We have a brand new home -- we've been in it only 2 months. Unfortunately, the builder installed a downdraft cooktop and it vents externally about 2 feet off the ground on the side of the house. The external vent has operable louvered flappers which field mice have learned how to scale the brick wall an nudge the flappers open and scurry into the kitchen via the cooktop. The cooktop is an Electrolux product with an 8" pipe. I have looked at the big box stores for some sort of critter guard and one vent specialty store and scoured the web and cannot find anything that will work. Screening inside the louvered flappers is not an option due to the size of the flappers and the internal mechanism.
We have 2 house cats who have taken to hunting the kitchen nightly, but even they cannot control the influx of field mice. We are in a new neighborhood south of Houston that was a giant cow pasture prairie.
I think downdraft ventilation is a horrible system and am tempted to rip it all out and put in a proper cooktop and overhead vent system --- we have owned several hoses and have NEVER had rodent issues. The builder indicates that the vent is installed according to code and this is just the problem of living at the edge of town!