Squirrel In Water Heater Vent. It looks like you got your suspect well before i did. All water heater venting systems use a vent duct or pipe—also called a chimney or flue—to bring exhaust gases from the water heater to the outdoors.
Water heaters with power venting are recommended for tightly built homes and homes with no chimney, where a good natural draft is hard.
What size tank is it? Holy shit, that brings back unpleasant memories. High efficiency tankless water heaters will either need a category iii stainless steel vent or pvc. The contractor that put it in used part of the existing 3 concentric vent from the furnace we removed used.