven mouth and door seal, no matter how well the oven is insulated its heat loss here is considerable. The smaller the oven, generally the higher the heat loss around the mouth because the mouth area is a higher proportion of the ovens inner surface area. The mass of the flue gallery is also a consideration as it acts as a heat sink and is cooler and not as well insulated as the oven chamber.
Generally, just as larger ICE engines are more efficient than smaller ones (a 6 litre motor doesn't use 6x the fuel of a 1 litre) so the larger the oven the more fuel efficient, relative to size, it becomes.

[UtahBeehiver]

A 42" oven is a large "party" oven. I built one but in hindsight I would have built a smaller one. With just the two of use, it is not efficient for me to fire the oven up for a pizza or two. So it depends on how you envision using the oven, how many people you typically will cook for, etc. David S ovens are on the opposite end of the size spectrum yet he can cook for a decent size party. Larger ovens consume more wood. I see you are from Provo, that is where I was born.

[david s]

Yes, as an oven’s fuel consumption is roughly proportional to the chamber volume a large oventakes a lot of fuel to heat. Because of the maths of volume a 10% increase in diameter means a 33% increase in volume and that also means a similar increase in materials and labour as well as weight which then requires a stronger stand. If you double the diameter the volume is increased by more than 10x. So there is a huge advantage in keeping it small. The downside is that you have less bragging rights “ mine is bigger than yours”

[paulkjrobbins]

Thank you for your advice everyone. I think I am going to go down to a 39". We will be using this for my family mostly (6 kids) but also want to have parties and other families over for fun as well. I don't see us having more than two pies in the oven at once no matter what we are doing with it. I think our primary concern is trying to get as close to a 3-4 day cook window from one firing while not going crazy with firewood. Sounds like decreasing the size won't affect that as much as my insulation layers. Here is what I have after firebrick:

Hearth: 5" 5/1 vericrete +2" FB
Dome: 3" FB in Gabled House Enclosure filled with vermiculite (≈ 4")
Door: 4" FB enclosed in hardiboard then finished with grout with a steel plate on inside and 3/4" oak on the outside (≈ 6")
Flue Gallery: 1/4" heatstop all the way around stuffed with boiler rope separating dome & gallery

Not sure about how to cover the rope channel in the floor heatstop to avoid catching on it. I might just put a little piece of 1/8" steel with rounded corners across it that I notch into the floor firebrick.

Russel, that's awesome you were born in Provo. My wife and I started our family here right before I graduated and now I am back teaching at the Y.

I really appreciate being able to get on here and benefit from all of your experience. Does anyone have any advice before I start laying brick? Thanks in advance.

[mongota]

I have a 42", the most I've had in it at one time is three pizzas. But for just pizzas, it's larger than I need. When cooking multiple pies, I usually cycle two pies through at a time. We do a fair amount of meal cooking as well, and for that it's right-sized. We can have several pots in at once, or a tuscan grill on one side with a half-sheet pan or a couple of pots on the other side. Or baking bread when we batch bake.

I could live with a 39" oven just fine.

I have 4" of board insulation under the oven floor and 4" of blanket over the dome. We've done several 3-day cooks but only a few 4-day cooks. As has been mentioned, minimize conductive loss through the door and gasket the door to minimize convective loss around the door. My oven has a "stone dome" exterior, a vermiculite-filled house might offer even better thermal performance.

[UtahBeehiver]

Just to make sure, you will need a concrete structural hearth underneath the 5 to 1 vcrete layer. It is also advise you place a few weep holes in the structural layer. Some builders place some 1/2" pvc with the ends duct taped over just below the finish structural surface then when cured use a piece of rebar and drive up from the bottom to knock out the thin layer of concrete. Silicon some screen on the bottom side to keep the bugs out.

You should post your design and layout so the members can give you feedback. A lot easier to change on paper!

