Need advice on chimney and outer arch construction

[Jayson]

Hi all,

My 43" dome is almost fully enclosed (should be this weekend at least) and now my attention is turning to the outer arch and chimney. I need advice on the best way to approach this.

I have clay bricks cut and ready to build the outer arch, along the same design as the inner arch. This will leave a nice vent between the two arch's and the two arch's should support the chimney ok I am guessing. I notice a lot of builds use firebricks for the outer arch and chimney, then a facade of clay bricks in front for looks. I want to avoid the firebrick outer arch and chimney to reduce the dept of the entrance and I want an exposed clay brick chimney for looks.

Will I have any heat related issues with just using clay bricks for the front arch and chimney instead of firebricks?

Will I have any cracking issues with the mortar between the firebricks and the clay bricks, considering that the chimney will be clay bricks and will sit on top of the inner firebrick arch and an outer clay brick arch. Will the mixing of birck types in a reasonably high heat area pose any issues I should address?

When I lay the clay brick outer arch & chinmey should I use refractory cement or standard mortar. So far the whole dome hase been constructed using refractory cement as mortar.

I read on the fb forum a bit that the transition between firebrick and clay brick seems to be a common area of cracking and I want to minimise the chances of this.

Any advice from those in the know? I have attached a few photos below of the inner arch...

[Karangi Dude]

Hey Jayson,

This is a curly one, I used sandstock bricks for my outer arch and used fire bricks for the chimney built on top of the door and vent arches. I made the mistake of mortaring the outer arch to the vent arch and consequenly after the fisrt big fire the outer arch cracked not badly but enough to disappoint me, the courses above the arch cracked a little as well and they have a gap between them and the chimney. I don't have a pic of the gap but it would open up when the oven was hot and close again as the oven coolded down. I solved the problem by putting a 1200mm length of rebar in a channel that I cut in between the top two courses of the enclosure I cut out the ends of the bricks to the depth of the frog and put the hollowed faces toward each other, put in the rebar and filled with strong mortar. I should have left a 10mm gap between the two arches and filled it with insulation. David S has talked of this happening to a lot of ovens that don't have a thermal break. Fire bricks and the sandstock bricks expand at different rates. In your case it could even be worse because you intend using the clay bricks as part of you chimney.

Cheers Doug

[brickie in oz]

Originally Posted by Jayson

Any advice from those in the know?

A few cracks wont hurt, my biggest concern would be the lack of provision for lateral support for the chimney.

How high is the chimney going to be?

[brickie in oz]

Originally Posted by Karangi Dude

In your case it could even be worse because you intend using the clay bricks as part of you chimney.

Any brickwork that isnt bonded properly will crack, the addition of heat will make it more so no matter what brick you choose.

[david s]

A brick chimney also has a couple of disadvantages. Firstly it is adding a lot of weight onto your arches and secondly there is a problem of heat differential between the inner surface of the chimney bricks and mortar and the outside surface at ambient temp. leading to stresses. This can be overcome by using a lightweight stainless steel flue pipe in the centre of your chimney or the stainless flue on its own. Probably extra expense though.

[SableSprings]

I used firebricks for the inside arch and for the span to the outside arch of regular brick. I used refractory cement for the firebrick portions, but switched to regular mortar for the red brick and subsequent regular brick for the chimney around the flue tiles. Note that I cut two flue tiles at an angle and joined them with refractory cement to make a wider bottom & narrower top (top back to the 8" diameter). The only visible crack on the outer arch is where I messed up and didn't offset the joints of a set of bricks between two courses. It's pretty minor and doesn't seem to expand/contract with heating. I hope the photos give you an idea of how I did this chimney.

Mike

[david s]

Mike,
Great job, to me that chimney looks like it will work really well

[Jayson]

Thanks for the replies, this has helped me make up my mind. I will build the outher arch and chimney entirely out of firebrick then a third arch facade in clay brick. I think I'll leave a small air space and fill with high temp silicone. Any thoughts?

Also when I say chimney, I mean just a few courses to sit the flu on. Not planning on a big one so, should hold up OK. That said, Birckie, your point about lateral support is a good one so I'm reckon I can hide a couple of brackets behind the final facade and cover it with insulation to support the sides of the outer arch. I'll post some pics as it progresses.

Up to the last 2 courses now and getting ready to 'fire up'.

[david s]

Last time I looked at high temp silicone it contained carcinogens so I opted for a weak vermicrete mix instead.

Try to make your decorative arch both wider and higher than the inner one. This will make it better to work the oven when reaching in.

[SableSprings]

The small air gap you'll have between the firebrick and facade will be much more effective if you don't fill it (especially with hi temp silicone). Also on the lateral chimney support, I just put in a couple extra pieces of rebar down the holes in my side chimney bricks and again at an angle on the outside embedded in the insulation cladding (perlcrete for mine). Seemed to tie it all together quite nicely.