New Oven Flue Size

[david s]

Vermicrete or perlcrete is the name given to either of them mixed with cement. The two are interchangeable. Although perlite has a better insulating capacity (lower thermal conductivity) than vermiculite, once you start adding cement to the stuff there's no measurable difference. The grain size of both varies, it is available in different grades, see what you can get. I've found fine vermiculite requires more water and both course perlite and vermiculite more difficult to work with so now always use medium grade perlite and vermiculite. I have found that a better mix is obtained if it contains 50% perlite and 50% vermiculite. Be careful the perlite usually contains some fine dust that's a lung irritant. I simply hold my breath when pouring it. Oven builders use all sorts of ratios, but remember that the more cement you add to the mix, although making it stronger, also reduces its insulating value. Roughly if you double the strength you halve the insulating capacity. I use 10:1 over the dome, (5:1 under the floor) any leaner I find too difficult to apply on to a vertical surface. If I could obtain the silicon coated stuff at a similar price to the uncoated stuff then I'd use it, because removing the water takes time. Regarding render you can use either a cementious or acrylic render. I use a cement based render because I don't really trust the strength of the acrylic stuff, I may be wrong.

[MattP]

Thanks again for your explanation David.
Could you please confirm if 25mm thickness of either vermicrete or pericrete is sufficient over the insulation blanket (2x 25mm).
Also you say you use a cement based render above the vermicrete/pericrete layer, could you please indicate thickness and mix ratio.
Finally, you state you use a 5:1 mix of vermicrete or pericrete or both under the floor, is that a layer under the base tiles (above the CaSi board) ? If so should I also build such layer or is the 50mm thick CaSi sufficient for my application.

[david s]

Yes, 2" of blanket should be enough with some vermicrete over it as well. So the layer of perl/vermicrete over it just provides a firm substrate to render on to. 25mm is more than enough for that. I use a proprietary render (Rockcote PM100) because you can do one thick 15mm layer in one go, rather than three layers with other renders (max thickness 10mm). To reduce cracking and to allow a thicker one application method, I also add AR glass reinforcing fibres randomly mixed in it because it's way faster than chicken wire reinforcing. There are plenty of other solutions though, others may disagree with my method, this is just what I've developed.
You could do vermicrete and cal sil under the floor, but it's usually either or. Vermicrete is cheaper than cal sil, but at 5:1 not as good an insulator so you need to make the layer thicker. So it's usually 100 mm of 5:1 vermicrete or 50 mm ca sil.

[MattP]

Hi David, I have modified the design again to incorporate a granite/bluestone strip at the front and hence have modified the flue gallery. Could please inform if this will work properly.

[UtahBeehiver]

Take a look at your door height to dome radius ratio, typically it should be optimally around .63-.65% of the internal dome radius. Right now it is hovering around 300/425 = 70.6%.It is not a deal killer just something to consider while you are in the planning stages.

As far as the cast portion, you are in good hands with David.

[david s]

Good point Russell, I didn't check that calculation, just thought it looked about right.You would be better to allow the top of the oven mouth follow the radius of the dome, as the idea is to keep the flames inside the dome rather than allowing them to escape too easily. [ATTACH=CONFIG]n390776[/ATTACH]
Matt, I've attached some pics of my flue gallery to explain what I described about its design. Mine is pretty extreme in reducing weight but the photos explain it better than words.

[MattP]

Thanks Russell and David.
I have considered the suggestions made and have reviewed the design. Please refer to Rev G attached.
Interesting flue gallery casting David - certainly lightweight (assume approx. 20mm thick?). Also interesting seeing strengthening ribs on the side. Is it in operation currently? In light of your photos I am now considering casting the flue gallery as a precast and the dome in-situ in combination with the precast gallery. It would make life easier to cast however, the challenge will obviously be the formwork required and the joint at the interface with the dome. What material have you used for formwork to cast the flue gallery David? Relevant photos would be appreciated.
I appreciate your feedback.

[david s]

Yes, that's much better. I've made my mould in fibreglass in three pieces, this is way too much trouble for a one off casting. I suggest you cast the dome first, in situ, then build form work with polystyrene and or sand in front of it, then cast the flue gallery, trowelling the mix against the mould. This should not be difficult after the experience of casting the dome first.

[MattP]

Thanks David.
Interesting information re fibreglass mould.
Forgot to mention in my previous post - in order to achieve the recommended design profile of the dome internal top radius, reducing thermal mass near the entrance, and complying with the recommended door height, the door is now approx. 50mm within the arch floor (795mm as opposed to 850mm).
55mm reduction for the cooking surface (in one direction) is not an issue I am just thinking of workability/functionality of the oven.
Any comments?

[MattP]

Hi David, due to constructability issues related especially to the flue gallery of previous design (tapered/funnel effect) I have reviewed the design again. Could you please refer to attached and please comment on design.
I trust the sectional elevation may make things easier to understand.

[david s]

It will work, but it would be far more efficient if you make the 135 measurement (front view) much larger at the bottom so there's greater volume there and the shape is more funnel like. At least try to eliminate the sharp corner. Polystyrene is easy to shape with a sharp knife if you are not confident to do it in sand.

[MattP]

Hi David, following your advise I undertook to build the flue galley in polystyrene - photos attached including front and door opening timber formwork. I have made the venting recess 40mm high as opposed to 30 in my design. I intend on using a threaded rod to hold the tapered cylindrical recess to aid removal following cast and removal of front arch. The venting recess pieces I can hold but I am having trouble deciding how to remove following the casting - any ideas?
I understand from your previous post 40mm thick castable is quite strong. Is it possible to decrease to say 30mm around the flue gallery/venting recess? Currently I have 50mm vertical at the door and 40mm around the flue gallery and venting recess. I would appreciate your thoughts.

[david s]

Without an outside mould I'd be worrying about trowelling against that mould near the top section without it slumping off. As suggested previously it would be far easier to cast the flue gallery separately. You could lay it on its side with a sand mould for the bottom half. 30 mm is ok, mine is thinner than that, but as you observed I have added buttressing where it needed some more strength.

[MattP]

Thanks David. I had anticipated casting issues with vertical/near vertical faces hence I propose some outside formwork. Also I am planning formwork at the front entrance so in effect it will be like casting a concrete structure with formwork surrounding it for the majority of faces. An open plastic bucket is the outside formwork for the flue gallery in combination with the internal polystyrene. I would prefer to cast the whole oven in situ at once. What are your thoughts. Please refer to attached for explanation.

[david s]

That looks like a workable solution, but it may be a bit tricky loading in the mix between the bucket and the inside polystyrene mould. Use a stick to work the mix into the space a little at a time. I've PM'd you my solution