Is replacement cost of a/c unit an entire home business repair expense?

[MichaelMc]

That is going to depend on more facts than I have access to, but here are some guidelines.

If you are replacing a wall-mounted or window-mounted single room air conditioner, that is going to benefit only the home office space, that would be a direct expense. If the cost is under $2,500, you may expense it in the current tax year as Business Equipment.

If you are replacing an entire HVAC system that serves the entire house, that would normally be an addition to the cost basis of the home. For purposes of your home office, you can set it up as a business asset, just like your house, itself, and depreciate it over its MACRS useful life, taking only the portion of the depreciation defined by the ratio of the area of your home office to the area of the entire home.

For more information, please see IRS Pub. 587 Business Use of Your Home and IRS Pub. 946 How To Depreciate Property.