Wow, you should have seen this mess!!! We did a new house in the country for this guy and his brother-in law did the plumbing. We went back today to install the outdoor unit and saw what this guy did. He common vented a side vent water heater into the 3" PVC furnace vent with a "T". I told the HO about it and he said "bubba the hack " said it would be fine, how often does the furnace water heater run at the same time anyway. Well it is staying the way as far as I'm concerned it can stay that way until "bubba" comes back and fixes it.