How to keep mice and rats out of your outside AC unit
Rodents seek shelter inside outdoor air conditioning units during the winter to nest, which can chew through wiring, fry the system, and cause expensive damage. This leaves homeowners dealing with unexpected HVAC failures and potential secondary infestations.
What the numbers show
Counted from the threads quoted below. How we count
- This comes up 26.2× more often in r/HomeImprovement than across the site as a whole.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
How to keep mice and rats out of your outside AC unit
- 1Checking inside the unit weekly through the top fan grilleDid this help you?
- 2Peeking in and vacuuming out leaves and pine needlesDid this help you?
- 3Sealing entry points with hardware cloth or steel woolDid this help you?
- 4Using cat hair or cat waste around the perimeter as a scent deterrentDid this help you?
- 5Using expanding spray foam with cardboard or drywall sawsDid this help you?
- 6Using rust-resistant copper mesh combined with windows and doors spray foamDid this help you?
- 7Using duct seal putty where the lines enter the homeDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“We have some HVAC lines that I think rodents may be entering through. We had roof rats last year and I’m trying to prevent another infestation this winter.”source ↗
“we had an opossum come up ours and have babies in the attic which was fun”source ↗
“Same issues here.”source ↗
“According to the AC guy who fixed it, a family of rats from the woods moved into their 3 year old AC unit in the previous autumn (2025), set up their overwinter home and must’ve completed the circuit and fried themselves before my folks put the tarp on for the winter, and my dad didn’t notice it.”source ↗
Where this came up
People with this problem also raised
- 4How to stop birds from nesting in high dryer vents without cleaning a clog
- 3How to clean a mouse nest out of car vents
- 3How to stop mice from coming up floor drains
- 2How to protect an outdoor AC unit for winter
- 3Keeping squirrels from breaking back into the roof
- 7How to seal window gaps around an air conditioner against insects