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  • #16
    Thanks, David. How can I strengthen my vermiculite base? Parts of the bottom edge are still crumbly and it's a bit worrisome. Can I make a stronger mix of the vermicrete, say 3:1, and just skim the outside to firm up the crumbly bits?

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    • #17
      Yes, of course, but probably easier to use a sand/cement 3:1 mix.
      Kindled with zeal and fired with passion.

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      • #18
        Thanks, will do!

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        • #19
          Worked on my gallery form this weekend, but did not have time to do the cast.

          Meanwhile, I got a 6" x 36" double-walled SS chimney and realized it's VERY heavy. I was planning to use David's tab method (rather than using an anchor plate), but I'm worried about the little tabs supporting the weight. I realize they'll be more additional vermicrete and a stucco layer to help support the weight, but still concerned... I'm thinking of drilling three or four SS screws into the base of the chimney and having them stick out an inch or so. These would rest on the top of the hole in the cast and support the weight, rather than the thin tabs. I'm sure it will work, but it is a bad idea?

          Thanks,


          Joe

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          • #20
            I’ve never had a problem with the seating of the pipe with my tab system and some are pretty tall at 2.5m (3 sections of pipe). My pipes are not heavy, but maybe yours are thicker. Mine are made of 0.55mm 304 stainless. Because screws are likely to come loose at the threads from thermal expansion cycling I think you’d be better to either make the tabs a little wider or just cut more of them, say 6 rather than 3
            Kindled with zeal and fired with passion.

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            • #21
              Got the gallery cast this weekend. Went fine but I think I made my chimney form a bit too big. I think I was worried about some of the posts I have read about the chimney expansion causing cracking in the casting.

              Did a test fit the next day with the actual chimney and there's probably 1/4" gap around the whole pipe. Sorry, didn't get any good pics.

              What's the best method to fill? Vermicrete or stuff with fire blanket?
              Attached Files

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              • #22
                Originally posted by Lotus19 View Post
                Got the gallery cast this weekend. Went fine but I think I made my chimney form a bit too big. I think I was worried about some of the posts I have read about the chimney expansion causing cracking in the casting.

                Did a test fit the next day with the actual chimney and there's probably 1/4" gap around the whole pipe. Sorry, didn't get any good pics.

                What's the best method to fill? Vermicrete or stuff with fire blanket?
                That will be ok. You could expect an expansion of the pipe to about 3mm extra in diameter.
                If you use my method of pipe attachment, then no anchor plate or drilling is required. You could stuff some blanket in the gap, but I prefer some 5:1 vermicrete to seat the pipe in position. If you build the vermicrete up around 150mm higher than the top of the flue gallery it will support the base of the pipe really well. The render can then surround the pipe (do the cardboard trick again, remove it when the render sets then fill the space with high temp silicone. This prevents the expanding pipe from cracking the render as well as preventing water entry at that point. If it's high enough away from the fire the high temp silicone copes ok.

                Click image for larger version  Name:	P2160559 copy.jpeg Views:	0 Size:	10.3 KB ID:	449945 Click image for larger version  Name:	P6070734 copy.jpeg Views:	0 Size:	10.2 KB ID:	449946 Click image for larger version  Name:	P1130302.jpeg Views:	0 Size:	36.5 KB ID:	449947
                Instead of using two terracotta caps as I do to allow the over dome insulation to vent to the atmosphere you could drill some holes in the base of the pipe, as illustrated, so water vapour in the insulation layers can escape to the vent pipe.(I've not tried this but can't see why it wouldn't work. It seems to me to be a far more elegant solution than an exposed vent.
                Click image for larger version  Name:	IMG_0163.jpeg Views:	0 Size:	18.8 KB ID:	449948
                Last edited by david s; 09-26-2022, 09:37 PM.
                Kindled with zeal and fired with passion.

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                • #23
                  Just finished filling the space around the chimney with some 5:1 vermicrete. I wrapped some thin cardboard around the chimney. How long before the vermicrete sets and I can remove the chimney/cardboard?

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                  • #24
                    It depends a bit on your weather (temperature), but 24 hrs should be ok. The vermicrete is ok to be hard up against the pipe as there is some elasticity in it and you want it to support the pipe. The cardboard is only meant to keep the more brittle render away from the pipe. The drying fires should be done before applying the render. When doing the cardboard trick I cover it with some plastic or oil it to prevent the render sticking to it. This makes removal easier.
                    Last edited by david s; 10-01-2022, 11:38 AM.
                    Kindled with zeal and fired with passion.

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