Combustion air needed for 6" flue

[Pete S]

I'm in the home strech on getting my fuel oil tank evaporator finished! I'm mounting the door and looked to see how much room is below the door for an "ash door" combustion air inlet.

This inlet will follow the traditional thought process of coming in below the grates.

I don't have a ton of room, and the base of the tank is of course radiused, BUT here's my question.

I know it's a different principal, but my wood stove [Jotul] has a 3" combustion air inlet, with a 6" flue.

What would be the seet spot for square inches of combustion air for my setup firguring the 6" flue as a cross sectional area of about 28.26 sq. inches.

Thoughts/experience/advice appreciated.

Pete
P.S. Bought a bunch of buckets and can't WAIT to tap!

[maple flats]

On an evaporator for wood fired you need the same area or even a little more than the stack area. For example, on my evap, I have a 12" (6x6x3.14= 113 sq ") stack as indicated by the manufacturer and the draft door is 7x18 (7x18=126 sq ") The draft door when boiling on natural draft is wide open as soon as a fire is extablished and remember, there should be no stack damper. You want to get maximum air to the fire. Even then sometimes you will see a ball of fire at the top of the stack. This is not flame going all of the way, it is non combusted wood gas that re ignites as it gets oxygen. With forced draft you just need the right blower.

[Pete S]

Thank you Sir!

That makes total sense!

So about a 10%+ increase would do the trick,......and run'er wide open.

I'm guessing too much more would be a waste of open space, as well,...............would that build static pressure within the arch and be looking for leaks? (or am I thinkin' too hard on this?)

Thanks Again!

Pete