Steam Vent Idea - comments please

[umabatata]

I Have an idea...
In an igloo WFO it seems that a lot of cracking in the shell is caused by the build up of pressure due to water turning to steam.
People acknowledge that the refractory absorbs water, and that you need to dry the oven out a bit after long periods of not using it.
Others say - cracks are a fact of life, the steam sill find a way out, etc
Others put a vent on the top of the dome.
If there is even a little pressure My thought is that the steam will go to the place of least resistance, so I suggest makign a hole in the chimney cowling giving the steam a place to get out without giving my dome a little top hat. The alternative is to make a similar small vent right in the middle of the back of the dome. I want to avoid the top of the dome as it looks pretty sharp right now.

The steam would have to move through the insulation (ceramic fibre blanket), so some might condense on the way, but would it get out eventually?

Thoughts, comments and experiences appreciated.

[SableSprings]

There have been several threads/builds that have used a moisture venting system. I couldn't find them in the new web site, but they basically involved a small piece of metal tubing or small open ended can that was embedded in the dome's stucco. The vent was always at the apex of the dome...not near the venting area. Most implementations simply involved placing the small tube so that it provided a route for moisture to leave the dome's insulation. The tube was simply bent over a little more than 90 degrees to keep water out (or notched on the sides a little and then another can or pipe end piece placed over it to keep out the rain).

As far as I remember, the intent was to allow moisture to readily escape during curing or after long periods of wet weather without a firing. In all cases, the vent did not allow/keep the insulation dry...it simply aided in moisture release when and if the insulation got wet again (humidity, rain, cracks in the outer cover, etc.). I totally understand not wanting to lose the look of the dome, however if you want to have a steam release for the dome, a small piece of copper tubing on top (or just behind the dome crest) of the dome won't be very noticeable...in fact you might have fun with making a top piece for the dome...the cherry on top so to speak.

Remember that stucco is not a "waterproof forever" material. It takes a fair bit of maintenance. In my humble opinion, if you are in a wet climate the best long term solution to keeping moisture out of your oven's insulation layers is to actually put a roof over the oven.

[umabatata]

Thanks for your comments
I've got another layer of coloured stucco to go on the dome, then might look at a mortar waterproofing sealer product or similar.

I could make a little hat for the dome, but I just wondered if anyone had tried anything else. My logic being that if the cracking often comes from pressure I still wonder if the position of the steam vent matters. A vent in the apex of the dome is the most obvious solution, but I'd be keen to know if anyone has success (ie. little or no cracking) with a steam vent anywhere else

[Gulf]

I believe that DavidS vents his internally back to the flu. And, that may be the best route. I do believe that a vent (somewhere) is needed. But, the vent at the apex doesn't really need a "top hat". (Though, as Mike added, it can be a fun decorative piece made to add to the "looks" of the dome) The threaded receiver can be quite small (3/8ths" to 1/2"). After the initial drying, all that is needed is a threaded plug to seal the hole, water tight, until such time as it may be needed later on. That plug and receiver shouldn't be very noticeable to most.

Edit: I also agree with Mike about a cover for the dome. It not only helps keep the water out. It will help extend the number of days per year that the oven can be used.

[umabatata]

Thanks Gulf.
It is probably too late now, but I had considered a hole in the flue while I was building.
I guess I could drill through into the flue, and mortar over the external hole.

[Gulf]

Thanks Gulf.
It is probably too late now, but I had considered a hole in the flue while I was building.
I guess I could drill through into the flue, and mortar over the external hole.

I hope that DavidS will respond to that. I'm just winging what I think that he did .

[david s]

This is my solution. I make two terra cotta collars that go around the flue pipe. Moisture does not go up the flue but beside and outside the pipe.The bottom one has a larger internal diameter than the flue pipe so moisture in the insulation layer can find its way out. The top one has a hole a few mm larger than the flue pipe and that gap is filed with high temperature silicon. The moisture finds its way out of the porous terracotta and the less than perfect gap between the two collars. I can't see that the vent has to be placed at the top of the dome as steam should find its way out wherever it is, although the top gets hottest fastest so it should probably go in at least the top half of the dome.There is a fair bit of mucking around fabricating with clay because of shrinkage calculations on drying and firing etc. but for a one off something fabricated in stainless would work as well. This system also prevents the render/stucco from cracking around the expanding flue pipe, which is often also a problem area for water entry.

[umabatata]

Very cunning David. Very cunning indeed. I also have a good collar around the chimney like yours, so might try venting out the back of the chimney collar. I intend to drill a few 10mm holes upwards on an angle through the mortar shell so the steam can get out, and the rain cant get in. Although I really like your design - so might have a think about how I might do that....
Thanks for sharing your idea - and good looking pizza ovens by the way.

[cnegrelli]

This is my solution. I make two terra cotta collars that go around the flue pipe. Moisture does not go up the flue but beside and outside the pipe.The bottom one has a larger internal diameter than the flue pipe so moisture in the insulation layer can find its way out. The top one has a hole a few mm larger than the flue pipe and that gap is filed with high temperature silicon. The moisture finds its way out of the porous terracotta and the less than perfect gap between the two collars. I can't see that the vent has to be placed at the top of the dome as steam should find its way out wherever it is, although the top gets hottest fastest so it should probably go in at least the top half of the dome.There is a fair bit of mucking around fabricating with clay because of shrinkage calculations on drying and firing etc. but for a one off something fabricated in stainless would work as well. This system also prevents the render/stucco from cracking around the expanding flue pipe, which is often also a problem area for water entry.

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Are these terra cotta collars of your own making?

[david s]

Yes, by press moulding them. The trick is for the lower one to have some clearance from the flue pipe so it can expand without damaging anything, while the upper one has some high temp siliconbetween it and the pipe.Steam can escape between the two caps because there's always some gap with the less than perfect fit.

[Man Food]

Hi I'm a Wood Oven and forum virgin and have read this thread with interest. Currently I am building a 110cm diameter oven in my back garden - Big learning curve!

I have also thought about the build up of steam in my oven as it will never be completely waterproof, so I have come up with a simple vent made from some 15mm brass plumbing fittings and two 125mm grinder discs. The idea is to embed the whole fitting in the top of the oven forming a sandwich with the cutting discs to make it secure.

Here are are a couple of photos to show you what I am thinking of doing - what do you more experienced oven builders think? Is this a stupid waste of time?

[Man Food]

Some more info - This vent idea is not designed to go through the ovens brick dome into the actual oven, but rather simply vent between the render coat and the C/f insulation layer - will this work to disperse any steam created and help prevent the dome cracking? Lots of self doubt here....

[Gulf]

Imo it will work. However, I would skip the cutoff blades. Same idea to lock it between the threads. But, get a piece of mesh cut in a circle. I used 1/4" hardware cloth. You only need one to embed it securely in the render. Also, you can use pvc to save $$$ .

[Man Food]

Thanks Gulf, that makes sense - I will try to fit it over the next couple of days and post a pic of it in place

[Man Food]

I installed the steam vent tonight - see the attached pics. I hope it works!