A: Hi MM, The Chimney Balloon will still work in a fireplace that
does not have a cap or top damper on the chimney. The precipitation does
not effect the balloon at all. As a matter of fact, an open chimney top
is often the case with large older constructions buildings with
fireplaces (especially pre 1900's).
However, there is something to be said about what a large amount of precipitation does to the inside of your chimney. Bricks are porous and absorb moisture, so when your chimney is getting wet on the inside and outside it can be a hard on the bricks and tuck pointing. Sealing off the flue with a chimney balloon will eliminate the air current that flows through the chimney that assists the chimney brick drying it out.
If this is a fireplace you use I would recommend you look into a metal screened cap for it. If this is a fireplace that you do not use at all and will not use in the foreseeable future, you may want to cover the chimney crown with some type of simple metal or wood weighted lid to keep the precipitation from having free access to the inside of the chimney. See the attached picture of a home made chimney cover made with plywood and a birch log screwed together. If you do cover the non-used chimney up-top on the chimney crown make sure to leave a large clear note down below in the fireplace that the chimney is plugged at the top. You don't want any unsuspecting person to start a fire in that fireplace with that cover on there.
If you have a vented gas fireplace I DO NOT recommend putting a Chimney Balloon at the top of the chimney at all. The balloon is too far away from the heat source at the top of the chimney. If someone accidentally and unknowingly lit the fireplace and the balloon was too far away to burst from the heat, that would result in a deadly CO backup in the house. So if you have a vented gas fireplace dont risk it. - Jason