"dean" wrote in message

I have a wood furnace in my basement with central forced air, and
right now (all winter so far) I have been leaving the basement door
open so that the cold air return is sucked down the stairs.

I'm considering connecting up the returns to the furnace so as to make
a closed system, and I wanted to know if this makes much of a
difference in fuel usage and/or house temperatures. Right now, the
furnace has a struggle with a 50-degree difference outside vs. inside,
so if its 20F outside, I can almost not reach 70F inside.

Closed loop or open, some sort of return is needed. You'd be better having
someone look at the actual layout of hte house nad heating system. In order
to get the hot air into the living space, you have to be able to get the
cold air out. If you have a multi story house, the basement door is
probably not enough for a good air flow pattern.