On Wednesday, January 27, 2016 at 11:44:09 AM UTC-5, Bob F wrote:
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..

My returns are on the wall and the grille is "thick" enough that the face is
flush with the face of the baseboard.

I cut a cover from those grey anti-fatigue floor mats from HF.

I cut a rectangle that is about a 1/2" larger than the face of the grille
and then attached thin strips of pad to seal the area above the baseboard.

I put a heavy box against the cover to hold it tight against the grille
and wall.