Comment by jasode

>Except you can write off those $1400 units as $2400 of losses that will offset profits

No, the IRS doesn't allow tax deductions for the "missing $2400 rents" of unrented units. IRS only allows deducting ongoing expenses such as "maintenance, insurance, taxes, and interest," :

https://www.irs.gov/pub/irs-pdf/p527.pdf