I live where it is about 36 degrees during the day and 22 at night. I have closed my flue. Checked all my connections of the plumbing to the vents and they are connected. Doors and windows sealed properly. I have closed all my vents upstairs and blocked off the return upstairs. My downstairs is still quite cool and the upstais is still warm. Almost 8 degrees different! No matter what I do to the vents the downstairs is always noticeably cooler then up. The house has always been this way. The non-digital thermostat is 2 rooms away from the return downstairs. What do I do??!!