We have our thermostat set at 75 degrees (higher than we would normally set it) and it will not get above 65 degrees on the main floor and yet the upstairs is about 85 degrees. We do have a lot of windows on both floors, but most of them have some sort of window treatments covering them once the sun sets. Also, on the first floor the heat comes out of vents in the ceiling, but upstairs the vents are in the floor. Could this be the problem? We do not have a basement and our great room is built on a concrete slab, so can we put vents in the floor? Our furnace is also about 18 years, maybe that is the problem. Please advise. Thank you!

You tried closing upstairs vents to push air out the bottom? Making sure there aren't dampers to the lower levels that are closed or dampers to the upstairs that could be closed?

Can you feel air coming out of the downstairs vents? Is the pressure similar to what you feel upstairs?

Yes, the upstairs vents are closed and the downstairs vents do have warm air coming out of them. The downstairs vents are different than "typical" vents and the pressure is not as strong, but they are all open and seem to be working properly. Thanks for your answer.

Where is are the cold air return vents located? If there is a big one close to the floor downstairs, it will pull all the cold air into your furnace to be heated. If there isn't one downstairs, or if it is small and near the ceiling, OR if it is blocked by furniture or something, all the cold air in your house will pool downstairs with nowhere to go! If that is the case, adding a return vent at the downstairs floor level will help a lot.