Removable chimney

[Thomobigands]

Morning all. I finally finished the dome last night! Will upload some pictures soon as my build thread is non existent!.

Anyway, just the chimney now to overcome. Another user suggested a removable chimney to make it easier to cover the oven when not in use but to get the height you need for the draw. I like this idea but was left scratching my head around trying to fix the single skin metal flue I have purchased to the oven without cracking it etc.

The idea that I have come up with instead is to buy and cut a 150mm diameter clay drainage pipe and homebrew that in place to the arch with the help of a few surrounding bricks. My 6" flue should then hopefully be just a big bigger to slide over the pipe whenever I want to use the oven. If the metal expands then it has room to do so as it is not fixed in place. Aside from not using the oven in gale force winds, does anyone envisage a problem with what I am proposing? I would be really interested to hear any thoughts on this? The clay pipe would be around 12 inches high. no more.

[SableSprings]

The thing to be aware of is (from the way I'm reading this), you are going to have a fairly small entry to your 6" flue pipe. The clay tiles I've used run between 10-15 mm thick (0.25- 0.6"), so you are throttling your flue pipe entry down significantly. Because of the tile's thickness, I suspect you won't actually be able to slip the metal pipe into the drainage tile. These tiles are normally measured by the outside diameter...and I suspect you're thinking inside diameter.

When we take the chimney down for winter covering at the cabin in Canada, the lower 2' section is left in place (we have a total of 8' of chimney) and we put a bucket over it as a cap. Then we double tarp the entire oven (going over the bucket) and tie it down pretty well...lots of high winds and snow up there during the winter. I really like the sleeve idea and I think if you could find a larger clay pipe so the SS fits down inside of it, that would work well. You could simply put three SS short screws into the SS pipe so they rest on the top of the clay pipe when the SS flue is in place. A bit of fiberglass/ceramic rope (stove gasket) pressed in around the top of the SS/clay joint would pretty much eliminate smoke "escape".

It also seems like the normal base plate that is sold for connection to the oven vent on some Forno Bravo modular ovens would allow you to simply (or reasonably simply) lift off the chimney pipe from the base when you are closing for the season.

Hopefully, there's some food for thought here...

[Thomobigands]

Thanks for the reply!

I take your point about reducing the diameter of the flue taking into account the thickness of the tile/pipe. Hadn't thought of that but will make a difference. Having said that, I only have a 36inch internal diameter oven so maybe I could get away with a slightly reduced flue?

Also, I think you may have misunderstood. My plan was to slide the single skin metal flue OVER the clay pipe, not inside it. That way, the metal can expand and not damage the clay sleeve. That was my thinking.

Although, someone on a different forum has said that when he used a clay pipe as a flue, it cracked all the way up after the first real exposure to heat! and I dont want that either. I see other people use clay pipes and pots without any problems though?

Am now wondering whether to just cut a short section of my metal flue off, fix that to the oven with bricks/homebrew/fireplace cement etc and then fit the larger section as and when I need to (the end of the pipe is tapered so I think I can use that as my quick connection method).

Decisions decisions!!!!?!?!?

[SableSprings]

Thanks for the reply!

I take your point about reducing the diameter of the flue taking into account the thickness of the tile/pipe. Hadn't thought of that but will make a difference. Having said that, I only have a 36inch internal diameter oven so maybe I could get away with a slightly reduced flue?

My understanding is that you would be pressing the upper limits of the smaller vent's ability to work well for the 36 inch oven. I have a 39" oven and an effective 6" diameter (8" clay flue pipe) chimney. With my setup, I do get occasional smoke escaping out the front...I think you would experience that as well with the reduced flue diameter.

Also, I think you may have misunderstood. My plan was to slide the single skin metal flue OVER the clay pipe, not inside it. That way, the metal can expand and not damage the clay sleeve. That was my thinking.

Although, someone on a different forum has said that when he used a clay pipe as a flue, it cracked all the way up after the first real exposure to heat! and I dont want that either. I see other people use clay pipes and pots without any problems though?

No, I understood the metal flue over the clay pipe...that's where the restriction would "happen". Any smoke spillage would be because of the initial, smaller vent diameter (the inner clay piece). I have all clay flue tiles for my collection area and chimney stack so I can't address the effect of a thin metal pipe inside a clay flue. I really don't think the metal 6" pipe will even fit inside a 6" clay pipe (again because I believe the clay pipe dimension is outside diameter not inside). I would strongly suggest taking your metal pipe section to the "brick yard" first to see if it will fit inside the clay pipe section.

Am now wondering whether to just cut a short section of my metal flue off, fix that to the oven with bricks/homebrew/fireplace cement etc and then fit the larger section as and when I need to (the end of the pipe is tapered so I think I can use that as my quick connection method).

Decisions decisions!!!!?!?!?

This "short pipe" idea sounds like a good option. I'm still unclear if you are using a base plate for your metal pipe to attach to the oven vent (masonry) interface. I attached a picture of what I'm talking about as a base plate.

[Thomobigands]

I have seen the anchor plate but wanted to try and avoid the added expense as the good ones are a little pricey. Plus I only have a couple of inches at the front of the oven so worry about hiding the plate. Hopefully I have managed to attach some photos to show you. The chimney us just balanced for now.

[SableSprings]

How about taking some bricks and a grinder to make an outer sleeve for the metal pipe? The bricks could be mortared together to form the ring and then you could just slide the metal pipe down into position.

