Masonry to Metal Flue transition

[AaronBarnett]

Hello All,

I have completed the "bones" of my brick oven and now it's on to the flue design. I have finished around 12" of masonry chimney and need to transition to a metal flue to run through the corrugated roof of my pergola. I have done the normal google searches and this topic seems very vague, there's a lot of warnings about cracking and expansion around the transition area but not many explanations on how to avoid this problem. Any advice or more importantly links or pictures would be very useful. The process of building the flue and running it through the roof seems straight forward enough, I just want to be sure to take care of the existing masonry flue and oven in general.

[david s]

The second two castable ovens I built both had cracking problems where the cartable surrounded the base of the stainless pipe. I concluded that the problem was the far more conductive stainless steel pipe expanding before the refractory that surrounded it had time to expand itself. Now I cast the flue gallery about 2mm bigger in diameter to accomodate the expansion of the pipe without the accompanying stress it places on the surrounding refractory. Rather than using an anchor plate which is not only expensive, but usually requires drilling which once again introduces expansion. stress and invites cracking. For the seating of the pipe I simply cut three tabs and bend them out 90 degrees which provides an easy method to adjust for dead vertical, then pack around the seating with a 5:1 vermicrete which is strong enough to seat the pipe as well as sealing the join, but still remaining flexible enough to withstand the expansion.

If you are penetrating a roof you should use a double flue and stay at least 50mm away from any timber rafters or battens. I like to fill the gap between the two pipes with some blanket or loose perlite to a height of at least 300mm.

[JRPizza]

Do you have a picture of what your 12" of chimney looks like? Many of us had a brick lower portion that we transitioned to a square for an adapter plate where we attached a double wall pipe.

Also, are you poking up through an existing roof or are you going to build one?

[Shawnr]

I don't know if mine is correct or not but maybe you will get some ideas from it. The guy at the Woodstove store (they install too ) suggested it to me. I was hesitant at first but think it will be ok. Four Tapcons fasten into the firebrick. I doubled up the width of the firebrick just to add support. Hopefully, the pictures show it well enough. The insulated chimney just attaches to the adapter with a snap ring.

[david s]

Unless the tapcons are stainless, the heat will corrode them. Another approach is to trim the anchor plate severely and then brick over it, thus avoiding both drilling and fixings.

[Shawnr]

david s Damn! I did not know that. Thanks. Now that the roof is on, and it is closed in, I guess I will wait to see how long that takes. I will be able to access it through the roof if I have to, although that would involve some precarious scaffolding if I don't want to perch on the roof. Another option might be to put a metal band on the chimney and secure it to the trusses, which are pretty secure. I put a bead of mortar under the mounting plate as a sealant so if I can mechanically secure the chimney in place, perhaps that will be enough.

[AaronBarnett]

david s I Iike the idea of the tabs cut out of the metal flue. Any advice on casting the flue gallery? Do you cast it separately then place it, or build a mold onto the exsisting brick flue? Also Do you use vermiculite for the entire thing or only to seal it after installing the metal section? This is exactly the kind of info I was looking for! Thank you!

I’m not worried about clearances for the metal flue I have all that figured out and ready to go, I just want to be sure to do my due diligence for the oven itself.

someone asked for pictures. Here is what I’m working with 30” interior.

[david s]

Check out some of the cast builds on “Other Oven Types”. Although I cast the flue gallery separately then fit it to the oven it is easier to form a sand mould in front of the oven mouth and cast it in situ. A cast enables you to make it much thinner than brick and to easily do compound curves. The entry will be subject tobumps and abrasions so needs to be cast from a dense material like castable refractory or homebrew. The vermicrete is used to seal the joint between the pipe and the flue gallery as well as insulating the whole of the outside of the gallery.