How to get brick oven HOT?

[edjc]

I built an outside dome brick oven: Bricks, Insulation, Cement vermiculite insulation layer on the top.

48 " inside round.

Smoke Vents out the front door.

My questions:
Should I build then start my fire near the door?

How long might I expect to burn the fire in order to get it pizza oven hot?

I'm in cold, snowy New England

[boerwarrior]

Re: How to get brick oven HOT?

yes, start the fire near the front door and then push it back when you have good heat.

Is your oven cured? You didn't mention insulation in the floor.

If your oven is cured and well insulated all the way round, you should expect to be able to get it really hot in 1-2 hours. It takes me exactly 75 minutes to get my oven to temperature (for pizza) when it is 40 degrees out. But my oven is only 32 inches.

[edjc]

Re: How to get brick oven HOT?

Cooking surface is fire brick

Beneath that is base: 3" of cement and vermiculite base

[stonecutter]

Re: How to get brick oven HOT?

I built an outside dome brick oven: Bricks, Insulation, Cement vermiculite insulation layer on the top.

48 " inside round.

Smoke Vents out the front door.

My questions:
Should I build then start my fire near the door?

How long might I expect to burn the fire in order to get it pizza oven hot?

I'm in cold, snowy New England

If your oven is dried, are you using dry wood?

What is your flue size? How tall?

How wide is the throat? An undersized throat is arguably the biggest reason smoke bypasses the vent area and comes out of the outer opening.

[david s]

Re: How to get brick oven HOT?

All ovens will tend to be smokey when they're new and wet too.

[edjc]

Re: How to get brick oven HOT?

No chimney in my design

My oven is 20 days in of 29 day cure time

My wood dry split hardwoods

How long might I expect to have to pre burn my fire in order to heat oven up for pizza cooking

[david s]

Re: How to get brick oven HOT?

You don't say how thick your floor or walls are, but if you have had fires going for 20 days it should be dry. Most ovens will start burning the carbon off the roof of the oven after about one hour at which point you should have the dome filled with flames. After around an hour and a half the whole dome should have gone white. If cooking more than a few pizzas keep firing for around two hours. If it takes longer than this then your oven may still be a bit wet or your walls are really thick.

[wotavidone]

Re: How to get brick oven HOT?

How wide is the throat? An undersized throat is arguably the biggest reason smoke bypasses the vent area and comes out of the outer opening.

I have tried to draw this out in paint. For the purposes of drafting, smoke can be considered a liquid, which has velocity as it come out of the oven. It needs to be turned to go up the flue, and it won't do a sharp right angled turn.
The flue is a compromise, we try to make it fairly big, but we don't want to have to far to reach into the oven.
The layout on the left in my crude drawing is what most of us build, wide across the oven, and not very deep.
It doesn't leave much distance for the smoke to make the turn, especially when we have the most smoke and the least draft, at the beginning before things heat up, and we ask the smoke to spread laterally.
I reckon the layout on the right would work better because it would leave more length for the smoke to make the turn, but it would mean we have to reach a long way to get into the oven.
I note though, that the oven being discussed here vents out of the front and doesn't have a chimney, so I guess my ramblings are irrelevant.
My oven started as just a dome with an opening in it. I had a stainless steel hood perched above the opening to funnel the smoke away. The theory was good in still air, but in practice there almost never a completely wind free day here. I ended up building an arch in front of the oven. This made a big difference to heat up times. With the new arch, I was also able to use what most people call a blast door. This excludes the wind, while letting combustion air in the bottom cut out. End result, when it's windy I get a much faster, smoke free heat up. Not a lot of difference on a still day.

[david s]

Re: How to get brick oven HOT?

