I have a ranch with a walk out basement around 4500 square feet. I built in 2012 and on 90 degree days the upstairs can't keep up. When they ran the computations we said a 4 ton would do the job, but when it hits 85 or above the unit can't keep temp. Here are some basic facts. Bryant evolution, 4 ton, 4 zones. Two upstairs two downs stairs. My contractor swapped out the other unit and replaced with a five ton unit. I think there is not enough return and the unit just can't keep up. He is telling to cover all my windows and that will help but it is a contemporary home and it was not designed to have blinds and drapes everywhere. We spray foamed the entire house. My question is how much return capacity should I have and what questions should I be asking. He just cranked up the blower and the system is a whistling., which is unnerving.