On Wednesday, January 27, 2016 at 7:50:07 AM UTC-6, Stormin Mormon wrote:

So, if the air leaking out of the back bedroom is
colder since the vent is closed, you still need to
put the same ammount of heat into the house?

If the heat vent is closed and the cold air return is not blocked...there will be negative air pressure in that room that will draw in some warm air. How much would be the question...