Indoor oven on a gym ball

[crolande]

Hello all you oven constructors!

My plan is to build an oven 80 cm diameter by using this excellent forum recipe, home brew. As the build is inside I have made a wooden construction for the oven to be placed on. A lot of questions appears along the way and one particular is the heat from the oven and downwards to the wooden stand. We don't want to see the house end up in fire.

The idea is to use a 50 mm CaSi bord and probably 50 mm of perlcrete or vermicrete under sand and refractory stones. Would that be sufficient insulation downwards? What do you think about the floor temperature by the way? Too cold?

Looking forward to your ideas.

I have to apologize for my spelling and sentence construction as I am just a retired swede.

[UtahBeehiver]

Looks at threads by David S, he is our resident cast expert. As far as insulation that should be sufficient for min. temperature issues but the layers should be as follows, oven floor, sand/clay mix for leveling only using as little thickness as possible, CaSi board (post specs before you buy, there are all types out there), 5 to 1 p/v crete is the recommended ratio.

[crolande]

What's your opinon about my questions David S?

[david s]

Provided you have adequate insulation under the cooking floor the heat won’t worry a timber stand. But be very careful because ovens are generally heavier than builders allow for. A timber stand should ave diagonal bracing included in its construction. With two layers of 40 mm calcium silicate board (or the equivalent) the bottom of the insulation will only get mildly warm , so wouldn’t worry the timber. Coals spilt out of the oven mouth would be another story.

[crolande]

Provided you have adequate insulation under the cooking floor the heat won’t worry a timber stand. But be very careful because ovens are generally heavier than builders allow for. A timber stand should ave diagonal bracing included in its construction. With two layers of 40 mm calcium silicate board (or the equivalent) the bottom of the insulation will only get mildly warm , so wouldn’t worry the timber. Coals spilt out of the oven mouth would be another story.

Thank you David S for your ideas about the timber construction and above all, the presumed heat.

Since much less is written about ovens made indoors, it sometimes feels like walking on thin ice. There are so many extra parameters to consider, the Swedish legislation on indoor fireplaces not least, phew! The reason for my decision to build indoors anyway is solely the Swedish climate. We have winter a large part of the year and keeping it indoors makes it possible to use it even in winter.

Here are my construction of the timber stand: 90 x 50 mm late-grown northern pine built with 30 mm distance between the beams and then diagonally slanted supports, I think the construction should hold for an oven of say 300 - 350 kg. But I have not done any calculations. On top of that 15 mm plywood followed by 50 mm CaSi board. Then 50 mm percrete.

Comments are welcome!

[fox]

I would replace or add to the plywood with cement board (hardyboard) and use two layers of ceramic fibre board.
Or make a wooden frame and cover it with all cement board.
You could use precrete or similar, it wont have the same insulating properties as two layers of ceramic board but it will stop any airborne fibres floating around your indoor space!
Cement board is a standard way to cover wood near a heat source.
Any form of ceramic fibre should be encased especially if used inside a home as the fibers can be dangerous to your health!

[crolande]

I would replace or add to the plywood with cement board (hardyboard) and use two layers of ceramic fibre board.
Or make a wooden frame and cover it with all cement board.
You could use precrete or similar, it wont have the same insulating properties as two layers of ceramic board but it will stop any airborne fibres floating around your indoor space!
Cement board is a standard way to cover wood near a heat source.
Any form of ceramic fibre should be encased especially if used inside a home as the fibers can be dangerous to your health!

Thank you fox for mentioning the idea of fibre cement. It has crossed my mind as I am well aware of its characteristics. In Sweden we have something called Cembrit in various thickness. Cembrit also provides extra strength as it can interact with the plywood slab. Tomorrow I will purchase Cembrit although it's fairly expensive.

[david s]

Thank you David S for your ideas about the timber construction and above all, the presumed heat.

Since much less is written about ovens made indoors, it sometimes feels like walking on thin ice. There are so many extra parameters to consider, the Swedish legislation on indoor fireplaces not least, phew! The reason for my decision to build indoors anyway is solely the Swedish climate. We have winter a large part of the year and keeping it indoors makes it possible to use it even in winter.

Here are my construction of the timber stand: 90 x 50 mm late-grown northern pine built with 30 mm distance between the beams and then diagonally slanted supports, I think the construction should hold for an oven of say 300 - 350 kg. But I have not done any calculations. On top of that 15 mm plywood followed by 50 mm CaSi board. Then 50 mm percrete.

Comments are welcome!

