We have recently moved house and currently have an open fireplace -
something we didn't have before. Our cats are very interested in the
opening and on one occasion one of the cats ran up the chimney - in
fright - and it was a few hours before she finally emerged...filthy!
We are not going to use the fireplace for an open fire and would like to
block off the opening. Does anyone have any experience of chimney
balloons as we have heard that these are quite effective?

A problem with the one we had was rain. It was in a bedroom fireplace with a
clear view to the sky and in heavy rain some came down the chimney and
produced a very loud drumming noise. This was eventually solved with a disc
of plywood in the chimney top which needed a roof expedition.

The balloon would fall out every so often and after ten plus years got a
slow puncture. It has been replaced with a bin bag full of bubble wrap.