A repair would be an expense. It would fall under the maintenance and repairs category.
Was this an AC for your business or for your home? If it was for your home, you cannot deduct it, unless you have a home office, then you would prorate the expense based on your home office to the rest of your home. This would be taken under the home office category for maintenance and repairs.
If this was at your business, then this would be deducted as an expense under Other Common Business expenses Repairs and Maintenance.