Second broken Flue Tile

[christo]

the original flue tile used as a chimney in my oven cracked the entire length along its back (closest face to center of oven).

Figured it was a fluke and replaced it.

I am day 5 of drying fires and noticed that the replacement flue tile has cracked almost identically. I have not placed a big fire in the oven yet. So far internal air temperature of oven has peaked at 350 F using an oven probe. No radical fires yet.....

The side of the tile that cracked is closest the oven so it would see highest heat.

The tile was bought from the same supply house, 3 months apart so it's not likely the same tile run - but could be.

Any thoughts here? Bad batch of flue tiles? Heat as it comes out of the oven creates too much differntial and expansion causes the crack?

I will probably try a different suppliers flue tile, but have a sneaking suspicion that the next one will crack as well....

As always, thanks for the input!

Christo

[CanuckJim]

Re: Second broken Flue Tile

Christo,

A question that might have something to do with it. Is the tile in direct contact with flame or did you build up a few bricks first, then sit the tile on them. You might have gotten two defective tiles, quite possible, but maybe the tiles are not rated for direct exposure to flame. Just a thought.

Jim

[christo]

Re: Second broken Flue Tile

Good question.

The flue tile is isolated by a brick arch with a transistion piece made similar to one that Dmun Dave suggested.

It's a good distance from direct flame.

Attached is a pic of what it looks like. the upper tile is the one cracking consistently.

One thing that is odd but still plausible is that the crack showed up in the same oritentaiton relative to the oven. If it is a bad run of flue tiles, I had a 25 percent chance of getting it right so it's still within reason.

My signature pic has the interior lights turned on - the fire is that little thing in the back....

thanks

[CanuckJim]

Re: Second broken Flue Tile

Christo,

Now I see. I can't find any reason the first tile would have cracked, let alone the second. Sorry I can't be more helpful.

Jim

[wlively]

Re: Second broken Flue Tile

I would tend agree with you that two cracks in the same place is no fluke. The most of the heat coming off the dome travels up the interior side of the flue opeing (ask the burned hair on my arm how I know ). Combine that with that side also being in contact or close proximity to the dome and you have a very high temp gradient across it. That seems the most likely explanation.

So then, what to do? I think you have to either abondon using the tile in that spot or replace it with something else. If you still want to use the tile you could try replacing the verticle transition with something built out of refractory bricks or KS-4. That may give you enough mass to help balance the temps out, especially if it is at least 14" or so tall. You could also try using mortar to lay a few verticle lines or beads in a slightly twisted line on the inside walls of your vent. These may help induce a turbulence into the flue to help evenly distibrute or mix the heated air.

I wish I could give better answers, but I used KS-4 and duravent so no direct experience.

[Pannabecker]

Re: Second broken Flue Tile

After reading this thread I am concerned about my terra cotta flu and the future of it once I enclose it. Today I cut fire bricks to use as my transition between vent and flu. I plan to mortar the bricks together with refractory mortar, then mortar the flu to the bricks. My chimney will be 3 2 foot sections of clay flu because it has to be higher than my 10 foot high pergola. The sections are so heavy and I'm concerned about the weight. I'm also trying to figure out a way to enclose it with the space allowance around it. It's so close to the front of my oven and doesn't allow for a gap around the flu and then block to enclose it. I tried to attach pictures but I'm having a hard time figuring out how to do so. Once I place the flu on the fire bricks there is only 2 inches of space to the front wall if the enclosure, not enough space for the 1/2 inch air space and a brick or block. Any ideas?

Do I attach the sections of flu liner with refractory mortar only or it there some kind of heat resistant tape that holds them together? I suppose if it's all level and plumb it shouldn't go anywhere but it still concerns me that just a single layer of mortar is holding these big sections together.