Yes, it works really well. Too well in fact. The resulting temperature rise from ambient to 400C in less than an hour is the outcome. It would be the equivalent of firing a kiln with the burner on full from the beginning. The problem is that the rapid temperature rise creates uneven expansion which can lead to cracking. If you want to be kind to your refractory don't use a "blast door" below around 250 C or at around 400 C.+ These are both temp ranges where damage is likely to occur from rapid temp rise? If you don't really mind cracks in the oven then ignore this advice, your oven won't fall to bits. I prefer to take it slow and steady and allow the refractory to adjust itself in its own time. My mobile oven has quite a few cracks because when I hire it out people decide to see how hot they can get the thermometer and how much flame they can get up the chimney, while my oven at home (same design) is in pristine condition.

[wotavidone]

Re: How to get brick oven HOT?

Yes, it can be scarily quick. I address this in much the way you have mentioned. I only made the door because I needed a draft excluder, not as a way of accelerating the burn, and don't put it on until the initial thermal shock is over.
The outer door has a way of directing the air straight onto the burning timber, hence the moniker "blast door" and things get bloody hot bloody quick. I can't limit the air much, since I don't have any fancy adjustable vent on the door, so I limit the fuel in the oven once the door is on. I really must come up with some adjustable set up.
Until I made this door, I had an oven that would heat to white in less than an hour on a still day, and struggle to get to temperature at all on a windy day, even though I took care to build it with the door facing away from the prevailing wind. The outer arch was a big improvement on its own, though.

When your oven is exposed to the elements, especially when the air is below freezing, (I simply cannot imagine what living in that sort of climate would be like) a door like this would help a lot to retain heat and exclude the cold winds.
Of course you need an outer arch and a chimney before you can use it.

[mirassou]

Re: How to get brick oven HOT?

If you have wood that isn't cured, it'll seem like it takes forever. Kindling+cured wood gets my oven up to temps within an hour.

[stonecutter]

Re: How to get brick oven HOT?

It needs to be turned to go up the flue, and it won't do a sharp right angled turn.

If smoke does not travel up the throat into the flue even when cold, then in most cases, the throat is incorrectly sized.

The thermal dynamics in an oven, even a cold one at the beginning of a fire, force the gas and smoke up into the throat and flue. As another example, masonry heaters have multiple right angles built in them, in series...and they still draw when they are cold.

The flue is a compromise, we try to make it fairly big, but we don't want to have to far to reach into the oven.

You can't really over flue an outdoor oven, unless you have a very deep, cavernous vent area.

......... wide across the oven, and not very deep........we ask the smoke to spread laterally.

That is where most throats go wrong...they are not wide enough along the width of the oven opening. See below....


I reckon the layout on the right would work better because it would leave more length for the smoke to make the turn, but it would mean we have to reach a long way to get into the oven.
\
That would work but the problem, as noted, is that it creates a large vent area. And that will create new problems such as less visibility, limited accessibility, and draw issues even with the large throat like in your drawing on the right, if the flue is undersized.

I don't see the issue of smoke roll-out being cased by the throat width from front to back, but with the throat width across the oven opening - left to right. From the measurements of my current and past builds, the 60%-64% ratio for opening height seems to apply to the throat width (across the opening) too. My current oven has a 17.5" opening and my throat is 10.75" across...close to 62%

I don't see any reason to be conservative when building the opening for the throat, because any heat which exits the oven opening arch is lost, and has no benefit outside of assisting draw.

[wotavidone]

Re: How to get brick oven HOT?

I don't see the issue of smoke roll-out being cased by the throat width from front to back, but with the throat width across the oven opening - left to right.

We've hijacked this thread a bit. We aren't now talking about "how to get a brick oven HOT."

I should have made it clear I am thinking in terms of semicircular arch type oven openings.
I think, if you are talking about an oven opening that is rectangular, or one of those openings that has straight, buttressed sides and a shallow arch across the top, then yes, a wide opening is best.
However, for an oven with a semicircular arch type opening, like I have, it isn't IMO.

My point is the combustion gases are flowing at some speed as they come out of the oven and the semicircular arch channels the gases into a fairly concentrated stream in the centre which is moving fairly quickly. For that sort of flow, openings that are wide (from left to right) and narrow (from front to back) are not the best way to go, from a gas-flow point of view.

