Using Whole house fan with a central air.

I'm not an expert here but do you have registers to "pipe" the central air to the second floor? The whole house fan will just move the cool air from your downstairs right out of your house. This would be throwing money out the window.

The best method is to get the AC working properly. Be sure you have plenty of insulation in the attic and that it is well ventilated to start with. The sun will really heat it up as you have seen.

Be sure filters are clean and not restricting air flow, blower is operating at the proper speed. No leaks in duct connections, return vents are working properly and open.

Running a whole house fan with AC is going to cost a bundle. You dehumidify the air with the AC and the fan is just going to suck in warm moist air You don't want that.

Be sure all the vents on the second floor are open all the way in rooms you are using, closed partially in any room of the house not being used. Close down the first floor vents just a little and see if it forces more air upstairs. Allow some time between adjustments to see what affect it is having.

Most people keep the vent registers wide open in every room. There is a reason they are made to be adjustable, that is to balance the heating and cooling through the house.

The best thing would be to get the central air balanced, or dampered properly. Close vents downstairs and force more up. Or put power dampers on and thermostat the upstairs.

What is the temperature outside? If it is hotter than the inside, which it could be until 9 or 10PM or later, you'll just make it hotter inside to run a whole house fan.

Plus adding all that humidity, which is uncomfortable in itself and makes it harder to cool the air.

There are not one answer fits all for your questions. However here are a few notes:

A properly designed and installed A/C system will cool all areas of a home properly. Yours appears it was not properly designed to start with. Additional ducts and returns may be needed. You may need additional cooling capacity or not.

A whole house fan can be a plus or minus. If you live where humidity is low and the nights are cool, then using the fan as the evening cools off can save you money and might solve your problem as well. If you live in a humid area, like I do, it would certainly greatly increase your cooling cost and might even reduce your overall comfort, as well as damaging fine furniture with rapid changes in humidity.

Is it possible you don't really have a whole house fan, but rather have a attic power vent to move the hot air out of the attic. If so it may or may not help. Sometimes they can cause circulation problems an actually make matters worse. Usually good static venting of the attic along with proper insulation between the attic and the living area is best.

A HVAC professional can take measurements and do the math (Manual calculations) to determine what you should have and compare that to what you have. They then can make some recommendations based on those numbers and their experience in the area.

Another possible issue is a "helper" A/C. Maybe a small window unit to assist your central unit in those areas that need a little help. I have one in my exercise room due to the temporary extreme loads it gets. I am considering something similar for my bed room as a temporary fix until my current HVAC dies and I fix the problem properly. It is getting about that age now.

A fan control can measure humidity...

Nick