My build, New Zealand.

[david s]

Yes, I wondered about that. Strangely though, it draws really well. I suspect that, even though the throat is on the small side, the height and size of the flue compensates for the slight undersize in throat.
Will fire again on Friday (tomorrow by us) so more temperature tests, this time with the door.

Yes the flue diameter and height is what creates the power of the draw and as long as the throat is no less than about 2/3 of the cross sectional area of the flue pipe, it will simply act as a Venturi at the restriction . Observe the smoke at that point and you should see it accelerating there. All good.

[MarkJerling]

unfortunately stainless has a propensity to warp badly under heat, particularly if heated unevenly as occurs with an internal facing for a door.The edges hiding behind the door rebate staying considerably cooler than the middle facing the hot oven. The thinner the stainless sheet used the greater the warping. The warping is likely to result in sealing of the door against the oven mouth. If it is screwed to plywood you may also get problems with excess heat transfer to the plywood.

Thank you. Only the "cold" side is screwed to the outside ply, so I don't think we should see any heat transfer to the ply. But, will let you know tomorrow when I test it! As to warping, the inner face may bulge but the rest of the box should be stiff enough to hold things relatively straight. But, we'll know tomorrow night. Fingers crossed!

[david s]

In further news, the door construction is almost complete. I need to fit some seals and I need to fit handles. The basic door construction is a 45mm thick steel "box" (with galvanized steel for the side facing the ply, as that's what I had lying about, but 316 stainless facing the inside of the oven) which is screwed to a 32mm ply face. Inside the steel box I have a two layer insulation system: Furnace blanket and 32mm thick furnace insulation bricks.

The bricks are interesting. As they are secondhand, I'm not sure what they're made from, but a test with a piece of the stuff, heating one side to red hot with an oxyacetylene torch, showed that the other side stayed cold, so I feel
that, combined with the furnace blanket it should work well for door insulation. Also, there was no off-gassing in the test, so I'm pretty confident that the door should not impart any strange flavours to the food!

unfortunately stainless has a propensity to warp badly under heat, particularly if heated unevenly as occurs with an internal facing for a door.The edges hiding behind the door rebate staying considerably cooler than the middle facing the hot oven. The thinner the stainless sheet used the greater the warping. The warping is likely to result in sealing of the door against the oven mouth. If it is screwed to plywood you may also get problems with excess heat transfer to the plywood.

[MarkJerling]

The comparison of flue requirements for a cross draft system, as used in a front glued oven, with that of an updraft system as used in a standard fireplace, is unsound. This is because a decent draw is required to pull the flame sideways and slightly down before exiting. An 8” x 2.7m high flue
is pretty big and would do for a 45” ID oven.

Yes, I wondered about that. Strangely though, it draws really well. I suspect that, even though the throat is on the small side, the height and size of the flue compensates for the slight undersize in throat.
Will fire again on Friday (tomorrow by us) so more temperature tests, this time with the door.

[david s]

The comparison of flue requirements for a cross draft system, as used in a front flued oven, with that of an updraft system as used in a standard fireplace, is unsound. This is because a decent draw is required to pull the flame sideways and slightly down before exiting. An 8” x 2.7m high flue
is pretty big and would do for a 45” ID oven.

[MarkJerling]

In further news, the door construction is almost complete. I need to fit some seals and I need to fit handles. The basic door construction is a 45mm thick steel "box" (with galvanized steel for the side facing the ply, as that's what I had lying about, but 316 stainless facing the inside of the oven) which is screwed to a 32mm ply face. Inside the steel box I have a two layer insulation system: Furnace blanket and 32mm thick furnace insulation bricks.

The bricks are interesting. As they are secondhand, I'm not sure what they're made from, but a test with a piece of the stuff, heating one side to red hot with an oxyacetylene torch, showed that the other side stayed cold, so I feel that, combined with the furnace blanket it should work well for door insulation. Also, there was no off-gassing in the test, so I'm pretty confident that the door should not impart any strange flavours to the food!

[MarkJerling]

Here's a few photos of the throat to the flue. I see there's much talk about a flared flue throat but I have instead decided to use the old English method of a relatively narrow throat and smoke shelf design.
As we know that a 200mm dia. (8") flue is more than big enough for a 510mm (20") wide x 317mm (12.5") tall fireplace opening, all that remained was to "guess" the throat size. Somewhere, I have an ancient British construction book which gives the relationship of fireplace opening to throat to flue size but I could not find that so I had to "guess" based on what looked right. I recall that 100mm was considered more than enough for a throat width, so I settled on a throat of 300mm x 95mm (12" x 3.75"). Had I not "guessed", I would have made it 317mm x 100mm (12.5" x 4") or the full width of 510x75mm but really, I was pretty close, especially considering the height of the flue (2.7m / 9').

As can be seen from the flames, when I'm not overloading the fire (which does cause some smoke to come out of the door opening) then the flue draws beautifully and there's no smoke out the door. The trick is to start with a small fire and to build it up gradually to a medium size fire. Large fires are not needed, but longer burn times are, as that heats the fire bricks to depth.

[MarkJerling]

Your correlation between floor and dome space temperature is excellent. Good stuff. I hope to emulate.

Thank you! Door made, I just need handles and then I can test how the temperature profile goes with a closed door. (After cooking pizza, of course)

[KISS4me]

Your correlation between floor and dome space temperature is excellent. Good stuff. I hope to emulate.

[MarkJerling]

Here's the heat profile, also posted elsewhere:

[MarkJerling]

Hi Mark

A great build and I like the simplicity. I am far from an expert, but I don't reckon the cracks are much to worry about. Good heat retention by the sounds of it too. Impressive.
A few questions about the bricks used for oven floor:

1. 42mm or standard brick thick?
2. Cut with an angle grinder?
3. Pizza bottoms browning nicely?

Steve

Hi Steve

Thank you. Cracks have not got worse, so all good!
To answer your queries:
1. The floor bricks are all very large nightstore heater bricks, roughly 300x240mm (12x9.5") and 45mm (1.75") thick.
2. Cut with a diamond bladed angle grinder to fit in between the walls, yes.
3. Yes, floor temperature seems to be good.

Kind regards,
Mark

[KISS4me]

Hi Mark

A great build and I like the simplicity. I am far from an expert, but I don't reckon the cracks are much to worry about. Good heat retention by the sounds of it too. Impressive.
A few questions about the bricks used for oven floor:

1. 42mm or standard brick thick?
2. Cut with an angle grinder?
3. Pizza bottoms browning nicely?

Steve

[MarkJerling]

Pizzas have come out great, by the way!

[MarkJerling]

My lower courses are tidy enough, but at the top where I worked over a former, I have quite untidy mortar. I've been working from the inside tidying that up, but, in doing so, I have noticed three cracks. Need I be concerned about these?

[MarkJerling]

Tools made, so we're having pizza tonight!
I still need to make an insulated door for the oven, then we can test it with bread etc. For now, it's pizza only.