OAK question when using a thimble; 2 1/2 be made into 3 inch?

I am doing an install on a stove requiring a 4 inch output. I have thimble similar to this one, http://www.dynamitebuys.com/store/cart.php?m=product_detail&p=5966 , that has a 2 and 1/2 diameter knockout to be used for an OAK. Can I make this hole larger so as to hookup the 3 inch OAK it calls for? Would I be cutting into the clearance on the thimble, piping? The chimney is PellentVentPro.

I'd do it if it looked like it would work, but someone will come along and mention voiding the UL listing or something like that.

Thanks Smoke. Has anyone else done this?

As long as the thimble pass through section that the vent passes through is not changed, you should be fine that pass through provides the clearance to combustibles needed do not do anything that modifies that portion of the thimble.

It is the third layer of the vent that excludes any material coming in contact with the vent itself.

Sorry Smokey. I am not sure I understand your answer.
I am not planning any change to the main pass through section in the thimble. The 4 inch diameter section remains untouched and the outer section cutout (approx 7+") to that remain unchanged. This chimney system has a 1 inch clearance to any combustable. What I would like to do is make the 2 and 1/2 inch cutout plate larger by 1/2 inch and I would cut at an angle so as to keep the original distance from the pass through. Here's a drawing that might explain it better.


Please let me know your thoughts. Anyone?

*sigh* Smoke Show nailed it for me......ANY modification made to a UL listed system is technically incorrect, and you void the UL listing in doing so. Why not simply use the correct part? (yea, lol, I'll take the heat for being a harda$$ on this one)

Lousyweather, the issue is if making a 1/2 inch adjustment in what the thimble is designed for and if it will make a difference? We are talking about making an air opening larger, not making the venting area smaller. I hope this is understood correctly with the enclosed drawing.

ANY alteration. Do you have any qualitative data to prove that it is in fact safe? ANY modifications technically invalidate the UL listing.

LW is technically correct, this is like the how do I make it fit in a much deeper wall.

You can call the venting manufacturer. and ask.

Why not get a 3->2.5" reducer and try it first? The smaller opening is honestly not that much of a difference... and once the pipe is filled with air, it shouldn't be that much of an issue. Run it and see if the stove is starved for air. I doubt it will be.

The test is to determine if any combustible material placed in direct contact with the pass through wall will ignite.

That pass through wall becomes in essence the third wall in a vent system (makes it a zero clearance triple walled vent).

This may be a good first try. Thanks for the input all.

The 3" has a cross section area of 7.06 square inches and the 2.5 " has a cross section area of 4.90 square inches give it a try just remember the 3" has 1.44 times the cross section area.

Smoky has alot of time now since the Zumba classes have ended!

I can see running on high that it might get a little starved, but on a medium setting that will fill 90% of your burning, it should suffice.

Unfortunately it has to be run on high to set the damper. Since the air is not being taken from the same place as it was and the damper is the course fuel/air mixture control it will have to operate on high. No way around it. The only question is will it be air starved on high due to the difference in the venting at the exit, one way to find out try it.

But please set the stove up correctly after you install the OAK. It prevents all kinds of issues later on..