Brick Arch entryway problem

[lwood]

The arched entryway to my oven is being pushed off the oven dome by the expanding metal of the flue. I only used half bricks to form the archway and no reinforcement around the flue. I am ready to tear off the archway and re-do it but not sure how to fix it. Should I leave a space btw the archway and the flue to accommodate the expansion, also thinking about adding perlcrete to the outside of the archway. Any thoughts? Will get pics.

[Les]

Re: Brick Arch entryway problem

I need a picture. It's hard to believe that the flue is the cause. The flue should not be attached to the dome - it's sitting on top of the entry. Anyway, pictures help us all to understand what is going on, I'm sure we can solve this.

Les...

[david s]

Re: Brick Arch entryway problem

I had this problem when I built my second oven. I used a stainless steel flue pipe that was about 0.9mm thick. It gets hot first and expands. It cracked the castable refractory surrounding it. I now use thinner material which is 0.55 mm thick and I surround it with the slightly flexible vermicrete. No more problems at the join of the flue to the inner oven or to the outside shell. Take a look at the drawings page on my website.
davespizzaovens.com.au

[lwood]

Re: Brick Arch entryway problem

Here are a couple of pics

[lwood]

Re: Brick Arch entryway problem

I'm certain it's flue expansion, because when the oven is really hot, that gap widens to maybe 1/4" and I can't push it in to close the gap.

[dmun]

Re: Brick Arch entryway problem

If you rebuild that, i would expand the space where the metal flue interacts with the masonry and stuff it with some refractory blanket, for expansion. A quarter of an inch expansion seems like a lot to me, but I don't have time to do the calculation now.

[lwood]

Re: Brick Arch entryway problem

I didn't measure, 1/4" may be an exaggeration, but that's the flue bowing out, not the metal expanding 1/4". That's exactly my thought, leave more space and stuff it with flexible insulation. Thanks, I guess that's a heads up for everyone to use .55 mm stainless for their flue matl.

[Tscarborough]

Re: Brick Arch entryway problem

For the correct construction detail, Google "chimney cap construction detail".

That is certainly your problem, and it is pretty easy to fix without tearing apart the oven.

[david s]

Re: Brick Arch entryway problem

This is my solution. Seems to work pretty well.

[lwood]

Re: Brick Arch entryway problem

david s...so you would put perlcrete extending onto the OD of the arch also? Is that correct?

[david s]

Re: Brick Arch entryway problem

Yes. I make the mixture about 7:1 which gives it some strength, but still some flexibility. You could replace the cement with lime if you're worried about it getting too hot and the portland cement deteriorating.

[lwood]

Re: Brick Arch entryway problem

Thanks David, those are the magic words I've been waiting to hear. I have been contemplating extending the perlcrete over the od of the arch and adding another inch of perlcrete to the entire exterior of the dome.

When the oven is very hot and totally heat soaked, the exterior still is getting well over 120F maybe 150F. Will have to try and measure next time it is hot. This is unexpanded perlite and I am under 10 firings above 700F, so maybe it's still steam. If I do anything, I should do it all at one time and then do some re-curing....right?

[david s]

Re: Brick Arch entryway problem

After applying the perlcrete or vermicrete, allow it to dry for a week, let the sun and wind do much of the work of eliminating moisture. The colour of mixture will lighten considerably as it dries. Then start firing again.

[lwood]

Re: Brick Arch entryway problem

Great thanks david

[altamont]

Re: Brick Arch entryway problem

I might have another idea for you.

The cracking might not have much involvement with the metal ductwork - it might also be caused by thermal expansion causing the arch to push upwards.

My suggestion is to eliminate both concerns. If possible, allow about 1/4 inch clearance between the vertical portion of the metal and the surrounding 'hard' material (brick, mortar, castable, whatever). Then very lightly stuff some ceramic fiber into that gap.

In the refractory (firebrick) industry you would have just created an expansion gap, allowing the various materials to expand with heat and contract on cooling without interfering with each other. The lightly packed ceramic fiber offers a very good way to keep hot flue gases from escaping through the gap.