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...

[brickie in oz]

Re: Need advice on chimney and outer arch construction

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]

Re: Need advice on chimney and outer arch construction

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]

Re: Need advice on chimney and outer arch construction

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]

Re: Need advice on chimney and outer arch construction

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]

Re: Need advice on chimney and outer arch construction

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

[Jayson]

Re: Need advice on chimney and outer arch construction

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]

Re: Need advice on chimney and outer arch construction

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]

Re: Need advice on chimney and outer arch construction

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.

[Jayson]

Re: Need advice on chimney and outer arch construction

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.

Thanks david s appreciate your point about the high temp silicone. I like your idea of the outer arch being wider. I have angle cut the bricks for the side arch so that the arch opening as already wider at the front. The clay bricks I am going to use for the decorative outer arch are curved on one corner which will make it wider again. I'll post some pics when I'm done.

[Jayson]

Re: Need advice on chimney and outer arch construction

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.

Thanks Mike, I'll be sure to add some side support. Love the Dragons Den by the way!!

[jeckysmith]

Re: Need advice on chimney and outer arch construction

I am going to build a new home and i joined this community to get advice about chimney and outer arch construction. Please help me, advices are appreciated.

[Massimilliano]

Re: Need advice on chimney and outer arch construction

Hi guys
I have read of using clay pipe chimneys. Are these standard pipes or are they made of refractory clay?
Has anyone ever made their own using clay? I thing I might give it a go, using a PVC pipe as a form, however, I have a concern that I might not be able to get the pipe off the form.
Does anyone have an idea of how thick the clay pipe wall should be?
Grazie in anticipo

[SableSprings]

Re: Need advice on chimney and outer arch construction

I built my chimney with brick around a flue liner. The liner is available in Oregon (USA) in various diameters (both rectangular & circular) and about 3/4" to 1" (20-25 mm) thick. Sections tend to be about 1' (30-32 cm) long. The material appears to be a finer grade, fired clay that looks a lot like regular clay brick. Although you could make a clay tube to act as a flue, I think you'd need to fire it for the needed strength & durability. (If I was going to try making enough for my chimney, I'd cut a short length of PVC lengthwise and insert some plywood circles to hold the shape of the original. Then after the clay was formed around the pipe and dried thoroughly, I'd take out the plywood and then the PVC sections would be easily removed.) After making the flue sections, I'd build the old fashion, single use (Chinese & Roman) temporary firing kiln to fire them with an open flame/coals.

Frankly, although making the flue liners would be interesting...I'm not sure if you'd get the strength, consistency, and durability of the commercial product. In either case, welcome to the forum and good luck with the project which ever way you decide to go.

[Massimilliano]

Re: Need advice on chimney and outer arch construction

Thanks for that good advice. A problem we have in New Zealand is that it is almost impossible to buy the sort of products you are talking about. I guess this is a drawback of having such a small population.
Most of our local brickworks have either closed down or make decorative bricks. When is comes to pipes it is just about all plastic. The few clay pipe suppliers have all said their products won't stand the heat.
I think I will have a crack at making my own from a clay cement, sand, and lime mix I have been advised works well for making pizza oven domes (as a substitute for fire bricks). Your idea for a mould is brilliant.
Cheers
Max