Don't use the coaxial. Viessmann offers a 2" rigid PPs. The pipe is the same pipe that is used inside the Coaxil. You can then use what ever you want for fresh air. Pop out one of the knockouts on the top and use PVC for the fresh air. The PPs is not expensive at all. Your in NJ and so am I. Curious as to where you got the boiler.

The concentric is made by M&G and they bring it in from across the pond. M&G does not offer concentric other then thru OEM.

"The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."

cgi.ebay.com/Biasi-Riva-Standard-Concentric-Vent-Kit-/220678214671?pt=LH_DefaultDomain_0&hash=item336172540f will be fine, correct me if i am wrong but 2.5/4" = 60/100mm ?

The reason i need to use a concentric vent is i am also installing a rinnai tankless water heater, so i'd rather have 2 vent holes than 3 or 4 .

This is my first boiler installation, I got the unit from pexsupply online website, i am converting from oil so i decided to go with decent boiler, and after few searches, reviews .. i picked the viessmann, i had to design my setup of boiler and water heater, and i am also splitting my house into 2 zones.

i am located in morris county, if you have any questions please dont hesitate to ask...

I am the local distributor of Viesssmann in the territory. You own the vent system and any problems caused by not using an approved vent system. Viessmann purchases and boxes the Coaxial from across the pond and ships it into the US with the boilers. If you are a trade contractor be more then happy to quote you the Viessmann venting. Viessmann inner is not 2.5". Please see page 17 of the attached. I'd also be curious as to the serial number of that boiler. Its on the white label on the left hand side on the outer side of the boiler cabinet not the serial number on the plate that is on the heat exchanger.

I used a piece of baxi coaxial venting in my own house, it was left over from a job and I was going quite a distance (12 ft or so) and was just short on the viessmann venting so I used it. It looks almost identical to the viessmann and is the same materials: PPS inner and enameled aluminum outer. I really don't see what the problem would be using it but you would probably be putting yourself out on a limb if anything went wrong.

Zach hits the nail on the head. As professionals, we owe you the consumer/homeowner the best and proper advice. This is America and you can do what you want, but be assured, you own the boiler and the vent system if you use another mfg's venting on the boiler.

"The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."