1 room in the house upstairs remains 11 degrees cooler than the remaining rooms. I don't even know where to start with this issue. The thermostat is on the opposite side of the house. But there is a room even further away than the problem room that doesn't get quite this cold. The problem is the worst at night. How can I even out the temperatures?
The reason can be one of a few. Keep in mind that the position of the thermostat plays no part whatsoever. Its like having a light switch that is 1 ft away or 100 ft away from the actual light. It is only used to turn on and off the light.
With that, here is what should be considered.
1) Location of the duct distribution box ( This is where the air enters and then sent to other rooms.
2) A possible leak from not being sealed properly with duct tape.
Is this a new problem or has always been this way?
You can try simple trouble shooting such as close off other vents to see if indeed air does reach the room. If it does, then adjust other registers/vents until well balanced.
If indeed no air is circulated to the room after closing off other vents, then that means something is wrong in the attic. If you are able I would check in there first and look for openings or even the possibility of an animal chewing through the ducts..
Do not ask me how but I have even seen animals get in the ducts with no entrance holes and ultimately pass in the duct and reek up the house.
Should be able to easily find the problem based on the 2 aforementioned inspections and trials. Also look to see if they are all insulated as well.
Ductwork usually has a vent damper below where it rises up from the basement. If it is a manual damper, it probably looks like a wing nut see this picture
It's possible most of the dampers are closed or partially closed, and the really cold room is fully open.
If you have dampers like this, you can loosen the wing nut and adjust the damper with a flat screwdriver. Usually the blade orientation matches the orientation of the damper. So if the blade is parallel to the duct, it is fully open.