First, how far away from your window are the intake vents for the HVAC system? Depending on distance and relative locations of the HVAC system fresh air intake, the amount of vapors you would be discharging through your duct work would probably be sufficiently diluted before reaching the intake. In addition, the actual air velocity exiting the ductwork may not be sufficient to cause vapor entrainment into the HVAC system.
Second, if a spray booth is totally out of the question, consider simply opening your door and window where you are going to be spraying, and putting a small fan behind you as you spray to help dilute any vapor concentrations, and in addition, give consideration to wearing a respirator while you spray.
Gip Winecoff