Weak air conditioner air flow and very hot upstairs.

With a two story house you have to be aware that cold air flows downwards so any cold air coming from an ac unit will flow downstairs( normally down the stairway) so unless you can close off the down stairs then the up stairs unit will be cooling downstairs as well. Ac units have a thermostat located in the air intake and it cools until the cold air has risen to that point. In effect the space above the ac unit will be hotter than the floor. Knowing how they work ,you can see that the upstairs unit will never reach the set temperature because the cold air is continually heating from the house and will never get to the top thermostat.
So your solutions are
1-- fit controls that restrict the cold air from leaving the upper floor (Doors)
2--don't run the upper ac unit unless you are living up there (waste of money
)3 place an ac in each upper room and keep the room door closed

How large a space are you hoping to cool? One room? One LARGE room? One SMALL room? One floor of a two-story house? A whole four-story house?

What's your climate like where you live? Very hot, dry, year-round? Cool most of the year but two weeks of hot during the summer? Just a little too warm in summer? Blazing hot all summer, including nighttime?

How's the insulation in the building you want to cool? Is it a plain steel building with no insulation? Is it a trailer? Is it a frame house with 4" walls and R13 fiberglass insulation? An old frame house with no insulation? An adobe building? A timberframe house with 8" of polyisocyanurate foam insulation?

All these are important to know when you select an air conditioner.

For most of the USA, assuming a frame house with 4" of fiberglass insulation, a 10,000BTU air conditioner will cool one large room or two small rooms pretty well, or a 5,000BTU AC will do one medium-sized room. Different places, and different house designs, and different types & amounts of insulation, will all have a profound effect on how much AC you need/want.

jbrown - Hot air rises, Cold air falls. If you have ceiling fans upstairs, turn them on to run counterclockwise at highest speed you can stand. Make sure all the windows are closed upstairs. The ceiling fans will help to keep the cool air upstairs and pull some from downstairs.

If you don't have ceiling fans, invest in a box type fan that you can place at the head of the stairs (that's the top of the stairs) run it on HIGH and it will act like an exhaust fan, by pulling cool air upstairs. It may take a while, but it will help cool the upstairs to some degree.

Hope this helped you. Please let me know. Thanks.

Assuming that there are ducts ran upstairs and downstairs, it all depends on the square footage of the living spaces, i.e. bedrooms, living room, dining room, etc. You need an average of 1 cfm per sq ft. For every 1 ton of air, you have an average of 400cfms. So your 5 ton unit will cool/heat 2000 sq ft. A 16 seer unit with a 95% furnace is a nice buy and you will notice the difference for sure. Hope this helps!

the techs probably checked the units operation,but did they get in the attic to see that the duct work on the return air side of the unit is tight and not sucking in the attic air.

The one on top probably has no water in it or the pump is not working. Look closely at the front to ascertain if water is being pumped.If not turn electric off and remove one of the side panels with a 1/4 driver. In the reservior there should be water. If not you need to turn water on to unit or adjust/replace float. Also is it's warmer than 100 degrees outside you're unit might be fine...it has limitations.

That depends on to many items like what city is located, how and what type of insulation is in the house, how many windows it has and if they facing to the sun etc.The best way to know what you need is make a heat balance and they will tell you exactly the amount of BTU you need for the house But a 5 ton unit is probably enaugh unless you live in a very hot place in the Caribean.

Some of this depends on where the air handler/furnace is located. Most often in two story houses, it's in the attic upstairs, or in a closet upstairs. A duct routes air from the furnace to the downstairs supply duct, so it is a distance from the air handler.
When the unit first initiates a heating cycle, that duct is going to be full of cool air, and the duct itself will be cool. So the furnace has to run long enough to push the cool air out, as well as warm the duct up, before your going to feel warm air at the registers downstairs.

Keep in mind, heat rises. And, your thermostat only monitors the immediate area where it's located. In your case, upstairs.
Two story houses are problematic because of this. And one way around it is to have a damper system installed that distributes the air upstairs or downstairs based on a thermostat located in those spaces. There would be 2 dampers, and 2 thermostats (one upstairs, one downstairs). Each stat would control a damper, and the call for heat or cooling.

Let me know if you would like to consider a system like this, and I can point you to components to use. I've put several system like this in.

It sounds like a restriction in your ducting. Feel the A/C vents in the hot rooms to see if cold air is blowing out of them. If the ducts are restricted, then it is possible that the air conditioning coils are freezing up from lack of circulation. Inspect the ducts if you can to see if there is any restriction.

Note that even if you feel air coming out of the ducts, it could be ambient as opposed to cold. The best thing is to measure the air temperature coming out of the ducts. It should be around 20 degrees colder than the ambient air at the intake of the A/C.

Cold air is denser than hot air, so is harder to move. All vents have some mechanism to regulate flow, so close all of the downstairs vents. Check to see if your furnace fan motor has more than one speed, & set it on maximum. Keeping the fan on all the time will help as well. If all else fails, you may need a more powerful fan motor.