Some of the larger homes I work in do have multi furnace situations to heat various areas of the home (7-15,000 SQF) I realize that most are not looking to install a second or third furnace.. but there are things you can do Zone heating!!! it may cost some $$$ but it may be worth it and I have no idea about your portfolios so..basically it consists of dampers that open and close according to the rooms temp a room is cold?? Close off the rest of the house and heat that one room... of course this could vary depending on the amount of zones
Just a thought and I know frig all about HVAC... Just what I see and soak up
Gerritt.