[loch]

what would the clearance of a 80/k condencing furnace that terminates with a concentric pipe through the roof to a existing gas vent pipe ( hot water heater ) 38/k natural draft

 

[hvac benny]

That information will be in the installation instructions for your furnace. The instructions will also tell you if a concentric termination is approved for your furnace (not all furnace manufacturers approve them). The Carrier installation instructions I have say 3 feet.