I had licensed plumbers install a Wei-McLain CGi-4E a few weeks ago. The venting problem was it goes into a flue along with a standard 50gal water heater. With my old boiler (an HE3), the 3" went into a 4" manifold that received the WH in a wye, and this worked fine. This new boiler is more powerful and they used just a Tee to connect the WH, now the boiler exhaust is coming out the WH hood.
I've contacted them about this, but since they're obviously challenged by this, I'd like to hear some ideas about how it really should be done, short of the obvious solution of cutting a separate hole into the brick flue for the WH. There so much piping right there that's a big hassle.