I recently purchased and am installing a 96% efficiency two-stage gas furnace. The old unit was an 80% efficiency unit using a 4" metal flue. This flue merges the exhaust of both the hot water heater and the old furnace.
With the new furnace however, I need to switch to 2" PVC and I am unable to connect this to the metal exhaust as the condensate created by the higher efficiency furnace is corrosive and will cause the exhaust flue to rust and create a carbon monoxide hazard.
As it so happens, however, I have been wanting to upgrade to a tankless hot water heater. I am aware that there are condensing and non-condensing tankless models, and that the condensing models also use 2" PCV.
Would it be safe to join the exhausts similar to the old setup with this configuration? Or do I need to run two separate exhausts? If I can merge the exhausts of these two appliances, Do I need to go larger above the y joint? How large? Would 3" be sufficient?
Best Answer
In answering @Harper-ReinstateMonica's question in the comments, I stumbled across something on page 12 of the manual which states "never common vent this appliance with another appliance or use a vent which is used by a solid fuel appliance."
So it looks like it common venting with a tankless condensing water heater would be generally unsafe, is not recommended by the manufacturer, and may be against code.