Critter in my heating equipment

For awhile, my neighbors who are also my friends, have complained to me that they have rodent concerns. They told me it seems like no matter what precautions they take, a racoon or possum will find their way around their apartment or in their trash, dragging garbage all around their lawn and driveway. Although the rodents were always near their home, I never had any concerns with them until one spine-chilling night about a month ago. I remember that I was coming home from work rather late and ecstatic to be home. As I was getting out of the car, I could hear loud rattling and moving from my neighbors bushes. When I got closer after walking on the path to my garage and towards their Heating plus Air Conditioning system outside, and out of nowhere ran out a raccoon! It ran away fast thank goodness, although I could see that it had gotten their outside heating vents pretty good. I told them about it the next day and in no time they had a Heating plus Air Conditioning technician there to inspect the unit.  Of course it was replaced considering the sizable chunks that were missing from the Heating plus Air Conditioning system. But, luckily for them, the Heating plus Air Conditioning expert placed a guard that would help protect their heating and cooling units from rodents, but I know this won’t protect their trash can.