As you've already built your stand you'll have to go with that, but masonry is the preferred and recommended material because it doesn't rot, shrink, expand, burn or rust. It's kind of there forever. The next option is steel, but that of course has issues with corrosion. In an environment that includes both heat and moisture, corrosion is accelerated. Timber is subject expansion and contraction, rottting, burning and insect attack. I presume you have a good reason for choosing the timber option, presumably because it's over a timber floor. If so make doubly sure that there is sufficient strength supporting the floor underneath. Ovens are nearly always heavier than builders estimate.
On plywood, I have done two repairs on ovens that were built on plywood over a steel base. In both cases the plywood (although called marine ply) had pretty much turned to mush. This was because, firstly both ovens were outdoors, but also accumulated moisture in the floor and under floor insulation kept the plywood both moist and hot. This condition accelerates deterioration. The remedy was to drill up a number of holes through the rotting ply so moisture could escape. Also in both cases cracks had developed which allowed water to enter into the insulation layers, particularly around the flue to outer shell join. This moisture finds its way by gravity to accumulate in the under floor insulation. In both cases the repairs were effective and the ovens working performance returned. Nothing could be done to reverse the rotting ply short of a total tear down. I don't want to scare you and your oven is indoors so these problems largely don't apply, but are worthy of consideration.

[crolande]

Looks at threads by David S, he is our resident cast expert. As far as insulation that should be sufficient for min. temperature issues but the layers should be as follows, oven floor, sand/clay mix for leveling only using as little thickness as possible, CaSi board (post specs before you buy, there are all types out there), 5 to 1 p/v crete is the recommended ratio.

Thanks UtahBeehiver, I will consider to take your advice and follow it. But just for the sake of doing things right: When you mention sand/clay mix, do you mean home brew mix then?

[david s]

Take a look at the cast ovens section on this forum, there are a number of well documented builds that have used the homebrew as a dense castable mix. Because it's dense it is strong, but also heavy, The gym ball requires casting over it then lifting it to relocate. That will be extremely difficult for your 80cm internal diameter dome. A far better method is to use a sand form and cast in situ, removing the sand when the cast is complete.Most cast ovens have walls of 50-80 mm thickness. 50mm is strong enough and provides sufficient all round performance and heat storage with good fuel economy. If you want it stronger and more thermal mass, go thicker.

[crolande]

Yes David S, it might be a problem. it would be interesting to know how much the 5 cm thick "dome" of the Home Brew weighs.

[crolande]

The weight of the dome Home brew-made 5 cm thick can be calculated approximately. The volume of a dome can be calculated by the formula V=4πr³/3. But since half the ball is uninteresting in the context of weight we have to divide the resulting value by two.

First dome: Diameter 80 gives us the value =0,27 m³. Second dome (with 5 cm Home brew) diameter 90 value = 0,38 m³ . Don't forget to divide these values by two to get the volume of half the sphere, that is in the first case 0,135 m³ and the second 0,190 m³ .

If we then subtract the second value from the first we get the total volume of the 5 cm Home Brew 0,055 m³ . So what about weight then. In this context, we have to start from the weight of concrete because this calculation is highly approximate. The weight of concrete is 2400 kg/m³ with no reinforcement.

That gives us the approximate weight of the 5 cm Home Brew around a 80 cm gym boll: 2400 x 0,055= 132 kg. The opening of the oven is not included. But I would gess around 40 kg. That would give a very approximate total weight of 172 kg.

Should be possible to handle on four guys.

[Mahone]

Hi colande,
Good luck with your oven. The homebrew works well. I have cast it over a gym ball and moved them into place. It works well, you can jiggle the brew against the ball, and get a pretty void free cast. Moving them is not easy, lol. This is pretty much what you are proposing, 32 inch, roughly 2.5 inch cast. It would be a challenge for 4 guys to move, it's big, awkward, and heavy, for everyone to keep their hand on it and lift, not pull, is difficult, but possible. We had 6 guys, 5 lifting most of the time, had to carry it 300ft from my shed to the truck, and sat it on a couple 2x4s so nobody got their fingers pinched. I would recommend casting it as close as possible to your stand, in a garage that you can back a truck up to is alot easier. Carrying it through a house would definitely be a challenge.

[david s]

The difficulty you describe about moving a heavy casting is one reason I advocate using a sand mould and casting the dome in situ. There is also the difficulty in sealing the base of the dome. The other main problem is that you only require the top half of the gym ball, so fairly complex formwork is required to set only the top half for the casting lay up. Digging a hole in the ground , placing the ball in it with a plywood base set halfway up the dome is probably easier than building a frame. The sand mould method also allows departure from a pure hemisphere. Many cast domes have vertical sides near the base and flatter domes. This is very easy to achieve with a sand form.

[crolande]

I am in no way questioning the thesis you put forward David S and the weight will probably be an issue. But now it's all set for casting on the ball and I'll see how I move forward taking your undoubtedly perfectly correct points into account. Another possibility is of course to cast let's say half way up let it harden then move to the final location and by means of sand or other form complete the casting.