Benjamin Franklin said, "An ounce of prevention is worth a pound of cure". I am applying this to the heating/air unit in our new house. I live in the country, and I have chickens in my yard. Both of those add up to the occasional creepy, crawly visitors - snakes. I have learned to live with snakes. If they leave me alone, I will leave them alone.
I never thought about snakes coming in through the pipes for heating/air lines until one of W's co-workers took a few days off work to get the snakes out of his house. The 4 foot plus rat snakes had crawled in through the pipes.
The opening is more than big enough for a snake or rat to crawl through. Rather than leave them an open invitation we opted to seal up the holes on both ends with spray foam.
I will be checking periodically to make sure the seals are intact. I found out that sealing up the areas is not common. I would have thought everyone did it. Are the pipes in your home sealed?