From a purely gas flow point of view, you need adequate width of course, but depth from front to back matters.
The fast moving, channelled gas flow has mass and momentum and needs a little distance to turn. Of course, the deeper you make it, the deeper your whole set-up is, and the further you have to reach in.
Hence my statement that it's a compromise, we have to make it big enough to do the job, small enough we can still reach everything easily. I meant to make that statement about the throat, by the way, not the flue.

Anyway, there is a top limit in flue size for natural (unassisted) drafting.
I don't think anyone's found that top limit yet, it's probably a lot bigger than any size that is practical to build.

I do agree that any heat that has come out of an oven has no other purpose than to help the flue gases rise up the flue.

BTW isn't this different to a masonry heater? I always thought that the protruding masonry bits in the chimney that make the hot gases turn on their way up the chimney, I don't know what they are called, were there to ensure that the hot gases contacted as much masonry as possible, so that less of your heat from your fuel is lost up the chimney?

[stonecutter]

Re: How to get brick oven HOT?

We've hijacked this thread a bit. We aren't now talking about "how to get a brick oven HOT."Well you are right this is a hijacking . Hopefully the OP doesn't mind a bit of extra material,after all, it does have to do with oven building.

I should have made it clear I am thinking in terms of semicircular arch type oven openings.
I think, if you are talking about an oven opening that is rectangular, or one of those openings that has straight, buttressed sides and a shallow arch across the top, then yes, a wide opening is best.
However, for an oven with a semicircular arch type opening, like I have, it isn't IMO.


No, actually, I am talking about all types of opening arches. In fact most of the freestanding ovens and beehives I built had either semi circular or a modified equilateral opening. There were only two which had segmental arches.

All things being equal, I don't see how a slight variance in an arch radius would cause any marked difference in the amount of exhaust leaving the oven. A properly sized throat and flue draw like a vacuum, even before the oven gas heats the flue. Seeing all of them in action ( along with building many,many fireplaces) is how my opinion was formed on this matter ( venting ).


My point is the combustion gases are flowing at some speed as they come out of the oven and the semicircular arch channels the gases into a fairly concentrated stream in the centre which is moving fairly quickly. For that sort of flow, openings that are wide (from left to right) and narrow (from front to back) are not the best way to go, from a gas-flow point of view.

From a purely gas flow point of view, you need adequate width of course, but depth from front to back matters.
The fast moving, channelled gas flow has mass and momentum and needs a little distance to turn. Of course, the deeper you make it, the deeper your whole set-up is, and the further you have to reach in.

The vents that I have seen that can't contain smoke usually have a throat with a short width span relative to the width front to back. My point is that you want to scale the width to the depth, while considering the oven dimension. I get what you are saying about a narrow opening and it is logical.


The reason I am not agreeing with a large boxy throat design is because it is not necessary to successfully vent all the exhaust into the flue....which makes it impractical to build. It wouldn't hurt anything by doing it, if it made a builder feel more secure about containing the exhaust, so that it doesn't stain the front of the oven.

I do agree that any heat that has come out of an oven has no other purpose than to help the flue gases rise up the flue.

BTW isn't this different to a masonry heater? I always thought that the protruding masonry bits in the chimney that make the hot gases turn on their way up the chimney, I don't know what they are called, were there to ensure that the hot gases contacted as much masonry as possible, so that less of your heat from your fuel is lost up the chimney?

The point of bringing up a masonry heater was to illustrate that smoke and combustion gases can and will vent out of properly built right angle throats and flues.

[david s]

Re: How to get brick oven HOT?

In my kiln I pull the flame not just sideways, but down.It travels up over two feet then back down again to exit the flue. Downdraft kilns are the most efficient and produce the most consistent temps in the chamber.

My view on the entry for a WFO is that it is of great benefit to make it as shallow as possible reducing the reach into the oven proper. To prevent smoke exiting the front and staining the arch you can compensate for this by making the flue larger. The tricky part is then designing an entry that is only as deep as the diameter of the flue pipe. This is far easier if the entry gallery is cast. It's a bit like a vacuum cleaner head that is wide but not deep to feed into a pipe.