Insects
Central air conditioning air returns provide an ideal hiding place for a variety of insects. spiders, houseflies or dust mites, but they often aren’t noticed because they don't present a problem until populations become significant. Bed bugs and fleas can also hide in air return vents, particularly if you have a large infestation. However, they don’t typically remain in the venting system for long periods, preferring carpets and bedding, which are usually near a host so that they can feed. If you notice a sudden increase in spiders or flies around your home, cleaning your ducts and venting systems might help to eradicate the problem.
Mice
Mice can easily enter your home. They can climb vertical surfaces and can squeeze through holes as small as a quarter inch in diameter. Once inside, two mice are capable of producing thousands of offspring because mice reproduce year round and a female can become pregnant with a new litter while the previous litter is still nursing. When no predators such as birds are present, mice populations become large and they’ll invade every space that is hospitable to them, including your air conditioning vents. Because your air conditioner air return vents carry warm air outside, mice might be attracted to this warmth. As long as enough food is nearby to feed them, mice can nest inside.
Rats, Squirrels and Chipmunks
While rats don’t produce as many offspring as mice, they do prefer to live indoors and can become a problem because they adapt to almost any environment. Rats, as well as chipmunks and squirrels, can make their way inside your home through gaps or openings as small as half an inch wide along eaves, chimneys, windows or in your foundation. Once inside, they can nest in your air conditioning return vents. Squirrels and chipmunks are less likely to live in an occupied building, but it is not impossible to find a nest in your vent system after a season of disuse.
Eradication
Poisoning is not recommended to eliminate animals that are living inside your air return vents. If they die in an area that is not accessible, their decomposing bodies cause odor problems and potential health risks. If you suspect that insects or animals are living inside your air conditioning vent system, it should be cleaned to remove the problem. The best method of cleaning your air conditioning vent system is to hire an expert who will have specialized equipment that can clean out all areas of your system. Experts also have access to chemicals that can kill contaminants such as mold.