This works for some...but the way some systems are laid out it can't be done (correctly) without going to powered dampers...mine was one of them.
The other thing people need to keep in mind is keeping air flow available to the wood furnace when the power goes out...especially if it happens right after loading a big load and the fire is well established, to the point of being "on cruise control"...the furnace is still gonna get hot, but without some gravity air flow things can get extremely hot, including the ductwork...that's why the CTC specs on ductwork.
Anyways, gravity dampers can block gravity air flow...again, depends on your system though.
yep, everyone's setup differs. I don't even have my wood furnace hooked up the return duct. Allows both my wood and LP furnaces to run at the same time if needed. I have a power open/spring close damper in my supply line connecting the wood furnace to the main supply duct and a gravity damper in the plenum of my LP furnace. The way mine is setup works fantastic for me, however, may not for someone else.