Karen

[Dave M]

Re: Second broken Flue Tile

I think I read someplce recently that clay flue liners are not supposed to be attached to their walls but free floating to allow for the large amount of thermal expansion. I wonder if thast also means that it should not be mortared to the angled flue tiles beneath it or the arch? The mortar could be holding it tight when it needs to move. Just a thought.

[dmun]

Re: Second broken Flue Tile

After reading this thread I am concerned about my terra cotta flu and the future of it once I enclose it. Today I cut fire bricks to use as my transition between vent and flu. I plan to mortar the bricks together with refractory mortar, then mortar the flu to the bricks. My chimney will be 3 2 foot sections of clay flu because it has to be higher than my 10 foot high pergola. The sections are so heavy and I'm concerned about the weight. I'm also trying to figure out a way to enclose it with the space allowance around it. It's so close to the front of my oven and doesn't allow for a gap around the flu and then block to enclose it. I tried to attach pictures but I'm having a hard time figuring out how to do so. Once I place the flu on the fire bricks there is only 2 inches of space to the front wall if the enclosure, not enough space for the 1/2 inch air space and a brick or block. Any ideas?

Do I attach the sections of flu liner with refractory mortar only or it there some kind of heat resistant tape that holds them together? I suppose if it's all level and plumb it shouldn't go anywhere but it still concerns me that just a single layer of mortar is holding these big sections together.

Karen

I'm fighting the same thing too. It seems strange that these huge heavy segments just float free inside the chimney with just a bit of mortar between them. The 8 by 12's that I'm using for the fireplace are hard to even pick up. A hint: a pair of C-clamps on the top of the flue tile, clamped to a pair of plywood shims gives you a handle to pick it up and place it.

If you need more space in the front of your oven, you tilt the flue back. Unless you are trying to create space to a wooden structure like I am, you shouldn't have to tilt it much.

[Pannabecker]

Re: Second broken Flue Tile

What would happen if the flue tile was too close to one side of the chimney, or even touching? I guess, now I know why people light fires in their ovens before enclosing them in block or brick. Karen

[dmun]

Re: Second broken Flue Tile

I think masons mortar the flue tile in place, then build the masonry structure around the flue, maintaining the desired clearance.

[Dave M]

Re: Second broken Flue Tile

This is slightly off topic, but related to clay flue construction: When installing a clay flue can you just wrap it in insurfrax type insulation then apply stucco directly to the insulation surface (wrapped in wire of course)? Or does there need to be an actual air gap around the flue pipe?

[Pannabecker]

Re: Second broken Flue Tile

From looking at the many pictures posted on this website it looks like many people wrap their flues it the blanket insulation. I'm wondering, if the oven is freestanding, nowhere near a house, does there need to be any insulation at all between the flue and brick chimney, or just the air space. Karen

[maver]

Re: Second broken Flue Tile

Code allows for an air gap as the only insulation in the chimney run with a brick surround - this is ok inside or out. Mine is an outdoor, and it's built with flu tile, an airgap, then a brick surround. Inside you have to be careful about clearance to combustibles, but the brick surround would usually provide the clearance you need.

Marc

[Dave M]

Re: Second broken Flue Tile

What do you do if you're doing an igloo style enclosure and plan to stucco up the flue? I was thinking I could use 1" insulation blanket wrapped in chicken wire and apply about 1/2" - 3/4" thick stucco layer over that.

[Dutchoven]

Re: Second broken Flue Tile

I didn't get to the last message of the thread...boy I hate that ...answer a question that has already been answered. I might suggest using a larger diameter flue tile if you're using clay and set that outside of the first with the insulfrax in between.
Best
Dutch

Dmun is correct as far as the many that I have had the chance to look at. The space is to allow the flue liner to expand and contract within the outer shell of masonry, as well as to allow it to be heated separate from the external masonry and that helps the draft. Sometimes the space between is filled with loose ceramic fiber or some other higher heat insulating material. To the best of my knowledge the pieces are connected to one another and there usually are codes governing their construction in earthquake zones.
Best
Dutch