Have 2 ton carrier gas furnace outside my house. Noticed heat from vent wasted. Could I
Vent this under the house, it would keep the floors warm since heat rises. Have never noticed exhaust fumes from this. Uses natural gas. Thanks stickman807

No, it may contain Carbon Monoxide which is odorless and deadly. The exhaust also contains water vapor, which wouldn't be good on the bottom of your house.

Bad idea, listen to KISS.

A third vote for bad idea. How old is your furnace? If you truly want to recover all the heat, look into a modern 90% efficiency furnace.