Install a mini-split upstairs, it will keep that zone the correct temperature using it’s own thermometer. It can be downsized a bit as the main HVAC unit will help it out using your existing ducting.

For exact design, I would say have one per bedroom or a multi-zone mini-split that can cover each bedroom.

Many (most?) mini-splits are DIY friendly as well, if you want to save a bunch of money and install it yourself.

Just piling on at this point, but we made 2 changes last spring that made summer so much more tolerable in our house.

1. More insulation. I bought a cheap thermal camera on Amazon and found entire closets and a bathroom with no insulation. Those rooms are a solid 10+ degrees cooler now.
2. More ventilation. Half my house didn’t have any soffit vents, but had attic vents. Adding soffit vents made that half the house 5 degrees cooler all on its own.

And we haven’t found ourselves needing it, but a mini split has popped up a lot here already and is a great idea.

The mini-split route idea is a great solution, but I am wondering if you have an unvented or poorly insulated attic that is a contributing factor to your problem. If your attic space isn’t setup properly and you throw a mini-split at the problem, it is sort of like pumping water out of a boat without fixing the leak that is flooding the boat. Even something as “simple” as a powered attic vent could help in HVAC performance and operating costs. If the hot air can’t vent or your insulation is inadequate, it just stacks down and heats the top floor.

The other thing I would be looking at is if the HVAC run up to the second floor is done properly or if the dampers are not right. A house that size really shouldn’t have much trouble if everything is balanced right.

Closing vents on the first floor and leaving the second floor open will increase the wear on the fan and may decrease efficiency if not enough air can be moved through the open vents. Knowing how bad an idea that is would require some measurements and math.

Having the nest with a second temp sensor would just force your HVAC to work harder to cool the upstairs. The single sensor nest can be a smart move to reduce costs with the higher degree of control.

Having dampers installed does more or less the same thing as closing vents, but with a higher cost. A zoned system is expensive and kind of silly on a house of that size.

I dont have HVAC specific advice for you, but I’ve lived in a well isolated house in a hot environment (42C max temperatures) and what we did was open basement windows and atic windows, and have a fan point out of the attic window. This created a draft that exhausted the hottest air in the house out, and while upstairs was always hotter than downstairs, it was much more bearable than without.

I imagine a similar setup would help you get the cooler air from downstairs up, and a big advantage is that you can try it with very little cost and commitment.

I don’t know how your house is configured, it might not be a solution for you. We have a pretty habitable attic, not those insulation filled dens I see in American houses, so it was pretty easy to try it for us.

Also, we opened and closed the windows every day, we didn’t leave it open overnight.

From the “redirect the vents” side of things, I’ve been doing this manually for the 7 years with no ill effects. Last year I added a Flair system and Ecobee to automatically balance using the registers. They have back pressure detection to prevent damage to the HVAC system so there’s always enough vents open. At least in my scenario it’s been a game changer for the third floor of our townhouse. As we’ve headed into warmer months our bedroom is actually cool in the evenings and the lower floors are normal temperatures. During the winter our living space on the second floor was cozy without blasting the bedrooms and making it too hot to sleep. With the number of vents I had it cost just over 1K to do, but that was way cheaper than it would have been to have the house and system rezoned.

I’m into smarthome stuff so now I’ve actually got room level presence detection going and tying that back to Flair with Home Assistant so we only cool or heat occupied rooms. Wife is a very happy camper in her now temperature controlled office, and it only targets the office when she’s in it.

Minisplits. I put one in every bedroom because even with a big new hvac system, the second floor was too hot.

They work wonders.

The nests (and many other thermostats) let you operate the fan independent from the AC.

I configured my fan to run 15 minutes every hour regardless of whether or not the AC/heat is running and it fixed all of my issues with the upstairs being way too hot.

Not an HVAC tech, but where are your air returns located? I would agree that keeping the fan on will help mix the air in the house. Closing all the registers on the lower floor might stress the system, but you could close a few I’d wager and be safe.

Just had to solve this problem myself recently. Same issue, upstairs stifling while downstairs stays nice and cool. For a whole year I just ran the system a bit colder, but that made the downstairs freezing in order to make the upstairs feel normal. Not to mention all of the energy waste running the system for so long, my home is 2300 ft.² so I went for the dual zone with a mechanized damper. You could save on the cost by installing a manual damper that you could adjust as needed so far the system marks very well and my upstairs can get nice and cold, while the downstairs is a cozy warm temperature. I paid an HVAC company $2,300 for this to be done, but the manual damper would have been just $600.

Other options I considered though were going to mini split route like the other comments suggest, or if your HOA is strict about even that, you could even use an indoor AC or two, and have the exhaust vents blowing out the window.

Turn on any upstairs bathroom fans. I find it gives the hot air an escape and allows more cool air to flow upstairs

I had the same problem, in a similar sized townhome. My solution was similar to your first option. I bought and installed a wireless thermostat. This was back in the late 2000’s, so it predates the “smart” thermostats like the Nest. It’s just a basic programmable thermostat you can move around.

It works well enough. In the summer, I can move it upstairs so the upstairs stays cool. In the winter I could move it downstairs, but generally I leave it upstairs anyway because that’s where the bedrooms are. I remember the thermostat was a bit pricy back in the day, but I’m still using it some 15 years later so I’ve gotten my money’s worth out of it.

Here’s a couple of cheaper ideas you can try before replacing your unit:

• I have a smart thermostat that lets me configure the fan to be on X minutes every hour. Helps circulation, evens out temps
• it has remote sensors, and I can tell it which to follow at any time or to average
• I have partly closeable vents, so I can direct more heat downstairs in winter, more cool upstairs in summer. This is not as effective as dampers and you need to watch shutting down too much airflow, but it’s cheap diy that can make a difference
• my brother swears by pointing a pedestal fan up the stairway

I use a portable AC - this is different from a window unit. The unit itself stands up inside your room, and it has a flexy hose that goes into a flat panel that’s about 10 inches high and expandable widthwise. You lift the window a bit, put the flat panel in the open spot, then close the window so the light pressure keeps the flat panel in place. It’s all on the indoors side of the screen, so it counts as being inside your house and nobody can complain.

(Assumptions: you have the typical American sliding windows, and your HOA doesn’t have rules about the inside of your house like curtain color or whatever)

Close the downstairs registers?

Same issue as you. Second floor bedroom above the garage gets really hot, but we can’t stick a window unit out the front of the house because it would be conspicuous, ugly, and prohibited by HOA.

What I do have is access to the attic part that is adjacent to that room. So what I’m thinking of doing is punching a hole thru the drywall of the bedroom into the attic, mounting a window type AC unit in there so the attic is the “waste heat” side. I would install a drip tray to handle the condensation, and maybe something else to vent additional moisture and heat out of the attic. They make solar powered fans you can install that pull air out the top of the roof.