I'm working on a four story condo building. We're replacing the entire exterior with Perm-a-Barrier waterproofing, insulation board and a rain screen cladding system. The existing gas-fired boilers are located in each residential unit and vent through a common pipe. Once our project is completed, accessing the waterproofing will not be easy and I want to install future vents for residents when they must replace their existing units. I have found two typical vent systems; one has two vents through the side wall (combustion and exhaust) and the other has a combo pipe that vents through the inner pipe and brings in combustion air from the outer. I found a concentric pipe that doesn't cost too much that will work and it would mean one less penetration through the waterproofing, but I am not sure if any boiler can be vented with the two-in-one pipe. I've gotten conflicting info from plumbing/heating contractors and haven't been able to find a Mech. Eng. who doesn't cost an arm and a leg. Any help would be appreciated.