[fox]

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…

[paulkjrobbins]

Yes, I already poured my hearth slab 2 weeks ago. 5" thick 4000 psi with 1/2" rebar 11" o.c with a center 8"x8" inch cinderblock pedestal in the middle. I didn't put in weepholes but I am not sure that I need them. If I'm thinking about this correctly, the moisture would be between my slab and vermicrete layer, Since I missed that tip, I figure I can always drill up from underneath later if needed. My CFB is seperated from my slab by 5" of vermicrete. Do you think that I will get water penetration through the vermicrete? I'm going to extend the gabled roof 24" past the block walls to give me more firewood storage on three sides and leave a large overhang in the front so I am not so concerned with moisture penetration and I can't see water percolating through to that extent. Is there a lot of steam penetration? From what I've read, these ovens cook really humid.
I'll draw up what I have this weekend and post it before I do anything else too permanent. I'm going to weld my IT tool together today. Hoping I can have this up and done before Halloween.
Best, Paul

[UtahBeehiver]

You are good. I forgot you are doing an enclosure and not an open dome. FYI, since you are considering dry filling the structure you can fill the corners with scrap dry wall or old plastic pop bottles to take up the corner volume. Also an access door may be good since the perlite or vermiculite could settle. Consider SS instead of carbon steel for the door, the K value is much lower 14 vs 54 respectively or in laymen terms heat will transfer 4,5 times less with SS vs Carbon Steer.

[MarkJerling]

Thank you for your advice everyone. I think I am going to go down to a 39". We will be using this for my family mostly (6 kids) but also want to have parties and other families over for fun as well. I don't see us having more than two pies in the oven at once no matter what we are doing with it. I think our primary concern is trying to get as close to a 3-4 day cook window from one firing while not going crazy with firewood. Sounds like decreasing the size won't affect that as much as my insulation layers. Here is what I have after firebrick:

Hearth: 5" 5/1 vericrete +2" FB
Dome: 3" FB in Gabled House Enclosure filled with vermiculite (≈ 4")
Door: 4" FB enclosed in hardiboard then finished with grout with a steel plate on inside and 3/4" oak on the outside (≈ 6")
Flue Gallery: 1/4" heatstop all the way around stuffed with boiler rope separating dome & gallery

Not sure about how to cover the rope channel in the floor heatstop to avoid catching on it. I might just put a little piece of 1/8" steel with rounded corners across it that I notch into the floor firebrick.

Russel, that's awesome you were born in Provo. My wife and I started our family here right before I graduated and now I am back teaching at the Y.

I really appreciate being able to get on here and benefit from all of your experience. Does anyone have any advice before I start laying brick? Thanks in advance.

Welcome! If you'd like to have good retained heat, then there's two bits of advice: You need thermal mass (so bigger is better) and you need to insulate, insulate, insulate. Feel free to look at my temperature graphs.

[paulkjrobbins]

Has anyone laid their floor bricks on end? ie. 4" thick instead of 2" thick. Does anyone have any input as to whether this is worth the extra cost?

[david s]

Yes plenty of builders, usually those who intend baking a lot of bread, have laid their bricks on the side to increase the thickness, thermal mass and more heat retention.Not sure about placing them on end, that might be overdoing it. The usual floor thickness is about 2" and has proved a reasonable compromise between adequate thermal mass and reasonable heat up time, but it really depends on what type of cooking you'll be doing mostly. As a general rule for our temperature range, heat penetrates dense brick or refractory at about one hour/inch of thickness and as the "higher the temperature, the greater the heat loss" it will take longer at pizza temperature. So if you don't mind using a lot more fuel and having to wait longer to reach temperature, then make it thicker, but the floor thickness should be much the same as the oven wall thickness for balanced cooking.

[paulkjrobbins]

Thanks David, I think I am going to lay them down flat. Vacation and work have got in the way of me doing much for a couple of weeks. Just welded together an IT from scraps I had around. it doesn't look pretty but it will do the job.