I have a 2 story house and about 20 years old. I recently bought this house.
I do not know so much about to maintain properly.

I have a delime that, when I put the thermostat to 70, the downstairs always get cold but the upstairs get hot. As long as the hot air comes from room vent, it feels warm and I tried to put the thermostat to 65, it gets cold in a short time, this happens only downstairs.

I checked if there are any leaks in the windows, doors etc, I did not see any, But I found cold air coming from Chimney, I closed the chimney cover but still air comes from inside.
I covered the chimney from outside with trash bag, helping little not much.

My questions are

Anybody know if there is any material I should use to cover chimney because I don't use it.

Is there a way I can check if the house is reasonably air tight, if not where and how to check?

Thanks