You possibly could have an improperly designed duct system. A quick band-aid type fix would be a duct booster, but it might also amplify any potential leakage or blockage problems. Or maybe you don't have enough returns in your downstairs. I had a similar problem in my 1st floor living room until adding two extra returns. Now way more comfy.

Moving the thermostat will not help this problem. You can't solve HVAC design problems by moving thermostat or putting in these remote sensor things. Basically these gadget makers are just interested in selling you more gadgets. If your location isn't so hot it might help, but typically in short order you see the limitations of such. I live in and fix air conditioning problems in a otherwise hot climate. (Katy, Texas area) You have quite a few challenges in what you have explained as your situation. There could very well be other problems than what I say in this post... *read carefully*. Design is a critical attribute as to how well or poorly an AC system will work. Location is another factor --- due to climate concerns. In a 2 story structure you have two separate living areas divided by floors. With only 1 AC system to cool both floors of a 2 story structure that is going to present problems. Because the thermostat that decides if the system is on or off is limited to sensing temperature from an area of about 5 feet around where the thermostat is placed. So let's put in sensors! Well as you have seen this only keeps the system running and dumping cool air into areas that are already cool making them colder. Let's move the thermostat? uh no, it will never work. If a sensor won't fix it neither will moving the thermostat will make a difference. Design: returns are useful in pulling hot air / keeping this air from being trapped. However, having a lower level with no return and HVAC system in the attic you're talking major expense to fix this the right way. If you were to figure out a way to run a new return from the lower level (where it's warm) then duct this to the system in the attic... this would help. Think boxed in chase on second floor to attic with filtered return grill cut in thru the ceiling to the first floor. (The return grill would ideally be in the ceiling on the first floor in the room that is warmest for AC season.) Yes, you will give up floor space on 2nd floor to accomplish this. Next step would be to zone this system for better control. First floor would be zone 1, Second floor would be zone 2. Ideally for a HVAC zone system to be all it can be would be to have at a minimum a 2 stage AC and 2 stage heat. Zoning is a complex thing. If it's not done right more problems will ensue. The system needs to be able to work well in every season... when it's not so hot outside as well as when the heat wave comes. Just as with the gadget makers there is an awful lot of garbage that has come out over the past few years in zoning controls. If you do it on the cheap, with the inexperienced it probably won't last long and problems could be quite severe. I have a 4 zone system in my single story home. I wouldn't put something in my own home if it didn't actually work. BUT, equipment selection as well as design are just as important. Design are the invisible things that very few people talk about when dealing with problems. They would rather sell you a gimmick. As I said earlier up in the post --- I service the Katy, Texas area.

Sounds like Freon related problem. (Without inspecting in person -- it's only a guess based on 26 years of actually doing this.) Typically the room farthest away from the HVAC unit will exhibit temp problems first. I service the Katy, Texas area.

However, if the AC is running to cool the upstairs down to 75, the downstairs will be between 65-68 degrees. There are two systems in this house, but they each do both the upstairs and downstairs on opposite sides of the house. Only the system for the main living area has AC. It is a 16 SEER Carrier. We are looking to replace all the heating and AC units and want to achieve these goals Equalize the temp between upstairs and downstairs Quiet the noise, both of the AC and also the heating/fan Improve efficiency How would this best be accomplished? It's better when you separate floors and use 1 HVAC system to cool upstairs and the other HVAC system to cool the lower floor. Because heat rises and cooling falls that lower level unseparated from the cooling system will always be too cool. You have two systems so it would be best to separate those to heat and cool one level / not a mixture of two levels. It sounds like this might be challenging due to cathedral ceilings and your disdain for spending money with the notation that these ductwork changes can't be done for <$30K. So you're at the point of letting the structure defeat you? Certainly I can understand why, it's not uncommon to find a line in the sand --- a line you won't cross no matter what. If that's the case, sell the "box" and buy a different one. Some homes are a challenge for more reasons than one. Cathedral ceilings, large walls of glass look oh so pretty... you get a mountain view (probably for your Colorado home) but once the heat comes that view gets distorted. Builders do all sorts of things to sell homes, you don't realize the downsides to what they do until you try to live there. ----------------- By separating these areas and zoning the upper floor to the upper floor and zoning the lower floor to the lower floor... how does that work? If you're not calling on the lower level there is no cooling. If the upper level is calling only that area is getting the cooling. Cool air will fall to the lower level but this amount will be much less than supply duct work blowing air down there. I service the Katy, Texas area.