unless the units are no longer functioning you can not properly expense the remaining balance.
2 options
1) allocate part of the sales price and pro-rata sales cost to them relative to their FMV . say HVAC FMV $4,000 total property FMV $400,000 then 1% of sales price and sales costs would be allocated
2) since FMV may not be easy to determine another alternative is to allocate just enough of the sales price and cost equal to the depreciated value so there is no gain or loss on the item
