I have a two story house, 30 years old. Temperature upstairs is five or six degrees hotter in summer than downstairs.

I understand that I can not zone the vents in this house. Would a variable speed A/C help? If the temperature downstairs is below the thermostat setting, can I get the fan to come on to cool the upstairs?

Would an attic fan against the gable help in addition to a turbine fan?

The cause can be many but usually the lack of a large enough high wall return air on the second floor is the major problem. Sorry to say but the cure in not very simple if yopu are going to try to get your old system to work correctly on the second floor.

The quick way to help the second floor is to ad a A/C unit in a window upstairs. The fan to remove attic heat will also work but you also have to have low vents to allow the outside air the circulate.

A variable speed A/C is of no value to help the second floor.

Why can't you zone the upstairs?