Yep, cpvc... I've never heard the 6" thing either. I see installs like this all the time... I suppose that doesn't make it right... just that it's common around here. It would seem that water heater manufacturers would have to postion the relief valve far enough away from the venting to allow for mainstream pluming materials to be used... Or at least if they wanted to sell their product they should.

For clarification.. it's a single family residence. Through the wall is a bedroom for that residence.