Since the pipe sits pretty nicely over the arch vent now, you could also just set some bricks (or splits) around the pipe and mortar them into position, using homebrew to fill the gaps...kinda like making a casting around the pipe to hold it upright. I'd layer the pipe in plastic wrap so the mortar didn't stick, but that might be worth thinking about. It would appear from the front to be a classic brick chimney that transitioned to a metal pipe. Just brainstorming here...but maybe something will strike your fancy.

[david s]

I think your set up as planned will work ok. The draw from the flue pipe is determined by its diameter and length. The restriction from the smaller diameter clay pipe will act like a Venturi which will not effect the draw significantly provided the clay pipe is fairly short ( no longer in length than double its internal diameter). If it works as it should you will get the smoke accelerating through the clay pipe. Maybe you can trim its length down. Also if you can shape the inner surface of it with a grinder to match the shape of a Venturi the flow will be improved.. So long as the clay pipe is insulated it should not crack. Uninsulated it almost certainly will.

http://www.tech-faq.com/venturi-effect.html

[Thomobigands]

I think your set up as planned will work ok. The draw from the flue pipe is determined by its diameter and length. The restriction from the smaller diameter clay pipe will act like a Venturi which will not effect the draw significantly provided the clay pipe is fairly short ( no longer in length than double its internal diameter). If it works as it should you will get the smoke accelerating through the clay pipe. Maybe you can trim its length down. Also if you can shape the inner surface of it with a grinder to match the shape of a Venturi the flow will be improved.. So long as the clay pipe is insulated it should not crack. Uninsulated it almost certainly will.

http://www.tech-faq.com/venturi-effect.html

Thanks for taking the time to reply. I am beginning to wonder if having a clay pipe is going to cause me problems. Especially if I am limited in its length when factoring in that I need to bury it in brick/mortar yet still leave enough exposed to slide the metal flue onto - which also means that certainly the exposed end would not be insulated and so would surely crack? Maybe I need to switch to a solely metal solution.

[Thomobigands]

How about taking some bricks and a grinder to make an outer sleeve for the metal pipe? The bricks could be mortared together to form the ring and then you could just slide the metal pipe down into position.

Since the pipe sits pretty nicely over the arch vent now, you could also just set some bricks (or splits) around the pipe and mortar them into position, using homebrew to fill the gaps...kinda like making a casting around the pipe to hold it upright. I'd layer the pipe in plastic wrap so the mortar didn't stick, but that might be worth thinking about. It would appear from the front to be a classic brick chimney that transitioned to a metal pipe. Just brainstorming here...but maybe something will strike your fancy.

Thanks for taking the time to reply. I quite like this idea. A couple of things spring to mind. If I wrapped the flue in plastic but maybe also something thin between the plastic and the flue, this would create a space slightly larger than the flue when I removed it, to allow for expansion - I wouldnt want the fit to be too tight or the metal could presumably risk cracking the small brick chimney during use.

Also, even if I blocked up the small gaps where the flue meets the arch at the bottom, would the weight of the chimney alone be sufficient to ensure that the smoke didnt leak out of the arch, under the chimney and up the gap around the flue?

[Thomobigands]

My revised plan?

[Thomobigands]

Sorry, I should have added that I am just going to use the 1m metal flue I have. I am going to cut around 300mm off the end to fix into the chimney.

The vermicrete- I guess I just thought that it would be better to cover it rather than leave it exposed to any rain. I thought that if I used a fire cement or sealant it would cover the vermicrete, be flexible to allow for expansion of the pipe and make a better seal around the lower metal pipe.

[david s]

I've done four ovens with removable flues and the pic shows how I make the sleeve that the pipe fits into. The wires allow adjustment of how tight to make the fitting. if your pipe comes with a crimped end, designed to allow fitting a cowl or extra length of pipe, you can cut that part off the pipe and set it in place so you can fit your pipe over the crimped section.

[Thomobigands]

Thanks for the comments, it is your idea that I keep coming back to in order to try and replicate it! but modify it slightly. It turns out that upon checking my flue, it is not recessed at one end as I thought. However, I have now purchased an inexpensive stainless steel connector that will do the same job as your hand made sleeve. It is actually about the same size as yours. The only difference being that rather than fix that to the oven, I am going to fix that to my removable flue and fix the short, cut off length of pipe to the oven within a small chimney, surrounding it with vermicrete.

I think I have it in my mind now as to how it is going to be set up so will try and put it all together at the weekend. I was then going to start the curing fires in a week or two prior to adding insulation but reading other posts tonight I am not sure that is the best way. Most of the dome has had several weeks to dry it has taken me so long so I think I am going to just get on with it - flue, chimney, blanket, vermicrete. Couple of weeks, render. Then fire.

I think........!

You make a decision then read other posts that leave you questioning everything again!!!

[david s]

That sounds workable. I prefer to insulate, then do the drying fires so the insulation helps even out the heat, stressing the refractory less. A cheap moisture metre poked into the vermicrete layer, assuming you're doing that, will tell you when it's dry. After its totally dry I do the outer shell then wrap it in cling wrap to hold the moisture in that layer for a week to enhance strength. It takes much longer and is riskier, to push the water out after you have rendered the outer shell.

[Thomobigands]

Thanks for the tips. Put the blanket and wire on tonight so vermicrete tomorrow. Thought I would post a couple of photos of how the removable chimney is working out .