I’m planning to retrofit my tank water heater to a Rinnai tankless condensing unit (180k BTU)

My furnace (assuming 80k BTU, need to confirm) has 2” PVC inlet and exhaust vents. I’d like to locate the water heater in the same room sharing the existing vents. I know you can vent multiple tankless heaters on a common vent, but looking for the code section for venting with a furnace. I’m in SC specifically.

ETA:   According to installation instructions, you can’t common vent with a non-Rinnai tankless heater.

Definitely not on a 2" PVC run.

The tankless I've always installed (Takagi, usually 199k BTU units) require 3" venting for 70' runs or less, and 4" PVC for runs up to 100'. ALL common venting requires at LEAST a 4" PVC pipe run.

I'm not hip to Rinnai but physics is physics so I'd say that's going to be a hard no.

I know when I looked at the our code, it has to be a Y-pipe and a non-reversing baffle is required in the past, but it is no longer allowed in new installations/constructions.

Finding a non-reversing Y-pipe is nigh-impossible these days unless its custom made

You would like to, but you can’t. No building code or manufacturer is going to allow that.

The problem is you need to make it 100% certain that your forced vent furnace won't blow exhaust backwards through your water heater into living space, and your WH can't blow exhaust through the furnace.

Previously the code allowed one way valves in the flue, now the code eliminates that possible (and deadly) failure point by saying no shared flue.

Just an FYI update… in SC you can common vent exhaust flues with Condensing furnaces and tankless water heaters in accordance with manufacturer’s guidelines. In my case a Lennox furnace and Navien tankless water heater.

That’s cool, but both manufacturers say no. Navien can be common vented with other Naviens when they’re tied together on the control system.