DO YOU LIVE WHERE IT GETS SUPER COLD IN THE WINTER?
If your attic gets really cold in the winter, you want to find a whole house fan that will keep that cold air up in the attic and not let it down into your rooms.
I have heard of some people crawling up into their attics to put blankets over their big propellers in the hallway. Others buy magnetic covers for the ceiling grilles.
If you want to avoid all that hassle, consider putting in the Tamarack R38 or R50 motorized door systems or the Invisco R60 dual damper system. Both will give you excellent thermal insulation all winter long. Invisco offers both a gravity operated damper and a motorized damper.
There is a specification for new homes that a lot of older homes would do well to follow. It is 2012 IECC spec. You can see more about it here. In a nutshell, 2012IECC changes the requirement for R38 in the middle of the US to R49. Both Tamarack and Invisco meet that requirement.