trader_4 wrote:
Thanks for all of your comments. It looks like the next step is to
block off the returns in the unheated rooms. I found a "Magnetic
Vent Cover" here that I will try:

http://www.lowes.com/pd_73204-34146-...d=3260659&pl=1

I bought one of those and it was totally worthless. The magnet is so
weak, the cover heavy, that it won't hold on.

If the vent is on the floor, some soft foam or even a folded towel would work,
with a board or couple books on it. On the wall, plastic sheet and a few magnets
would work fine for a return vent, since the negative air pressure of the return
will "suck" it into place when the fan runs. Make the plastic a little bigger
than the vent, ant it will seal against the wall too if there is leakage under
the vent..