If you believe it's as simple as asking on the worldwide web and recieving silver bullet solutions to each of your goals,it doesn't work like that. (A) As capabilities and sophistication of hvac system increase,the installer must be increasingly educated and experienced to properly install and/or maintain and repair it. Choose carfully because a poorly installed high dollar system is less effecient than a basic system that has been properly installed. (B) Unless you are trained in hvac desighn,you wouldn't understand it if an expert laid it out for you. (C) If your installer doesn't know how to accomplish your goals,what are the odds you can tell them how based on what you are told here? (D) Even if you are savy enough to program the system,it must be properly desighned and installed before it will do the dance that program tell's it to. The short answer is yes, a variable system is worth the additional programming complexity. As "a technical person",wouldn't you agree?

I have been through all these challenges and here is what I would do. Find a good resource that has the capability to assess your specific situation and recommend the design and best equipment to accomplish your goal. They can also recommend companies that install and maintain the systems. I would not relay on HVAC company to do this base assessment. They are selling products and service not necessarily system designs. We have had the so called multi phase system. It cost more in electricity, it was a bear to maintain and on going maintenance was more expensive. Every power blip caused us to have to reset the system which was a pain. I am pretty savvy electronic person, but the more “sophisticated” these systems get the more costly and troublesome. Get a good system engineer experienced in HVAC.

"I would not relay on HVAC company to do this base assessment."

A good HVAC contractor can assess the situation and propose approaches.

Heat rises, that's always a challenge with a two (or more story) structure with an open stairway. The best way to handle it is with separate systems for each floor. It's hard to rejigger existing ductwork when it's unfortunately done as you describe.

Often in these situations with an existing house, it may be the best solution would be to add a mini-split system to boost upstairs cooling. Some have new types of room vents that are more normal and attractive looking than the classical box mounted on a wall. If attic access allows doing so, you may be able to have a very presentable look for a supplemental system. Let an expert tell you what's possible. Good luck

"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."

Well, first of all, the systems are not set up to achieve what you want. To address your issue, one of the systems need to be set up for downstairs and the other needs to be set up for upstairs. This may or not be possible with your existing ductwork. Secondly, it's not clear how the single AC would cool the upstairs(?)

Thanks for the responses.

I'd love to find a good resource, whether an HVAC company or not. However, I do not feel confident in my ability to assess that resource's ability to design an appropriate system.

I've begun reaching out to several HVAC companies hoping one will stand out, but they are all single-manufacturer companies, so it's hard to get a reasonable cross-manufacturer assessment.

The existing ductwork cannot be changed for <$30k, including ceiling repair. That idea is out. The upstairs has no attic... all vaulted ceilings. The cooling requirements are not strong... it's rarely above 90 F for more than a few hours and always down to at least 75F overnight. That's how the undersized AC keeps up.

A minisplit system may be what we end up doing in the one upstairs bedroom. I just hoped to learn about how well variable speed fans work to spread around air and even out temperatures... and how loud they are when running at low speed. The one HVAC company I've spoken with at length believes a Carrier Infinity variable fan running continuously at low speed would cut the temp difference in half. He also believes we would barely hear it 10' away.

Are you up to the task of doing tasks hands on (not ac installation)? Are there budget constraints? The suplimental mini-split is a viable option.

Throwing in a mini split won't do a thing and will become the slave to cooling both levels of the house, cool air will still drop and warm air will still rise forcing the mini split to run almost continuously, and temperature differential between the two levels could even increase . It will be like trying to fill a sink with the drain open full of water. Your problem is also the most common and the answer is mixing and recirculating the air continuously to achieve any level of balance without sealing off the upstairs with doors and balancing the load with one system or using two systems.

The simplest and cheapest way to achieve mixing the air between the two levels is to turn the Fan to "on" and let it run. Or you could replace the thermostat with a new controller with multiple inputs. One to turn the heat and cooling on and off and two additional inputs for each floor to turn only the blower fan on if the differential is above a set level. Turn your fan to the "on" setting from auto and see if it makes a difference. Also ask your HVAC guy about changing the fan speed for the fan ON mode if necessary. You solution may be as simple as flipping a switch.

"One to turn the heat and cooling on and off and two additional inputs for each floor to turn only the blower fan on if the differential is above a set level."

This is something I asked the one HVAC company that's come out so far, and they said that Carrier Infinity controllers ($1200!) can't do this. I would think that's the advantage of having the EMC blower in a programmable multi-sensor system... to be able to tune parameters like this.

We may end up putting a mini split system into the one upstairs bedroom, just for some nighttime summer cooling in the bedroom that is not connected to the AC... not to do anything about the whole house problem.

Incidentally, I've begun a test with the fan on my current system, and so far, the temperature has been the same between upstairs and downstairs, off by only 1 degree. But the clouds are diffusing the sunlight so we aren't getting the typical sun-heating.

Sounds promising. We have had this issue in 3 homes. Hope this works for you.

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

1. Equalize the temp between upstairs and downstairs
2. Quiet the noise, both of the AC and also the heating/fan
3. 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.

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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.

Austin Air said "it's not uncommon to find a line in the sand --- a line you won't cross no matter what".

And it's impossible to help when folks asking for help choose to keep their lines secret even when prompted. Neglecting to answer these questions when asked above,could only mean that OP want's someone to tell him for free where,who or how to get what he wants for free and isn't willing to put any effort forth to bring it about.

Klem1 said above "Are you up to the task of doing tasks hands on (not ac installation)? Are there budget constraints?"

A person that was at all motivated to get their house comfortable would say something like "I'll spend as much time as neccessary doing what I can and I have $2,500 to put toward this?"