What is this vent called?
Lacking context, it's either a cleanout (normally at the bottom of a flue) or a disused thimble (place where a pipe used to connect to the flue) with a cover/plug in it (the half-rusted bit with the handle) - &quot;cleanout cover&quot; and &quot;end cap&quot; and &quot;flue plug&quot; are all possible names when searching. Add the diameter of the pipe/hole to narrow results towards ones that might actually fit.