When we turned on the forced air heat this autumn, we discovered that one or more of the cats had been using two of the ducts as urinals. Phew! Few things more olfactorily noxious that concentrated dried cat piss. Even after attempting to clean the ducts the smell has etched its way into the galvanized sheet metal [don't you just love the power of ionization?] We had to actually replace part of the duct-work.
So the solution we came up with was to create floor collages to cover the ducts but still allow the heat to rise from them. The objects are different heights, angles and positions from one another, so none of cats are comfortable climbing on top. we also made judicious use of mothballs, which the cats finds offensive to the smell. They also make our house have a stereotypical "old lady" smell.
But the floor collages seem to be doing the trick. Not only do the cats refuse to climb up on them, one of the cats - Jake - seemed positively spooked when he first saw them. Time will tell. I certainly hope they work.
I wouldn't want to have to get Archibald "Harry" Tuttle to repair the ducts, that's for sure.