Secure your vents and registers using cable ties or a tape measure. If possible, use screws to secure them to the wall.
2. Install a mesh screen over any entries into the vent system. This will make it difficult for cats to get inside and will keep them out of electrical components.
3. Make the general area around the vents less desirable for cats by placing furniture away from the ducts, removing clutter, covering openings that cats could use to gain entry (like doorways and windowills), and installing photoelectric barriers to keep them out of trouble.