comments:
The system is too small for the house, and the ducts are too small feeding upstairs. These are common issues. The giveaway clue that your house was likely built by an incompetent builder who didn't care about anything except profit lies the fact that you have a two story house built without two separate heat and air systems. If you measure the air temperature at the return vent (the intake/suction vent, usually the much larger one with a filter behind it) and measure the air temperature at the room supply vents while the system is cooling and stable, and you find that the difference in air temperatures in those two olaces is between 14 °F and 20 °F different, then you can be pretty confident that the guy is right – the system is functioning correctly for its capacity – and the problem is that the installation is inadequate. Partially closing some downstairs vents and removing the grilles from upstairs vents might improve the situation for you. Purists will object but I would argue that there's no reason to respect the design... of a bad design. Window air conditioners and central air can also work perfectly fine together to help cool hotter rooms in your situation, so adding a window unit to upstairs rooms that are two hot would not be an unreasonable action to consider. Walmart has these little guys for $156 right now. This is the smallest and one of the least expensive units around. It says it's good for rooms up to 150 ft² but you don't necessarily need it to be big enough to cool the room on its own, since the window unit does have some help from the central system, and this might be a solid and inexpensive strategy. It's what I would do, and have done. The ducting to my office, that someone built onto the side of the house, is tiny and inadequate, so I have a window unit in rhere to help the central get the job done. https://www.walmart.com/ip/18153521198?sid=38d124f9-ebba-4a14-802c-6417364b60c7
They’re for sale at Walmart for $156 and this is the exact model I have. I have two of them and they work great.