I am thinking of moving my thermostat to the second story hallway. I set the unit on 74-75 degrees and it is still very warm upstairs. You can feel the difference as you go up the stairs. This is also with the first floor ducts closed. All air is going upstairs. I am sure moving the stat upstairs will cause the unit to run longer costing me more, but I think we will be more comfortable in the long run.

Will my idea work or am I wasting time and money. I can pick up the old wire through a cold air return.

Only plemum rated cable or wiring method can be installed in a duct.

Seems you already did try the logical action by balancing the air duct system to force more air to the warm area.

THere may be a more difficult situation here, with improper insulation causing the second floor to loose what cooled air is delivered and the outside heat is not being kept out, and the cooled air cannot overcome and maintain temperature.

Try running some cable temporarily to the second floor along with the stat for a few days to see if your plan will help, however, I do believe you are correct in that the unit will continue to run longer to attempt to maintain temp. really should look at the insulation situation because all your cooling will be wasted.

Usually meens not enoughh
Return air

Simply turn the thermostat to a lower setting downstairs and you will get the same effect. No need to move it... It's going to be colder downstairs no matter where the t-stat is located...