Any advice on excluding bats from chimneys? The fireplace is rarely if ever used. The chimney is missing a cap, which blew off because the installer didn't tighten the screws. I wondered about just throwing a tarp over the top for a week, and then installing the cap. But I would guess the bats would squeeze through the grate anyway, is that right?

Is the chimney in good structural condition? What is burnt in it? Wood or gas?

Just like any other exclusion. If going down the chimney, you can put 1/2" screen or piece of plywood over top with a corner left open to attach bat door. Once bats are out, remove screen and install chimney cap. Just don't leave 1/2" screen in place, it's not fire code.

I've boxed them in with plywood, basically a plywood cap, standing tall enough to cut a large opening to hang the valve.

I thought a piece of sene net over it, the something I do everywhere else