Flue pipe question

[nino]

Good morning ,happy holidays, hope everyone is well.
I am installing a pizza oven that is rated 28000 btu/hr it comes with a 5 in flue. I will be using b vent to vent straight up probably between 3 or 4 ft max. I've read that a 4in vent can handle 3 to 4 times the rated btu of the oven. Question is can I reduce from 5 to 4 and use a the 4in to vent? Or would going from 5 to 6in be at all beneficial? You will probably ask why not just use the 5? Well the 4in double wall is available at local improvement store. But now that I think of it there's a Grainger not too far that probably has the 5. I have a buddy that tells me I overthink things too much this being said I would still want to know thanks in advance . .

 

[Twowaxhack]

4” B vent is rated for more than 28,000 btu. I’m not sure about the temps you’re running. Check with the manufacturer.

 

[breplum]

Never reduce a type B double wall flue smaller than the vent collar unless the mfr actually proffers such in the published literature.

 

[Rossando]

I Agree with the guys, check with manufacturer first. The reason why is the design of the flue pipe is to push gases up and away. Changing it to smaller size can create condensation issues or other things.