Size of opening for chimney - is mine too small???

[cheyne]

Hello,

Huge thanks to such an incredible community on here for the endless threads and resources that have helped me get to this stage on my oven.

I'm building a 42" oven and a 19" x 12.5" vent opening with a serpentine arch to attach the chimney anchor plate. I miscalculated the opening size for the chimney, and worry that what I have now would constrict airflow to an 8" diameter stove pipe. Right now the opening for chimney is only 6.5" x 6.5". Should I use an angle grinder on the bricks that are creating the serpentine arch to make the opening larger?

Thanks!!

[JRPizza]

I would do an internet (google?) search for "venturi fornobravo site:fornobravo.com". There is quite a bit of discussion regarding having a restricted flow leading to your chimney, but it is not something I have studied. If you want to get the area equavilent of 8" diameter pipe you will have lots of grinding to do.
You might be better off trying to "persuade" a few of your inverted arch bricks loose and re-position them - later when they are surrounded by the fore and aft bricks of the vent they will never move. I know it's hindsight, but I dry stacked mine and measured to make sure, since I built without a template.

[cheyne]

Thanks!

If the chimney opening area is equivalent to my 8" pipe, will I be ok? Or should it be slightly larger to accommodate the square to round transition and possibly decrease turbulence?

[UtahBeehiver]

The existing 6.5" square opening is only abt 42 sq in in area, assuming your 8" diameter stove pipe is 8" ID the area of the stove pipe is abt 50 sq inches so the brick opening is smaller in area than the stove pipe. In addition, some where on the forum, I recall seeing an efficiency diagram showing the square vent pipe is not as efficient as a round one so the opening of brick needs to be larger in area than the stove pipe. I do know that David S advocates the venturi theory but that is above my pay grade.

[JRPizza]

Whether you go with a venturi or widen up your opening to get at least 50(.27) inches of cross section, you will still need to widen the chimney at the top to accommodate the adapter plate if it is similar to the one I used. My plate had an 8" "sleeve" projecting out of the bottom, so I had to cut out some of my bricks to create an 8" circle. You can see this in the picture (it looks funny as the bricks aren't mortared yet. Is your plate flat on the bottom or is it similar to mine?

[david s]

Hello,

Huge thanks to such an incredible community on here for the endless threads and resources that have helped me get to this stage on my oven.

I'm building a 42" oven and a 19" x 12.5" vent opening with a serpentine arch to attach the chimney anchor plate. I miscalculated the opening size for the chimney, and worry that what I have now would constrict airflow to an 8" diameter stove pipe. Right now the opening for chimney is only 6.5" x 6.5". Should I use an angle grinder on the bricks that are creating the serpentine arch to make the opening larger?

Thanks!!

A 6.5 x 6.5 opening is 42 sq in. An 8" diam pipe is 50 sq in so the restriction is minimal. The power of the draw from the flue comes from its volume and the diameter is a stronger factor than the height. If you can grind off the corners of the brick to create a smoother smoke flow then the restriction acts like a Venturi and the smoke will accelerate at the restriction. You have a larger volume below the restriction which is more important. I think your oven will draw like a champ.

[cheyne]

Thanks everyone, ended up spending a few hours grinding it out, looks like it's going to work great!