vent opening size

[mooch]

I have been reading and searching. I am building a 42" pompeii. It looks like my oven opening will be 20"wide, 12.5 high at the top of the arch. My vent opening will be 4.5" wide (1/2 a brick) and between 12 and 18" wide. In the plans it calls for an 8" vent opening. Does that mean width? Will my opening be ok?

[azatty]

Re: vent opening size

If your vent is rectangular rather than square, go with rectangular flue liner. I believe you can get it in 6x12 on special order. The problem with your current plan is that you'll end up with a 4.5x12 inch throat mating to 8x8 flue liner, and you will effectively lose nearly half of your flue cross section.

You need to make the opening a full brick wide to accomodate 8 inch square flue. So measured from the front of the exterior arch to the back of the interior arch, the "barrel" will be approximately 18 inches (two firebricks) deep. That gives you 4.5 inches thickness for each arch, with enough room for a proper size vent opening.

[mooch]

Re: vent opening size

I cant beleive this to be true, on the plans the vent landing is 8", including the interior and exterior arch from how I read it. I have seen several build threads where the chimney opening is only 1/2 brick or so wide. I am waiting to mortar my first course in place but am waiting to finalize on these plans.

[deejayoh]

Re: vent opening size

You can make a transition from the rectangular shape to the round shape without losing any of the flue cross section. Sure, if you pop an 8" round flue on top of the rectangular cross section without any sort of transition it may be a problem. You just have to think through the way the gases will be flowing. Plenty of builders have done it.

I put an 8" duravent chimney on top of my 10" deep vent landing with no issues. My vent size was a 10" by 5 1/2" rectangle. I built that up so that it flowed into an 8 x 8 square by angling bricks in at the ends, and out at the sides. I have had no problems with venting. Yes, a little smoke comes out the front at startup (mostly depending on the wind) but once it's going I have zero issues.

Check out Ken's Kentucky Dome, he has a pretty good pictorial. I didn't do a great job documenting what I did, but I followed his pattern.

One more note: you will not be able to get an 8" duravent centered on a 8" deep vent - so you may have to rethink that bit. The anchor plate is 12" x 12". On mine, I had to hang the bricks supporting the plate off the back of the vent bricks by 2". For that reason, I would suggest going with a little bit deeper vent than you are currently planning

[david s]

Re: vent opening size

It sounds difficult to have a 4" deep vent transition to an 8" flue, but if you cast it then it is quite simple. although I'm sure you can also do it with bricks without too much trouble. The principle is to make sure your area at the bottom is greater than that of the 8" pipe, so have funelling to the pipe for smoothe flow. Going the full width of the entry is best IMO. The idea is to make the entry shallow for easier oven operation.

[mooch]

Re: vent opening size

ok, so an 8" diameter flue is around 50 sq. inches. If I make my vent opening 4.5" x 14" it would 63 sq. inches. Assuming I would lose a little bit depending on how I transition it I should be ok right?

[david s]

Re: vent opening size

If possible I think you should use the full width of your entry, unless of course the 14" is the full width.

[mooch]

Re: vent opening size

actually I was planning on using the whole length which is 20". I was more concerned about the depth of it only being 1/2 a brick length. But that would be 20" x 4.5" which sounds like it would be fine. After tapering the opening it would probably end up about 16-18" long I imagine.